Chapter 3  Life in Water

What's in a name? Often, names reveal what we think about the people and things closest to us, about friends, rivals, offspring, pets, old cars, or boats. Now, what sorts of names do we use for the place we all share, our home nlanet? Those names, whether in English (earth), Latin (terra), Greek or Chinese (地球, di qiu), all refer to land or soil, revealing that cultures everywhere hold a land-centered perspective. The Hawaiians, Polynesian inhabitants of the most isolated specks of land on earth, call the planet ka honua, an allusion to a level landing place or dirt embankment. This universal land-centered perspective may partly explain why portraits of earth transmitted from space are so stunning. Those images challenge our sense of place by portraying our planet as a shining blue ball, as a landing place in space covered not by land but mostly by water (fig. 3.1).

FIGURE 3.1 Earth from space.

Life originated in water but from our perspective as terrestrial organisms, the aquatic maim remains an alien environment governed by unfamiliar rules. In the aquatic environment, life is often most profuse where conditions appear most hostile to us: along cold, wave-swept seacoasts, in torrential mountain streams during the depths of winter, in murky waters where rivers meet the sea. The goal of this chapter is to make this realm more familiar; we'll take a look at the aquatic environment and its inhabitants and gain a general sense of the natural history of life in water that will prepare the way for more detailed and abstract studies of ecology.

CONCEPTS

l        The hydrologic cycle exchanges water among reservoirs.

l        The biology of aquatic environments corresponds broadly to variation in physical factors such as light, temperature, and water movements and to chemical  factors such as salinity and oxygen.

 

CASE HISTORIES:the hydrologic cycle

The hydrologic cycle exchanges water among reservoirs. Over 71% of the earth's surface is covered by water.This water is unevenly distributed among aquatic environments such as lakes rivers and oceans; most is seawater. The oceans contain  over 97% of the water in the biosphere,and the polar ice caps and glaciers contain an additionl 2%.Less than 1% is freshwater in rivers, lakes,actively exchanged groundwater .The situation on earth is indeed as Samuel Coleridge's ancient mariner saw it: "Water, water, everywhere, nor any drop to drink."

The distribution of water across the biosnhere is not static , however. Figure 3.2 summarizes the dynamic exchanges called the hydrologic cycle; The various aquatic environments such as lakes.rivers, and oceans plus the tmosphere, ice, and even organisms can be considered as "reservoirs" within the hydrologic cycle, places where water is stored for some period of time. The water in these reservoirs is renewed, or turned over.

FIGURE3.2 The hydrologic cycle.

As a result of the hydrologic cycle, water is constantly entering each reservoir either as precinitation or as surface or subsurface flow and leaving each reservoir either as evaporation or as flow. The hydrologic cycle is powered by solar energy which drives the winds and evaporates water, primarily from the surface of the oceans. Water vapor cools as it rises from the ocean's surface and condenses, forming clouds. These clouds are then blown by solar-driven winds across the planet, eventually yielding rain or snow, the majority of which falls back on the oceans and some of which falls on land. The water that falls on land has several fates. Some immediately evaporates and reenters the atmosphere; some is consumed by terrestrial organisms;some percolates through the soil to become groundwater; and some ends up in lakes and ponds or in streams and rivers, which eventually find their way back to the sea.

Turnover is the time required for the entire volume of a particular reservoir to be renewed. Because reservoir size and rates of wate  exchange differ,water turnover occurs at vastly different rates. The water in the atmosphere tums over about every 9 days. The renewal time for river water, 12 to 20 days, is nearly as rapid. Lake renewal times are longer, ranging anywhere from days to centuries, depending on lake depth, area, and rate of drainage. But the biggest surprise is the renewal time for the largest reservoir of all the oceans. With a renewal time of only 3,100 years ,the total volume of the oceans, over 1.3 billion km3 of water has turned over more than 30 times in the last 100,000 years or so, since the first Homo sapiens gazed out on the deep blue sea.

CASE HISTORIES: the natural history of aquatic environments

The biology of aquatic environments corresponds broadly to variations in physical factors such as light, temperature, and water movements and to chemical factors such as salinity and oxygen.

Our discussion of the natural history of aquatic environments begins with the natural history of the oceans, the largest aquatic environment on the planet. We continue our tour with environments found along the margins of the oceans, including kelp forests and coral reefs, the intertidal zone, and salt marshes. We then venture up rivers and streams, important avenues for exchange between terrestrial and aquatic environments. Finally, we consider lakes, inland aquatic environments that are similar in many ways to the oceans where we begin.

The Deep Blue Sea

The blue solitude of open ocean is something palpable, a sensation you can almost taste. As we have seen, the only terrestrial biomes that evoke anything close to the feeling of this place are the open prairies and deserts like the Namib, where the extensive dunes are called the "sand sea." But there is a difference between these terrestrial environments and the sea. On the open ocean, all is blue--blue sea stretching to the horizon, where it meets blue sky (fig. 3.3).

FIGURE 3.3 A view of the open ocean.

Experience with terrestrial organisms cannot prepare you for what you encounter in samples taken from the deep ocean. We dream of unknown extraterrestrial beings, some friendly and some monstrous, all with strange and shocking anatomy. We parade them through science fiction literature and films, while, unknown to most of us, creatures as odd and wonderful, some beyond imagining, live in the deep blue world beyond the continental shelves. Figure 3.4 shows one of the species found in the deep sea--a female deep-sea anglerfish with her male partner.

FIGURE 3.4 Deep-sea anglerfish.

Geography

The world ocean covers over 360 million km2 of earth's surface and consists of one continuous,interconnected mass of water. This water is spread among three major ocean basins: the Pacific,Atlantic and Indian. each with several smaller seas along ist margins.the largest of the ocean ocean basins, the Pacific, has a total area of nearly 180 million km2 and extends from the Antarctic to the Arctic Sea. In the Pacific Ocean, the major seas include the Gulf of California, the Gulf of Alaska, the Bering Sea, the Sea of Okhotsk, the Sea of Japan, the China Sea, the Tasman Sea, and the Coral Sea. The second largest basin, the Atlantic has a total area of over 106 million km2 and also extends nrearly from pole to pole. The major seas of the Atlantic are the Mediterranean, the Black Sea, the North Sea, the Baltic Sea, the Gulf of Mexico, and the Caribbean Sea. The smallest of the three oceans, the Indian, with a total area of just under 75 million km2, is mostly confined to the Southern Hemisphere. Its major seas are the Bay of Bengal, the Arabian Sea, the Persian Gulf, and the Red Sea. Figure 3.5 maps the world's oceans and their adjoining seas.

FIGURE 3.5 Oceanic circulation

The Pacific is also the deepest ocean, with an average depth of over 4,000 m. The average depths of the Atlantic and Indian Oceans are approximately equal, at just over 3,900 m. Undersea mountains stud the floor of the deep sea, some isolated and some in long chains that run as ridges for thousands of kilometers. Undersea trenches some of great depth and volume, rip through the seafloor.One such trench, the Marianas, in the western Pacific Ocean, is over 10,000 m deep---deep enough to engulf Mount Everest with 2 km to spare. The peak of Manna Loa in Hawaii is a bit over 4,000 m above sea level, a modest height for a mountain. But Mauna Loa hides a secret below its sea apron. The base of Mauna Loa extends 6,000 m below sea level, making it, from base to peak, one of the tallest mountains on earth. What new biological discoveries might await future ecologists along this undersea slope?

Structure

The ocean can be divided into several vertical and horizontal zones.The shallow shoreline under the influence of the rise and fall of the tides is called the littoral.or intertidal,zone.The neritic zone exends from the coast to the margin of the continental shelf where the ocean is about 200 m deep.Beyond the continental shelf lies the ocean zone .The ocean is also generally divided vertically into several into several depth zones. The epipelagic zone is the surface layer of the ocean that extends to a depth of 200 m,The mesopelagic zone estends from 200 to 1,000 m,and the bathypelagic zone extends from1,000 to 4,000 m.The layer from 4,000 to 6,000 m is called the abyssal zone ,and finally the deepest parts of the ocean belong to the hadal zone. Habitats on the bottom of the ocean,and other aquatic environments, are referred to as benthic, while those off the bottom, regardless of depth, are called pelagic.Each of these zones supports a distinctive assemblage of marine organisms. Figure 3.6 sketches the general structure of the oceans.

FIGURE3.6 Structure ofthe oceans.

Physical Conditions

Light

Approximately 80% of the solar_energy striking the ocean is absorbed in the firs 10 m.Most Ultraviolet and infrared light is absorbed in the first few meter.Within the visible range, red, orange, yellow, and green, light are absorbed more rapidly than blue light. Consequently, the open ocean appears blue--the wavelength most likely scattered back to our eyes. In the first 10 m, the marine environment is bright with all the colors of the rainbow; below 50 or 60 m it is a blue twilight Even in the clearest oceans on the brightest days, the amount of sunlight penetrating to a depth of 600 m is approximately equal to the intensity of starlight on a clear night. That leaves, on average, about 3,400 m of deep black water in which the only light is that produced by bioluminescent fishes and invertebrates. Figure 3.7 compares ther colors seen by a scuba diver in deep and shallow water to demonstrate the selective absorption of light by water.

FIGURE 3.7 Changes in light quality with depth: (a) the rich colors on a shallow coral reef: (b) the blue of the deeper reef

Temperature

The sunlight absorbed by water increases the kinetic state, or velocity of motion, of water molecules. We detect this increased kinetic state as increased temperature. Because more rapid molecular motion decreases water density,warm water floats on cold water. As a consequence, surface water warmed by the sun floats on the colder water below. These warm and cold layers are separated by a thermocline, a layer of water through which temperature changes rapidly with depth. This layering of the water column by temperature, which is called thermal stratification, is a permanent feature of tropical seas. Temperate oceans are stratified only during the summer, and the tbermocline breaks down as surface waters cool during fall and winten At high latitudes, thermal stratification is only weakly, if ever. developed. As we shall see, these differences in thermal conditions at different latitudes have far-reaching consequences to the ecological functioning of the oceans.

At the ocean surface, average annual temperature and annual variation in temperature change with latitude but. at all latitudes, oceanic temperatures are much more stable than terrestrial temperatures. The lowest average oceanic temperature, about - 1.5. is around the Antarctic. The highest average surface temperatures, a bit over 27. occur near the equator. Maximum annual variation in surface temperature, approximately 7°to 9. occurs in the temperate zone above 40°N latitude. Near the equator, as in the tropical rain forest, the total annual range in temperature, about 1 , approximately equals the daily range. The greatest stability in oceanic temperatures, however, is below the surface, where, at just 100 m depth, annual variation in temperature is often less than 1 .

Water Movements

The oceans are never still. Diverse currents transport nutrients, oxygen, and beat, as well as organisms, across the globe. These currents moderate climates, fertilize the surface waters off the continents, stimulate photosyntbesis, and promote gene flow among populations of marine organisms. For example, wind-driven  surface currents  sweep  across  vast expanses of open ocean to create great circulation systems calle gres that move to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.The great oceanic gyres  transport warm water from equatorial regions toward the poles moderating climates at middle and high latitudes.A segment of one of these gyres, the Gulf Stream, moderates the climate of northwest Europe all the way to Vardo, Norway, which lies above the Arctic Circle. We saw the climate diagram for Vardo in figure 2.34. Without the moderating influence of the Gulf Stream, the climate of Vardo would be more similar to that of Point Barrow, Alaska, or Tiksi, Russia, the other two sites shown in that figure. Figure 3.5 shows the locations and movements of the oceanic gyres.

In addition to surface currents, there are deepwater currents such as those produced as cooled, high-density water sinks at the Antarctic and Arctic and then moves along the ocean floor. Deep water may also be moved to the surface in a process called upwelling. Upwelling occurs along the west coasts of continents and around Antarctica, where winds blow surface water offshore, allowing colder water to rise to the surface. These various water movements are like undersea winds but with a difference: water is vastly more dense than air. How might this difference in density affect the anatomy, behavior, and distributions of marine organisms?

Chemical Conditititons

Salinity

The amount of salt dissolved in water is called salinity. Salinity varies with latitude and among the seas that fringe the oceans.In the open ocean,it varies from about 34 g of salt per kilogram of water (‰ or parts per thousand) to about 36.5‰. The lowest salinities occur near the equator and above 40°N and S latitudes, where precipitation exceeds evaporation. The excess of precipithtation over evaporation at these latitudes is clearly shown by the climate diagrams for temperate forests, boreal forests, and tundra that we examined in chapter 2 (see figs. 2.28, 2.31, and 2.34). Highest salinities occur in the subtropics at about 20°to 30°N and S latitudes, where precipitation is low and evaporation high--precisely those latitudes where we encountered deserts (see fig. 2.19). Salinity varies a great deal more in the small, enclosed basins along the margins of the major oceans. The Baltic Sea, which is surrounded by temperate and boreal forest biomes and receives large inputs of freshwater, has local salinities of 7‰ or lower. In contrast, the Red Sea, which is surrounded by deserts, has surface salin ities of over 40 ‰.

Despite considerable variation in total salinity, the relative proportions of the major ions (e.g., Sodium [Na+], Magnesium [Mg+2], and chloride [Cl-] remain approximately constant from one part of the ocean to another. This uniform composition, which is a consequence of continuous and vigorous mixing of the entire world ocean, underscores the connections between different regions of the planet.

Oxygen

Oxygen is present in far lower concentrations and varies much more in the oceans than in aeriaL environmemts. A liter of air contains about 200 ml of oxygen at sea level while a liter of seawater contains a maximum of about 9 ml of oxygen. Typicallu,oxygen concentration is highest near the ocean surace and decreases progressively with depth to some interme diate depth.The depth at which oxygen reaches a mininmum is usually less than 1,000 m, From this minimum, oxygen concentration increases_progressively to the bottom However some manne environments such as in the deep waters of the Black Sea and the Norwegian sill fjords are devoid of oxygen.

Biology

A century and a quarter of research on the open ocean has revealed close correspondence between physical and chemical conditions and the diversity, composition, and abundance of oceanic organisms. For instance, because of the limited penetration of sunlight into seawater, photosynthetic organisms are limited to the brightly lighted upper epipelagic zone of the ocean (see fig. 3.6). The most significant photosynthetic inhabitants of this zone,also called the euphotic zone are microopic organisms called phytoplankton that drift with the currents in the open sea. The small animals that drift with these same currents are called zooplankton. While them is no ecologically significant photosynthesis below the euphoric zone, there is no absence of deep-sea organisms. Fishes, ranging from small bioluminescent forms to giant sharks, whales, and invertebrates from tiny crustaceans to giant squid, prowl the entire water column, from the surface of the oceans to the bottom. There is life even in the deepest trenches, below 10,000 m.

Most deep-sea organisms are nourished—whatever their place in the food chain--by organic matter fixed by photosynthesis near the surface. It was long assumed that the rain of organic matter from above was the only source of food for deep-sea organisms. Then, about a decade and a half ago the sea surprised everyone. There are entire biological communities on the seafloor that are nourished not by photosynthesis at the surface but by chemosynthesis on the ocean floor (see chapter 6). These oases of life are associated with undersea hot springs and harbor many life-forms entirely new to science. Figure 3.8 shows the great density of organisms found on the ocean floors near an undersea hotwater vent.

FIGURE 3.8 Chemosynthesis-based community on the East Pacific Rise.

The deep ocean shines with the blue of pure water and is often called a "biological desert." This description suggests that the open ocean is an area nearly devoid of life---a wasteland, perhaps--that can be dismissed. While it is tree that the average rate of photosynthesis per square meter of ocean surface is similar to that of terrestrial deserts, the oceans, because they are so vast, contribute approximately one-fourth of the total photosynthesis in the biosphere. This oceanic production constitutes a substantial contribution to the global carbon and oxygen budget. So why "desert"? Oceanic populations live at such low densities that there is little in the open ocean that can be economically harvested for direct human consumption. J. H. Ryther (1969) estimated that the open ocean contains less than 1% of the harvestable fish stocks. Most fish are found along the coasts. We can, however, appreciate the open ocean from other perspectives.

The open ocean is home, the only home, for thousands of organisms with no counterparts on land. The terrestrial environment supports 11 animal phyla, only 1 of which is endemic to the terrestrial environment--that is, found in no other environments. Fourteen phyla live in freshwater environments but none are endemic. Meanwhile the marine environment supports 28 phyla, 13 of which are endemic to the marine environment. Figure 3.9 compares the number of phyla in terrestrial, freshwater, and marine environments.

FIGURE 3.9 Distribution of animalphyla among terrestrial, freshwater. and marine environments (data from Grassle 1991).

Does the greater diversity of phyla in the marine environment shown in figure 3.9 contradict our impression of high biclogical diversity in biomes such as the tropical rain forest? No, it does not. The terrestrial environment is extraordinarily diverse because there are many species in a few animal and plant phyla, especially arthropods and flowering plants. Still, the number of marine species may also be very high. J. E Grassle (1991) estimated that the number of bottom-dwelling, or benthic, marine species may exceed 10 million, a level of species diversity that would rival that of the tropical rain forest. We still have not documented the full extent of species diversity in the oceans.

Human Influences

Human impact on the oceans has been less than on other parts of the biosphere. For most of our history, the vastness of the oceans has been a buffer against human intrusions, but our influence is growing. The decline of large whale populations around Antarctica and elsewhere sounds a warning of what we can do to the open ocean system. The killing of whales has been curtailed, but there are plans to harvest the great whales' food supply, the small planktonic crustaceans known as krill. Although we may find them less engaging than their predators, the large whales, these zooplankton may be more important to the life of the open ocean. Another threat to marine life is the possibility of dumping wastes of all sorts, including nuclear and chemical wastes, into the deep ocean. In recent years, chemical pollution of the sea has increased substantially, and chemical pollutants are accumulating in deep-sea sediments. Assaults such as these will continue as long as the deep sea is considered by most to be a biological desert. The threats to this blue wilderness could be reduced by changes in human activities based on an appreciation of the great biological richness of the oceans and their critical importance to global carbon and oxygen budgets.

Life in Shallow Marine Waters: Kelp Forests and Coral Gardens

The shallow waters along continents and around islands support marine communities of very high diversity and biomass. Imagine yourself snorkeling along a marine shore, beyond the intertidal zone. If you are at temperate latitudes and over a solid bottom, you are likely to swim through groves of brown seaweed called kelp. Along many coasts, kelp grows so tail, over 40 m in some places, and in such densities that they resemble submarine forests (fig. 3.10).

FIGURE 3.10 Giant kelp forest off the Cabfornia coast.

If you snorkel in the tropics, you may come across a coral reef so diverse in color and texture that it appears to be a well-tended garden. But these are forests and gardens with a difference. Here, you can soar through the canopy with fish so graceful they are called "eagle" rays or float leisurely along with "butterfly" fish as they tend their coral "flowers." The colors on a coral reef rival that of any terrestrial biome (fig. 3.11).

FIGURE 3.11 Coral reef at Manado, Indonesia.

In a kelp forest and corm reef, chance meetings with large carnivorous sea animals seldom fade from the memory and are reminders that here you are not at the top of the food chain. The enchantment runs deep in these environments. The kelp that form the canopy and understory of the temperate submarine forest are not members of the plant kingdom but are, at least in some current classifications, gigantic photosynthetic protists. The corals that form the framework of the coral garden are not plants either but animals that secrete a stony skeleton. Corals are indeed animals but they depend for their survival on photosynthesis by photosynthetic protests called zooxanthellae that live in their tissues. These nearshore marine environments are worlds of surprise and enigma.

Geography

The nearshore marine environment and its inhabitants vary with latitude. In temperate to subpolar regions, wherever there is a solid bottom and no overgrazing, there are profuse growths of kelp. As you get closer to the equator, these kelp forests are gradually replaced by coral reefs. Coral reefs are confined to middle latitudes between 30° N and S latitudes. Figure 3.12 shows the global distributions of coral reefs and kelp forests.

FIGURE 3.12 Distribution of kelp forests and coral reefs (data from Barnes and Hughes 1988, after Schumacher 1976).

Structure

Charles Darwin (1842) was the first to place coral reefs into three categories: fringing reefs, barrier reefs, and atolls. Fringing reefs hug the shore of a continent or island. Barrier reefs, such as the Great Barrier Reef, which stretches for nearly 2,000 km off the northeast coast of Australia, stand some distance offshore. A barrier reef stands between the open sea and a lagoon. Coral atolls, which dot the tropical Pacific and Indian Oceans, consist of coral islets that have built up from a submerged oceanic island and ring a lagoon. Darwin's theory of the long-term development of reefs and the structure of fringing reefs, barrier reefs, and atolls is presented in figure 3.13.

FIGURE 3.13 Types of coral reefs.

Distinctive habitats associated with coral reefs include the reef crest, where corals grow in the surge zone created by waves coming from the open sea. The reef crest extends to a depth of about 15 m. Below the reef crest is a buttress zone, where coral formations alternate with sand-bottomed canyons. Behind the reef crest lies the lagoon, which contains numerous small coral reefs called patch reefs and sea grass beds.

Beds of kelp, particularly those of giant kelp, have structural features similar to those of terrestrial forests. At the water's surface is the canopy, which may be more than 25 m above the seafloon The stems, or stipes, of kelp extend from the canopy to the bottom and are anchored with structures called holdfasts. On the stipes and fronds of kelp grow numerous species of epiphytic algae and sessile invertebrates. Other seaweed species of smaller stature usually grow along the bottom, forming an understory to the kelp forest (fig. 3.14).

FIGURE 3.14 Kelp forest structure.

Physical Conditions

Light

Both seaweeds and reef-building corals grow only in surface waters, where there is sufficient light to support photosynthesis. The depth of light penetration sufficient to support kelp and coral varies with local conditions from a few meters to nearly 100 m.

Temperature

Temperature limits the distribution of both kelp and coral. Most kelp are limited to temperate shores, to those regions where temperatures may fall below 10 in winter and rise to a bit above 20 in the summer. Corals are restricted to warm waters, to those regions where the minimum temperature does not fall below about 18°to 20 and average temperatures usually vary from about 23°to 25. Reef-building corals are also sensitive to high temperatures, however, and temperatums above about 29 are usually lethal.

Water Movements

Coral reefs and kelp beds are continuously washed by oceanic currents. These currents deliver oxygen and nutrients and remove waste products. The biological productivity of kelp beds and coral reefs may depend upon the flushing action of these currents. However, extremely strong currents and wave action, as during hurricanes, can detach entire kelp forests and flatten entire coral reefs built up over many centuries. Periodic disturbance is a characteristic of both the kelp bed and the coral reef, and both may require some abiotic disturbance for their long-term survival.

Chemical Conditions

Salinity

Coral reefs grow only in waters with fairly stable salinity. Heavy rainfall or runoff from rivers that reduces salinity below about 27% of seawater can be lethal to corals. Kelp beds appear to be more tolerant of freshwater runoff and grow well along temperate shores, where surface salinities are substantially reduced by runoff from large rivers.

Oxygen

Coral reefs and kelp beds occur where waters are well oxygenated.

Biology

Coral reefs also face intense, and sometimes complex, biological disturbance. Periodic outbreaks of the predatory crown-of-thorns sea star, Acanthaster planci, which eats corals, have devastated large areas of coral reef in the IndoPacific region. In a Caribbean coral reef community, populations of a sea star relative, the sea urchin Diadema antiUarum, were infected by a pathogen and crashed to 1% to 5% of previous densities. It turns out that urchins, which eat both algae and corals, may benefit the corals. In the absence of urchins, algal biomass increased greatly, covering previously bare areas needed by young corals to establish themselves. Algal populations, no longer held in check by predation, compete for space with young corals. In the long run, reducing populations of urchins may reduce coral reproductive success. This is a good example of the complexity and indirect effects that characterize ecological relationships. Figure 3.15 shows one of these sea urchins on a coral reef in the Caribbean Sea.

FIGURE 3.15 The sea urchin Diadema on a coral reef

Corals also compete vigorously among themselves. Reminiscent of rain forest trees and vines, corals engage in a ceaseless struggle for light and space. The corals, however. add a new dimension to the straggle. They actively attack and kill neighboring corals of other clones that differ genetically from themselves.

Coral reefs and kelp beds are among the most productive and diverse of all ecological systems in the biosphere. Robert Whittaker and Gene Likens (I973) estimated that the rate of primary production on coral reefs and algal beds exceeds that of tropical rain forests. The center of diversity for reef-building corals is the western Pacific and eastern Indian Oceans, where there are over 600 coral species and over 2,000 species of fish. By comparison, the western Atlantic Ocean supports about 100 species of corals. Biotic diversity on reefs is also impressive on a small scale. A single coral head may support over i00 species of polychaete worms (Grassle 1973) and over 75 species of fish (Smith and Tyler 1972).

On the coral reef, the ecologist is faced with the same seeming paradox encountered in the tropical rain forest: overwhelming diversity and high primary production in an ecosystem that is nutrient-poor. For the coral reef and for the rain forest, ecologists explain that the answer lies with the organisms themselves and their biotic interactions, including mutualisms, and with rapid recycling and retention of nutriaents in the biological parts of the ecosystem.

Human Influences

Coral reefs and kelp forests are increasingly exploited for a variety of purposes. Tons of kelp are harvested for use as a food additive and for fertilizer. Fortunately, most of this harvest is quickly replaced by kelp growth. Corals, however, which are intensively harvested and bleached for decorations, do not quickly replace themselves. The fish and shellfish of kelp forests and coral reefs have also been heavily exploited. Once again, it appears that coral reefs are more vulnerable. Some coral reefs have been so heavily fished, both for food and for the aquarium trade, that most of the larger fish are rare. Unfortunately, some especially destructive means of fishing are used on coral reefs, including dynamite and poison, with disastrous results. In the Philippines, over 60% of the area once covered by coral has been destroyed by these techniques during recent years. While an appreciation of the threats to rain forests grows, there is less said of the plight of the rain forest's marine cousin, the coral reef, as it is changed from marine garden to wasteland. Again, the question is how can local people and coral reefs thrive together.

Marine Shores: Life Between High and Low Tides

At its shore, the ocean pulses, gurgles, crashes, hisses, and booms like a living being. The rise and fall of the tides make the shore one of the most dynamic environments in the biosphere. The intertidal zone is a magnet for the curious naturalist and one of the most convenient places to study ecology. Where else in the biosphere does the structure of the landscape change several times each day? Where else does nature expose entire aquatic communities for leisurely exploration? Where else are environmental and biological gradients so compressed? It should be no surprise that here in the intertidal zone, immersed in tide pools, salt spray, and the sweet smell of kelp, ecologists have found the inspiration and circumstance for some of the most elegant experiments and most enduring generalizations of ecology. The intertidal zone, the area covered by waves at high tide and exposed to air at low tides, has proved to be an illuminating window to the world. Figure 3.16 shows the tangle of diverse life that can be observed on a rocky shore during low tide.

FIGURE 3.16 A rocky shore at low tide.

Geography

Countless thousands of kilometers of coastline around the world have intertidal zones. From a local perspective, it is significant to distinguish between exposed and sheltered shores. Battered by the full force of ocean waves, exposed shores support very different organisms from those found along sheltered shores on the inside of headlands or in coves and bays. A second important distinction is between rocky and sandy shores.

Structure

The intertidal zone can be divided into several vertical zones (fig. 3.17). The highest zone is called the supratidalfringe, or splash zone. The supratidal fringe is seldom covered by high tides but is often wetted by waves. Below this fringe is the intertidal zone proper. The upper intertidal zone is covered only during the highest tides, and the lower intertidal zone is uncovered only during the lowest tides. Between the upper and lower intertidal zones is the middle intertidal zone, which is covered and uncovered during average tides. Below the intertidal zone is the subtidal zone, which remains covered by water even during the lowest tides. As we shall see in the next two sections, tidal fluctuation produces steep gradients of physical and chemical conditions within the intertidal zone.

FIGURE 3.17 lntertidal zonation.

Physical Conditions

Light

Intertidal organisms are exposed to wide variations in light intensity. At high tide, water turbulence reduces light intensity. At low fide, intertidal organisms are exposed to the full intensity of the sun. How might this variation in light intensity affect the distribution of photosynthetic organisms in the intertidal zone? How vulnerable are intertidal organisms to damage by sunlight, compared to organisms from other marine environments?

Temperature

Because the intertidal zone is exposed to the air once or twice each day, intertidal temperatures are always changing. At high latitudes, tide pools, small basins that retain water at low tide, can cool to freezing temperatures during low tides, while tide pools along tropical and subtropical shores can heat to temperatures in excess of 40. The dynamic intertidal environment contrasts sharply with the stability of most marine environments and presents substantial environmental challenges to organisms of marine origin.

Water Movements

The two most important water movements affecting the distribution and abundance of intertidal organisms are the waves that break upon the shore and the tides. The tides vary in magnitude and frequency. Most tides are semidiurnal, that is, there are two low tides and two high tides each day. However, in seas, such as the Gulf of Mexico and the South China Sea, there are diurnal tides, that is, a single high and low tide each day. The total rise and fall of the tide varies from a few centimeters along some marine shores to 15 m at the Bay of Fundy in northeastern Canada (fig. 3.18).

FIGURE 3.18 The Bay of Fundy at: (a) high tide, and (b) low tide.

The sun and moon and local geography determine the magnitude and timing of tides. The main tide-producing forces are the gravitational pulls of the sun and moon on water. Of the two forces, the pull of the moon is greater because, although the sun is far more massive, the moon is much closer. Tidal fluctuations are greatest when the sun and moon are working together, that is, when the sun, moon, and earth are in alignment, which happens at full and new moons. These times of maximum tidal fluctuation are called spring tides. Tidal fluctuation is least when the gravitational effects of the sun and moon are working in opposition, that is, when the sun and moon, relative to earth, are at right angles to each other, as they are at the first and third quarters of the moon. These times of minimum tidal fluctuation are called neap tides. The size and geographic position of a bay, sea, or section of coastline determine whether the influences of sun and moon are amplified or damped and are responsible for the variations in tides from place to place.

Intertidal organisms have a lot to withstand--not only exposure to air during low tide but also the pounding of waves breaking on the seashore. The amount of wave energy to which intertidal organisms are exposed varies considerably from one section of coast to another; this variation affects the distribution and abundance of intertidal species. Exposed headlands are hit by high waves (fig. 3.19), and they are also subjected to strong currents, which are at times as strong as those of swift rivers. Coves and bays are the least exposed to waves, but even the most sheltered areas may be subjected to intense wave action during storms.

FIGURE 3.19 Storm waves pounding a rocky headland.

Chemical Conditions

Salinity

Salinity in the intertidal zone varies much more than in the open sea, especially within tide pools isolated at low tide. Rapid evaporation during low tide increases the salinity within tide pools along desert shores. Along rainy shores at high latitudes and in the tropics during the wet season, tide pool organisms can experience much reduced salinity.

Oxygen

Oxygen does not generally limit the distributions of intertidal organisms for two major masons. First, intertidal species are exposed to air at each low tide. Second, the water of wave-swept shores is thoroughly mixed and therefore well oxygenated. An intertidal environment where oxygen availability may be tow is in interstitial water within the sediments along sandy or muddy shores, especially in sheltered bays, where water circulation is weak.

Biology

The inhabitants of the intertidal zone are adapted to an amphibious existence, partly marine, partly terrestrial. All intertidal organisms are adapted to periodic exposure to air, but some species are better equipped than others to withstand that exposure. This fact produces one of the most noticeable intertidal features, zonation of species. Some species inhabit the highest levels of the intertidal zone, are exposed by almost all tides, and remain exposed the longest. Others are exposed during the lowest tides only, perhaps once or twice per month, or even less frequently. On an even finer spatial scale, microtopography influences the distribution of intertidal organisms. Tide pools support very different organisms than sections of the intertidal zone from which the water drains completely. The channels in which seawater runs, like a salty stream, during the ebb and flow of the tides offer yet another habitat.

The substratum also affects the distribution of intertidal organisms. Hard, rocky substrates support a biota different from that on sandy or muddy shores. You can see an obvious profusion of life on rocky shores because most species are attached to the surface of the substratum (see fig. 3.16). The residents of the rocky intertidal zone you will likely see are sea stars, barnacles, mussels, and seaweeds. If there are sea urchins, you will notice their spiky presence. But even here, where low tide seems to freely yield the secrets of the sea, all is not obvious. Most organisms take shelter at low tide, some among the fronds and holdfasts of kelp and others under boulders. There are even animals that burrow into and live inside rocks. As we shall see when we discuss competition in chapter 13 and predation in chapter 14, biological interactions make major contributions to the distributions of intertidal organisms.

On soft bottoms some species wander the surface of the substrate, but most are burrowers and shelter themselves within the sand or mud bottom. Here, nature does not give up its secrets easily. To thoroughly study the life of sandy shores you must separate organisms from sand or mud. Perhaps this is the reason rocky shores have gotten more attention by researchers and why we know far less about the life of sandy shores. Beaches, like the open ocean, have been considered biological deserts. Careful studies, however, have shown that the intensity and diversity of life on sandy shores rivals that of any benthic aquatic community (MacLachlan 1983).

Human Influences

People have long sought out intertidal areas, first for food and later for recreation, education, and research. Shell middens, places where prehistoric people piled the remains of their seafood dinners, from Scandinavia to South Africa, bear mule testimony to the importance of intertidal species to human populations for over 100,000 years. Today, each low tide still finds people all over the world scouring intertidal areas for mussels, oysters, clams, and other species. But the intertidal zone, which resists, and even thrives, in the face of twice daily exposure to air and pounding surf, is easily devastated by the trampling feet. and probing hands of a few human visitors. Relentless exploitation has severely reduced many intertidal populations. Exploitation for food is not the only culprit, however. Collecting for education and research also takes its toll. Each Iow tide, the intertidal community offers itself up. Don't abuse this rare openness; walk lightly and took closely. Take photographs; make sketches and notes; take away only inspiration and renewal and you can come back for more when you need it.

Estuaries, Salt Marshes,and Mangrove Forests

Estuaries are found wherever rivers meet the sea. Salt marshes and mangrove forests are concentrated along lowlying coasts sandy shores and may, like estuaries, be associated with the mouths of rivers. All three are at the transition between one environment and another--salt marshes and mangrove forests at the transition between land and sea, and estuaries at the transition between rivers and the sea. Because these areas are transitions between very different environments, they have a great deal in common physically, chemically, and biologically. These are environments that pulse to the rhythm of lunar-driven tides and teem with life. Figure 3.20 shows a rich salt marsh landscape, and figure 3.21 shows the structurally complex environment provided by dense populations of mangroves and their many prop roots.

FIGURE 3.20 Salt marsh landscape.

FIGURE 3.21 Mangroves.

Geography

Salt marshes, which are dominated by herbaceous vegetation, are concentrated along sandy shores from temperate to high latitudes. At tropical and subtropical latitudes the herb-dominated salt marsh is replaced by mangrove forests. Mangroves are associated with the terrestrial climates of the tropical rain forest, tropical dry forest, savanna, and desert, due mainly to the sensitivity of mangroves to frost. Figure 3.22 maps the global distributions of salt marshes and mangrove forests.

FIGURE 3.22 Salt marsh and mangrove swamps (data from Chapman 1977, Long and Mason 1983).

Structure

Salt marshes generally include channels, called tidal creeks. that fill and empty with the tides. These meandering creeks can create a complex network of channels across a salt marsh (fig. 3.23). Fluctuating tides move water up and down these channels, or tidal creeks, once or twice each day. These daily movements of water gradually sculpt the salt marsh into a gently undulating landscape. Tidal creeks are generally bordered by natural levees. Beyond the levees are marsh flats, which may include small basins called salt pans that periodically collect water that eventually evaporates, leaving a layer of salt. This entire landscape is flooded during the highest fides and drains during the lowest tides. A typical cross section of a salt marsh is shown in figure 3.24.

FIGURE 3.23 Salt marsh from the air

FIGURE 3.24 Salt marsh channels.

The mangrove trees of different species are usually distributed according to height within the intertidal zone. For instance, in mangrove forests near Rio de Janeiro, Brazil, the mangroves growing nearest the water belong to the genus Rhizophora. At this level in the intertidal zone, Rhizophora is inundated by average high tides. Above Rhizophora grow other mangroves such as Avicennia, which is flooded by the average spring tides, and Laguncularia, which is touched only by the highest tides. Figure 3.25 shows zonation within a mangrove forest.

FIGURE 3.25 Tidal level and mangrove distributions.

Estuaries vary vertically and longitudinally, especially in regard to the amount of dissolved salts (see the section on salinity).

Physical Conditions

Light

Estuaries, salt marshes, and mangrove forests experience significant fluctuations in tidal level. Consequently, the organisms in these environments are exposed to highly variable light conditions. They may be exposed to full sunlight at low tide and very little light at high tide. The waters of these areas are usually turbid because shifting currents, either from the fides or rivers, keep fine organic and inorganic materials in suspension.

Temperature

Several factors make the temperatures of estuaries, salt marshes, and mangrove forests highly variable. First, because their waters are generally shallow, particularly at low tide, water temperature varies with air temperature. Second, the temperatures of seawater and fiver water may be very different. If so, the temperature of an estuary may change with each high and low tide. Salt marshes at high latitudes may freeze during the winter. In contrast, mangroves grow mainly along desert and tropical coasts, where the minimum annual temperature is about 20. The shallows in these environments can heat up to over 40.

Water Movements

Ocean tides and river flow drive the complex currents in estuaries. These currents are at the heart of the ecological processes of the estuary because they transport organisms, renew nutrients and oxygen, and remove wastes. Complex tidal currents also flow in salt marshes and mangrove forests, where they are involved in these processes and also fragment and transport the litter produced by salt marsh and mangrove vegetation. Once or twice a day, high tides create saltwater currents that move up the estuaries of rivers and the channels within salt marshes and mangrove forests. Low fides reverse these currents and saltwater moves seaward. Tidal height may fluctuate far from where an estuary meets the sea. For example, tidal fluctuations occur over 200 km upstream from where the Hudson River flows into the sea. The vigorous mixing, in more than one direction, makes these transitional environments some of the most physically dynamic in the biosphere. Penetration of light and water movements vary over short distances and in the course of a day. This physical variability is reflected in highly variable chemical conditions.

Chemical Conditions

Salinity

The salinity of estuaries, salt marshes, and mangrove forests may fluctuate widely, particularly where river and tidal flow are substantial. In such systems, the salinity of seawater can drop to nearly that of freshwa, ter an hour after the tide tums. Because estuaries are places where rivers meet the sea, their salinity is generally lower than that of seawaten In hot, dry climates, however, evaporation often exceeds freshwater inputs and the salinity in the upper portions of estuaries may exceed that of the open ocean.

Estuarine waters are also often stratified by salinity, with lower-salinity, Iow-density water floating on a layer of higher-salinity water, isolating bottom water from the atmosphere. On the incoming tide, seawater coming from the ocean and fiver water are flowing in opposite directions. As seawater flows up the channel, it mixes progressively with river water flowing in the opposite direction. Due to this mixing, the salinity of the surface water gradually increases down river from less than 1 ‰ to salinities approaching that of seawater at the river mouth (fig. 3.26).

FIGURE 3.26 Structure of a salt wedge estuary.

Oxygen

In estuaries, salt marshes, and mangrove forests, oxygen concentration is highly variable and often reaches extreme levels. Decomposition of the large quantities of organic matter produced in these environments can deplete dissolved oxygen to very low levels, and isolation of saline bottom water from the atmosphere adds to the likelihood that oxygen will be depleted. At the same time, however, high rates of photosynthesis can increase dissolved oxygen concentrations to supersaturated levels. Again, the oxygen concentrations to which an organism is exposed can change with each turn of the tide.

Biology

The salt marshes of the world are dominated by grasses such as Spartina spp. and Distichlis spp., by pickleweed, Salicornia spp., and by rushes, juncus spp. The mangrove forest is dominated by mangrove trees belonging to many genera. The species that make up the forest change from one region to another; however, within a region, there is great uniformity in species composition.

Estuaries and salt marshes don't support a great diversity of species, but their primary production is very high. These are places where some of the most productive fisheries occur and where aquatic and terrestrial species find nursery grounds for their young. Most of the fish and invertebrates living in estuaries evolved from marine ancestors, but estuaries also harbor a variety of insects of freshwater origin. Whatever their origins, however, the species that inhabit estuaries and salt marshes have to be physiologically tough. Estuaries and salt marshes also attract birds, especially water birds. In the mangrove forest, birds are joined by crocodiles, alligators, and, in the Indian subcontinent, by tigers. From both academic and practical perspectives, the ecological importance of these environments cannot be overstated.

Human Influences

Estuaries, salt marshes, and mangrove forests are extremely vulnerable to human interference. People want to live and work at the coast, but building sites are limited. One solution to the problem of high demand for coastal property and low supply has been to fill and dredge salt marshes, replacing wildlife habitat with human habitat (fig. 3.27). Because cities benefit from access to the sea, many, such as Boston, San Francisco, and London, have been built on estuaries. As a consequence, many estuaries have been polluted for centuries. The discharge of wastes depletes oxygen supplies, which physiologically stresses aquatic organisms. The discharge of organic wastes depletes oxygen directly as it decomposes, and the addition of nutrients such as nitrogen can lead to oxygen depletion by stimulating primary production. Heavy metals discharged into estuaries and salt marshes are incorporated into plant and animal tissues and have been elevated to toxic levels in some food species. The assaults on estuaries and salt marshes have been chronic and intense, but the interest and concerns of people grow steadily. In these resilient environments, hope comes with each change of the tide.

FIGURE 3.27 Fill and dredge operation on a salt marsh.

Rivers and Streams: Life Blood and Pulse of the Continents

We become aware of the importance of rivers in human history and economy as we name the major ones: Nile, Danube, Tigris, Euphrates, Yukon, Indus, Tiber, Mekong, Ganges, Rhine, Mississippi, Missouri, Yangtze-Kiang, Amazon, Seine, Zaire, Volga, Thames, Rio Grande. The names of these rivers, and many others great and small, ring with a thousand images of history, geography, and poetry. The importance of rivers to human history, ecology, and economy is inestimable. However, river ecology has lagged behind the ecological study of lakes and oceans and is one of the youngest of the many branches of aquatic ecology. In the past couple of decades, however, river ecology has, as all youthful sciences do, exploded with published research, competing theories. controversies, and international symposia and now claims a well-earned place beside its more mature cousins.

What might rivers offer to the science of ecology? Their most notable feature is their dynamism. In art and literature. this characteristic has made rivers symbols of ceaseless change. For example, Leonardo da Vinci wrote: "In rivers the water you touch is the last of what has passed and the first of that which comes. So with time present." The ancient Greeks said simply: "You never step in the same river twice." In ecology, we call dynamic ecosystems such as rivers "nonequilibrial." Nonequilibrial theory, one of the newest branches of theoretical ecology, may find, as has art and literature, precisely the metaphor it needs in the rivers of the world. The meandering pattern of the river shown in figure 3.28 suggests the dynamism of river ecosystems.

FIGURE 3.28 The meandering Okavango River, Botswana.

Geography

Rivers drain most of the landscapes of the world. When rain falls on a landscape, a portion of it runs off, either as surface or subsurface flow. Some of this runoff water eventually collects in small channels, which join to form larger and larger water courses until they form a network of channels that drains the landscape. A river basin is that area of a continent or island that is drained by a river drainage network, such as the Mississippi River basin in North America or the Congo River basin in Africa. Rivers eventually flow out to sea or to some interior basin like the Aral Sea or the Great Salt Lake. Figure 3.29 shows the distribution of the major rivers of the planet.

FIGURE 3.29 Major rivers.

Structure

Rivers and streams can be divided along three dimensions (fig. 3.30). They can be divided along their lengths into pools, runs, riffles, and rapids and, because of variation in flow, rivers can also be divided across their widths into wetted channels and active channels. A wetted channel contains water even during low flow conditions. An active channel, which extends out from one or both sides of a wetted channel, may be dry during part of the year but is inundated annually during high flows. Outside the active channel is the riparian zone, a transition between the aquatic environment of the river and the upland terrestrial environment.

FIGURE 3.30 The three dimensions of stream structure.

Rivers and streams can be divided vertically into the water surface, the water column, and the bottom, or benthic, zone. The benthic zone includes the surface of the bottom substrate and the interior of the substrate through depths at which substantial surface water still flows. Below the benthic zone is the hyporheic zone, a zone of transition between areas of surface water flow and groundwater. The area containing groundwater below the hyporheic zone is called the phreatic zone. Each part of a river or stream is a physically and chemically distinctive environment and each supports different organisms.

Physical Conditions

Light

There are two principal aspects of light to consider in relation to rivers and streams. First, how far light penetrates into the water column and second, how much light shines on the surface of a riven Streams and rivers vary considerably in water clarity. Generally, however, even the clearest streams are much more turbid than clear lakes or seas. The reduced clarity of rivers results from two main factors. First, rivers are in intimate contact with the surrounding landscape, and inorganic and organic materials continuously wash, fall, or blow into rivers. Second, river turbulence erodes bottom sediments and keeps them in suspension, particularly during floods. The headwaters of rivers are generally shaded by riparian vegetation. Shading may be so thorough along some streams that there is very little photosynthesis by aquatic primary producers. The extent of shading decreases progressively downstream as stream width increases. In desert regions, headwater streams usually receive large amounts of solar radiation and support high levels of photosynthesis. Figure 3.31 contrasts the environments of headwater streams flowing through a forest and a desert.

FIGURE 3.31 Headwater streams in. (a)forested Great Smoky Mountains: and (b) Sonoran Desert.

Temperature

The temperature of rivers closely tracks air temperature but does not reach the extremes of terrestrial habitats. The coldest river temperatures, those of high altitudes and high latitudes, may drop to a minimum of 0. The warmest rivers are those flowing through deserts, but even desert rivers seldom exceed 30. The outflows of hot springs can be boiling in their upper reaches, but populations of thermophilic bacteria live in even the hottest of these.

Water Movements

What is notable about a river is the continuous movement of water. Viewed environmentally, river currents deliver food, remove wastes, renew oxygen, and strongly affect the size, shape, and behavior of river organisms. Currents in quiet pools may flow at only a few millimeters per second, while water in the rapids of swift rivers in a flood stage may flow at 6 m per second. Contrary to popular belief, the currents of large rivers may be as swift as those in the headwaters.

The amount of water carried by rivers, which is called river discharge, differs a lot from one climatic regime to another. River flows are often unpredictable and "flashy" in arid and semiarid regions, where extended droughts may be followed by torrential rains. The flow in tropical rivers varies considerably. Many tropical rivers, which flow very little during the dry season, become torrents during the wet season. Some of the most constant flows are found in forested temperate regions, where, as we saw in chapter 2, precipitation is fairly evenly distributed throughout the year (see fig. 2.28). Forested landscapes can damp out variation in flow by absorbing excessive rain during wet periods and acting as a reservoir for river flow during drier periods. Figure 3.32 compares the annual flows of rivers of moist temperate and semiarid climates.

FIGURE 3.32 Annual flow of rivers in mois tempera e and semiarid climates (data from Calow and Petts1992).

Chemical Conditions

Salinity

Water flowing across landscapes or through soils picks up dissolved materials. The amount of salt dissolved in river water reflects the history of leaching that has gone on in its basin. As we saw in chapter 2, annual rainfall is high in tropical regions. Consequently, many tropical soils have been leached of much of their soluble materials and it is in the tropics that the salinity of river water is often very low. Desert rivers generally have the highest salinities. Figure 3.33 shows that the salinity of river water from different regions may show 10- to 100-fotd differences.

FIGURE 3.33 Salinities of tropical, temperate, and arid land rivers (data from Gibbs 1970).

Oxygen

The oxygen content of river water is inversely correlated with temperature. Oxygen supplies are generally richest in cold, thoroughly mixed headwater streams and lower in the warm downstream sections of rivers. However, because the waters in streams and rivers are continuously mixed, oxygen is generally not limiting to the distribution of river organisms. The major exception to this generalization is in sections of streams and rivers receiving organic wastes from cities (wastes with high biochemical oxygen demand or BOD) and industry. Only organisms tolerant of Iow oxygen concentrations can inhabit these sections.

Biology

As in the terrestrial biomes, large numbers of species inhabit tropical rivers. The number of fish species in tropical rivers is much higher than in temperate rivers. For example, the Mississippi River basin, which supports one of the most diverse temperate fish faunas, is home to about 300 fish species. By contrast, the tropical Congo River basin contains about 669 species of fish, of which over 558 am found nowhere else. The most impressive array of freshwater fish is that of the Amazon River basin, which contains over 2,000 species, approximately 10% of all the fish species on the planet.

The organisms of river systems change from headwaters to mouth. These patterns of biological variation along the courses of rivers have given rise to a variety of theories that predict downstream change in rivers and their inhabitants. One of these theories is the river continuum concept (Vannote et al. 1980). According to this concept, in temperate regions, leaves and other plant parts are often the major source of energy available to the stream ecosystem. Upon entering the stream, this coarse particulate organic matter (CPOM) is attacked by aquatic microbes, especially fungi. Colonization by fungi makes CPOM more nutritious for stream invertebrates. The stream inverte- brates of headwater streams are usually dominated by two feeding groups: shredders, which feed on CPOM, and collectors, which feed on fine particulate organic matter (FPOM). The fishes in headwater streams are usually those, such as trout, that require high oxygen concentrations and cool temperatures.

The river continuum concept predicts that the major sources of energy in medium-sized streams will be FPOM washed down from the headwater streams and algae and aquatic plants. Algae and plants generally grow more profusely in medium streams because they are too wide to be entirely shaded by riparian vegetation. Because of the different food base, shredders make up a minor portion of the benthic community, which is dominated by collectors and grazers on the abundant algae and aquatic plants. The fishes of medium streams generally tolerate somewhat higher temperatures and lower oxygen concentrations than headwater fishes.

In large rivers, the major sources of energy are FPOM and, in some rivers, phytoplankton. Consequently, the benthic invertebrates of large rivers are dominated by collectors, which make their living by filtering FPOM from the water column. In large rivers, there are also zooplankton. The fish found in large temperate rivers are those, such as carp and catfish, that are more tolerant of lower oxygen concentrations and higher water temperatures. Because of the development of a plankton community, plankton-feeding fish also live in large rivers. The major changes in temperate river systemspredicted by the river continuum concept are summarized in figure 3.34.

FIGURE 3.34

Most of the invertebrates of streams and rivers live on or in the sediments; that is, most are benthic. These benthic organisms are influenced substantially by the type of bottom sediments. Stony substratum in the riffles and runs of rivers harbor fauna and flora that are different from those in sections with silt or sand bottoms mainly because of differences in the structure and stability of these bottom types. River ecologists have recently discovered that a great number and diversity of invertebrate animals live deep within the sediments of rivers in both the hyporheic and phreatic zones. These species may be pumped up with well water many kilometers from the nearest riven We know very little about the lives of these organisms. Once more, nature has yielded another surprise.

Human Influences

The influence of humans on rivers has been long and intense. Rivers have been important to human populations for commerce, transportation, irrigation, and waste disposal. Because of their potential to flood, they have also been a constant threat. In the service of human populations, rivers have been channelized, poisoned, filled with sewage, dammed, filled with nonnative fish species, and completely dried. Because of the rapid turnover of their waters, however, they have a great capacity for recovery and renewal. The River Thames in England was severely polluted in the Middle Ages and remained so until recent times. During recent decades, great efforts have been made to reduce the amount of pollution discharged into the Thames, and the river has recovered substantially. The Thames once again supports a run of Atlantic salmon and gives hope to all the beleaguered river conservationists of the world.

Lakes: Small Seas

In 1892, E A. Forel defined the scientific study of lakes as the oceanography of lakes. On the basis of a lifetime of study, Forel concluded that lakes are much like small seas (fig. 3.35). Differences between lakes and the oceans are due, principally, to the smaller size of lakes and their relative isolation. Perhaps because they are cast on a more human scale, lakes have long captured the imagination of everyone from poets to scientists. For poets such as Henry David Thoreau (1854), they have been sources of inspiration and mirrors of inner truth. For scientists such as Stephen A. Forbes (1887), who wrote, "The lake as a microcosm," they have been mirrors of the outside world and microcosms of the ecological universe.

FIGURE3.35 Lake Baikal. Siberia, Russia.

Geography

Lakes are simply basins in the landscape that collect water like so many rain puddles. Most lakes are found in regions worked over by the geological forces that produce these basins. These forces include shifting of the earth's crust (tectonics), volcanism, and glacial activity.

Most of the world's freshwater resides in a few large lakes. The Great Lakes of North America together cover an area of over 245,000 km2 and contain 24,620 km3 of water. approximately 20% of all the freshwater on the surface of the planet. An additional 20% of freshwater is contained in Lake Baikal, Siberia, the deepest lake on the planet (1,600 m), with a total volume of 23,000 km3. Much of the remainder is contained within the rift lakes of East Africa. Lake Tanganyika, the second deepest lake (1,470 m), alone has a volume of 23,100 km3, virtually identical to that of Lake Baikal. Still, the world contains tens of thousands of other smaller, shallow lakes, usually concentrated in "lake districts" such as northern Minnesota, much of Scandinavia, and vast regions across north-central Canada and Siberia. Figure 3.36 shows the locations of some of the larger lakes.

FIGURE 3.36 Distributions of some major lakes.

Structure

Lake structure parallels that of the oceans but on a much smaller scale (fig.3.37). The shallowest waters along the lake shore, where rooted aquatic plants may grow, is called the littoral zone. Beyond the littoral zone in the open lake is the limnetic zone. Lakes are generally divided vertically into three main depth zones. The epilimnion is the warm surface layer of lakes. Below the epilimnion is the thermocline, or metalimnion. The thermocline is a zone through which temperature changes substantially with depth, generally about l per meter of depth. Below the thermocline are the cold dark waters of the hypolimnion. Each of these zones supports a distinctive assemblage of lake organisms.

FIGURE 3.37 Lake structure.

Physical Conditions

Light

Lake color ranges from the deep blue of the clearest lakes to yellow, brown, or even red. The color, which depends on light absorption within a lake, is influenced by many factors but especially lake chemistry and biological activity. In lakes where the surrounding landscape delivers large quantities of nutrients, primary production is high and phytoptankton populations reduce light penetration. These highly productive lakes are usually a deep green. They are also often shallow and surrounded by cultivated lands or cities. Dissolved organic compounds, such as humic acids leached from forest soils, increase absorption of blue and green light. Absorption in this range shifts lake color to the yellow-brown end of the spectrum. These acid-stained lakes are generally of low productivity. In deep lakes where the landscape delivers low quantities of either nutrients or dissolved organic compounds, phytoplankton production is generally low and light penetrates to great depths. These lakes, such as Lake Baikal in Siberia, Lake Tahoe in California, and Crater Lake in Oregon, are nearly as blue as the open ocean.

Temperature

As in the oceans, lakes become thermally stratified as they heat. Consequently, during the warm season, they are substantially warmer at the surface than they are below the thermocline. Temperate lakes are stratified during the summer, while lowland tropical lakes are stratified year-round. As in temperate seas, thermal stratification breaks down in temperate lakes as they cool during the fall. Where lakes freeze over in winter, the water immediately under the ice is approximately 0. Meanwhile, bottom water is a comparatively warm 4. The temperature at which the density of water is highest. In spring, once the ice has melted, temperate lakes spend a period without thermal stratification. As summer approaches, they gradually become stratified again. In high-elevation tropical lakes, a thermocline may form every day and break down every night! This dynamic situation occurs on the same tropical mountains where, as we saw in chapter 1, terrestrial organisms experience winter temperatures every night and summer temperatures every day. As in the oceans, these patterns of thermal stratification determine the frequency and extent of mixing of the water column. The seasonal dynamics of thermal stratification and mixing in temperate lakes are shown in figure 3.38.

FIGURE 3.38 Seasonal changes in temperature in a temperate lake (data from Wetzel 1975).

Water Movements

Wind-driven mixing of the water column is the most ecologically important water movement in lakes. As we have just seen. temperate zone lakes are thermally stratified during the summer, a condition that limits wind-driven mixing to surface waters above the thermocline. During winter on these lakes, ice forms a surface barrier that prevents mixing. In the spring and fall, however, stratification breaks down and winds drive vertical currents that can mix temperate lakes from top to bottom. These are the times when a lake renews oxygen in bottom waters and replenishes nutrients in surface waters. Like tropical seas, tropical lakes at low elevations are permanently stratified. Of the 1,400 m of water in Lake Tanganyika, for example, only about the upper 200 m is circulated each year. Tropical lakes at high elevations heat and stratify every day and cool sufficiently to mix every night. Patterns of mixing have profound consequences to the chemistry and biology of lakes.

Chemical Conditions

Salinity

The salinity of lakes is much more variable than that of the open ocean. The world average salinity for freshwater, 120 mg per liter (approximately 0.120‰), is a tiny fraction of the salinity of the oceans. Lake salinity ranges from the extremely dilute waters of some alpine lakes to the salt brines of desert lakes. For instance, the Great Salt Lake in Utah sometimes has a salinity of over 200‰, which is much higher than oceanic salinity. As we shall see in chapter 20 (see fig. 23.10), the salinity of desert lakes may also change over time, particularly where variations in precipitation, runoff, and evaporation combine to produce wide fluctuations in lake volume.

Oxygen

Mixing and biological activities have profound effects on lake chemistry. Well-mixed lakes of low biological production, which are called oligotrophic, are nearly always well oxygenated. Lakes of high biological production, which are called eutrophic, may be depleted of oxygen. Oxygen depletion is particularly likely during periods of thermal stratification, when decomposing organic matter accumulates below the thermocline and consumes oxygen. In eutrophic lakes, oxygen concentrations may be depleted from surface waters at night as respiration continues in the absence of photosynthesis. Oxygen is also often depleted in winter, especially under the ice of productive temperate lakes. In tropical lakes, water below the euphotic zone is often permanently depleted of dissolved oxygen.

Biology

In addition to their differences in oxygen availability, oligotrophic and eutrophic lakes also differ in factors such as availability of inorganic nutrients and temperature (fig. 3.39). Because aquatic organisms differ widely in their environmental requirements, oligotrophic and eutrophic lakes generally support distinctive biological communities. In temperate regions, oligotrophic lakes generally support the highest diversity of phytoplankton. These lakes are also usually inhabited by fish requiting high oxygen concentrations and relatively low temperatures, such as trout and whitefish. The benthic faunas of these lakes are rich in species and include the larvae of mayflies and caddisflies, small clams, and, along wave-swept shores, the larvae of stoneflies. Eutrophic temperate lakes, which tend to be warmer and, as we have seen, periodically depleted of oxygen, are inhabited by fish tolerant of high temperatures and low oxygen concentrations, such as carp and catfish, or fish that can breathe air in an emergency, such as gars and bowfins. The benthic invertebrate faunas of these lakes also tend to be tolerant of low oxygen concentrations; for example, midge larvae and tuhificid worms, common in such lakes, have hemoglobin that helps them extract oxygen from oxygen-poor waters.

FIGURE 3.39 Oligotrophic and eutrophic lakes

Much less is known about the biology of tropical lakes, however, a few generalizations are possible. Tropical lakes can be very productive. Also, their fish faunas may include a great number of species. Three East African lakes, Lake Victoria, Lake Malawi, and Lake Tanganyika, contain over 700 species of fish, approximately the number of freshwater fish species in all of the United States and Canada; all of western and central Europe and the former Soviet Union together contain only about 400 freshwater fish species. The invertebrates and algae of tropical lakes are much less studied, but it appears that the number of species may be similar to that of temperate zone lakes.

Human Influences

Human populations have had profound, and usually negative, ihfluences on the ecology of lakes. In addition to examples of ecological degradation, however, are cases of amazing resilience and recovery--resilience in the face of fierce ecological challenge and recovery to substantial ecological integrity. Because lakes offer ready access to water for domestic and industrial uses, many human population centers have grown up around them. In both the United States and Canada, for example, large populations surround the Great Lakes. The human population around Lake Erie, one of the most altered of the Great Lakes, grew from 2.5 million in the 1880s to over 13 million in the 1980s. The primary ecological impact of these populations has been the dumping of astounding quantifies of nutrients and toxic wastes into Lake Erie. By the mid-1960s, the Detroit River alone was dumping 1.5 billion gallons of waste water into Lake Erie each day. The Cuyahoga River, which flows through Cleveland before reaching the lake, was so fouled with oil in the 1960s that it would catch fire. In the face of such ecological challenges, much of Lake Erie, particularly the eastern end, was transformed from a healthy lake with a rich fish fauna to one that was, for a time, essentially an algal soup in which only the most tolerant fish species could live. With greater controls on waste disposal, the process of degradation began to reverse itself, and Lake Erie recovered much of its former health and vitality by the 1980s.

Nutrients aren't the only things that people put into lakes, however. Fish and other species are constantly moved around, either intentionally or unintentionally. For instance, the canals that were dug to connect the Great Lakes with each other and to bypass Niagara Falls inadvertently introduced two species of fish, the sea lamprey and the alewife, that seriously disrupted the biology of the lakes. Once in the Great Lakes, sea lampreys fed mainly on lake trout, lake hemng, and chubs. This predation, combined with intense fishing, devastated these commercially important fish populations. As these populations declined, alewife populations exploded. With exploding alewife populations came periodic and massive die-offs that littered beaches with tons of rotting fish. Massive efforts at controlling the sea lamprey by the United States and Canada have been reasonably successful.

These early introductions of fish into the Great Lakes were just a preview of future biological challenges, howeven The rogues' gallery of introductions to the Great Lakes, which now includes species such as the zebra mussel, the river ruffe, and the spiny water flea, continues to grow, and there appears to be no end in sight. As figure 3.40 shows, 139 species of fish, invertebrates, plants, and algae had been introduced to the Great Lakes by 1990.

FIGURE 3.40 Cumulative number of species introduced to the Great Lakes (data from Mills et al. 1994).

The population growth of many introduced species has been explosive and has had great ecological and economic impacts. One such introduction was that of the zebra mussel, Dreissena polymorpha, a bivalve mollusk native to the drainages emptying into the Aral, Caspian, and Black Seas. Zebra mussels disperse by means of pelagic larvae but spend their adult lives attached to the substrate by means of byssal threads. Their pelagic larvae allows them to disperse at a high rate. Though they spread throughout western Europe by the early 1800s, zebra mussels were not recorded in North America until the late 1980s. In 1988, they were collected in Lake Saint Clair. which connects Lake Huron and Lake Erie. In just 3 years, zebra mussels spread to all the Great Lakes and to most of the major rivers of eastern North America.

Locally, zebra mussels have established very dense populations within the Great Lakes. Shells from dead mussels have accumulated to depths of over 30 cm along some shores. Such dense populations threaten the native mussels of the Great Lakes with extinction. Zebra mussels are also fouling water intake structures of power plants and municipal water supplies, which may result in billions of dollars in economic impact. Biologists are working furiously to document and understand the impact of zebra mussels and other species introduced in the Great Lakes. Meanwhile, the governments of Canada and the United States are taking steps to reduce the rate of biological invasion of the Great Lakes. As a consequence of introductions of zebra mussels and other species, the Great Lakes have become a laboratory for the study of human caused biological invasions (fig. 3.4l).

FIGURE 3.41 Two invaders of the Great Lakes: (a) sea lamprey; and (b) zebra mussels.

APPLICATIONS AND TOOLS:

biological integrity--assessing the health of aquatic systems

How can we put our knowledge of the natural history of aquatic life to work? A major question that biologists often face is whether a particular influence impairs the health of an aquatic system. Natural history information can play a significant role in making that judgment. Given the complex array of potential human impacts on aquatic systems, what might we use as indicators of health? An answer to this question has been proposed by James Karr and his colleagues, who suggest that we consider what they call "biological integrity," which they define as "a balanced, integrated, adaptive community of organisms having a species composition, diversity, and functional organization comparable to that of the natural habitat of the region" (Karr and Dudley 1981). These researchers proposed that a healthy aquatic community is one that is similar to the community in anundisturbed habitat in the same region. The community should be "balanced" and "integrated." Deciding what constitutes this state requires judgment based on broad knowledge of the habitats in question and their inhabitants---that is, knowledge of natural history. If we could assess the health, as defined by Karr, of a community of aquatic organisms, we would have gone a long way toward assessing the health of the aquatic system in which this community is contained.

Moving beyond general definitions and broad goals, Karl developed an Index of Biological Integrity and applied his index to fish communities. Fish communities were chosen because we know a lot about fish and their habitat requirements and they are relativeely easy to sample. Karr's index has three categories for rating a stream or river:

   1. number of species and species composition, which includes the number, kinds, and tolerances of fish species;

   2. trophic composition, which considers the dietary habits of the fish making up the community;

   3. fish abundance and condition.

Under these three categories are 12 attributes of the fish community. The stream is assigned a score of 5, 3, or 1 for each attribute, where 5 equals best and 1 equals worst. The scores on all the attributes are added to give a total score that ranges from 12 (poor biological integrity) to 60 (excellent biological integrity). Notice that Karr has built a safeguard into his index. Judging several attributes of the fish community eliminates the bias that might creep in if assessments were made from only one or a very few attributes. We will examine the three categories of community characteristics in turn.

Number of Species and Species Composition

In this category the numbers of native and nonnative species are considered. The reason for including the number of native species should be apparent from our discussions, both in this chapter and in chapter 2. Heavy human impact generally reduces the number of native species in a community while increasing the number of nonnative species. The kinds of species that make up the community should also be telling, because some fish, such as trout, are intolerant of poor water quality while others, such as carp, are highly tolerant of poor water quality. The designation of tolerant versus intolerant species must be tailored for local, or at least regional, circumstance and requires a thorough knowledge of the natural history of the waters under study, as does scoring the number and abundance of species.

Trophic Composition

The dietary habits of the fish that make up a community reflect kinds of food available in a stream as well as the quality of the environment. The attributes rated in this category are the percentage of fish such as carp that eat a wide range of food and are called omnivores by ecologists, the percentage of fish such as trout and bluegill that feed on insects, called inseetivores, and the percentage of fish such as pike and largemouth bass that feed on other fish, called piseivores. Degradation of aquatic systems generally increases the proportion of omnivores and decreases the proportion of insectivores and piscivores in the community. Notice that here. again, scoring the attributes must be based on a thorough understanding of natural history.

Fish Abundance and Condition

Fish are often less abundant in degraded situations and their condition is often adversely affected. Two aspects of condition are considered for the index. First, what percentage of the individuals are hybrids between different species? Second, what percentage of individuals have noticeable disease, tumors, fin damage, or skeletal deformities--all strong indicators of poor environmental quality. Figure 3.42 summarizes the process of calculating Karr's Index of Biological Integrity.

FIGURE 3.42 Calculating an Index of Biological Integrity

The Index of Biological Integrity has been successfully applied to a large number of environmental situations. But, as with any tool, full appreciation of the details of the index only comes with use. Let's examine one application of the index.

An Application

Paul Leonard and Donald Orth (1986) tested Karr's Index of Biological Integrity in seven tributary streams of the New River, which flows through the Appalachian Plateau region of West Virginia. Leonard and Orth had to adapt the index to reflect conditions in their region. In their study streams the number of darter species, small benthic fish in the family Percidae, indicates high environmental quality, while increasing numbers of creek chubs indicate increasing pollution. In addition, high proportions of insectivores indicate excellent environmental conditions, while high proportions of generalist feeders, or omnivores, indicate poor conditions. High densities of fish were taken as a sign of high environmental quality, while the presence of diseased or deformed individuals indicated environmental problems.

Leonard and Orth assigned scores of 1 (worst conditions), 3 (fair conditions), or 5 (best conditions) for each of the variables they studied at each of their sampling sites in the study streams. They then summed the scores for the seven variables at each site to determine an Index of Biological Integrity. The minimum possible value was 7, poorest conditions, and the maximum possible value was 35, best conditions. They next made independent estimates of levels of pollution at each study site. Their estimates were based upon the daily discharge of municipal sewage and the local densities of septic tanks, roads, and mines. The study streams showed a wide range of environmental pollution due to sewage, mining, and urban development. Leonard and Orth found that the Index of Biological Integrity correlated well with independent estimates of pollution at each study site (fig. 3.43).

FIGURE 3.43 Pollution and the Index of Biological Integrity (data from Leonard and Orth 1986).

Many other investigators have tested the ability of the Index of Biological Integrity to represent the extent of environmental degradation in rivers and lakes. The index is effective in a wide range of regions and aquatic environments. The important point here is that natural history is being put to work to address important environmental problems. The foundation of natural history built in this chapter and in chapter 2 is useful now as we go forward to study ecology at levels of organization ranging from individual species through the entire biosphere.

SUMMARY CONCEPTS

Humans everywhere hold a land-centered perspective of the planet. Consequently, aquatic life is often most profuse where conditions appear most hostile to people, for example, along cold, wave-swept seacoasts, in torrential mountain streams, and in the murky waters where rivers meet the sea.

The hydrologic cycle exchanges water among reservoirs. Of the water in the biosphere, the oceans contain 97% and the polar ice caps and glaciers an additional 2%, leaving less than 1% as freshwater. The turnover of water in the various reservoirs of the hydrologic cycle ranges from only 9 days for the atmosphere to 3,100 years for the oceans.

The biology of aquatic environments corresponds broadly to variations in physical factors such as light, temperature, and water movements and to chemical factors such as salinity and oxygen.

The oceans form the largest continuous environment on earth. An ocean is generally divided vertically into several depth zones, each with a distinctive assemblage of marine organisms. Limited light penetration restricts photosynthetic organisms to the euphotic, or epipelagic, zone and leads to thermal stratification. Oceanic temperatures are much more stable than terrestrial temperatures. Tropical seas are more stable physically and chemically; temperate and high-latitude seas are more productive. Highest productivity occurs along coastlines. The open ocean supports large numbers of species and is important to global carbon and oxygen budgets.

Kelp forests are found mainly at temperate latitudes. Coral reefs are limited to the tropics and subtropics to latitudes between 30° N and S latitudes. Coral reefs are generally one of three types: fringing reefs, barrier reefs, and atolls. Kelp beds share several structural features with terrestrial forests. Both seaweeds and reef-building corals grow only in surface waters, where there is sufficient light to support photosynthesis. Kelp forests are generally limited to areas where temperature ranges from about 10°to 20, while reef-building corals are limited to areas with temperatures of about 18° to 29. The diversity and productivity of coral reefs rival that of tropicaI rain forests.

The intertidal zone lines the coastlines of the world. It can be divided into several vertical zones: the supratidal, high intertidal, middle intertidal, and low intertidal. The magnitude and timing of the tides is determined by the interaction of the gravitational effects of the sun and moon with the configuration of coastlines and basins. Tidal fluctuation produces steep gradients of physical and chemical conditions within the intertidal zone. Exposure to waves, bottom type, height in the intertidal zone, and biological interactions determine the distribution of most organisms within this zone.

Salt marshes, mangrove forests, and estuaries occur at the transitions between freshwater and marine environments and between marine and terrestrial environments. Salt marshes, which are dominated by herbaceous vegetation, are found mainly at temperate and high latitudes. Mangrove forests grow in the tropics and subtropics. Estuaries are extremely dynamic physically, chemically, and biologically. The diversity of species is not as high in estuaries, salt marshes, and mangrove forests as in some other aquatic environments but productivity is exceptional.

Rivers and streams drain most of the land area of the earth and reflect the land use in their basins. Rivers and streams are very dynamic systems and can be divided into several distinctive environments: longitudinally, laterally, and vertically. The temperature of rivers follows variation in air temperature but does not reach the extremes occurring in terrestrial habitats. The flow and chemical characteristics of rivers change with climatic regime. Current speed, distance from headwaters, and the nature of bottom sediments are principal determinants of the distributions of stream organisms.

Lakes are much like small seas. Most are found in regions worked over by tectonics, volcanism, and glacial activity, the geological forces that produce lake basins. A few lakes contain most of the freshwater in the biosphere. Lake structure parallels that of the oceans but on a much smaller scale. The salinity of lakes, which ranges from very dilute waters to over 200‰, is much more variable than that of the oceans. Lake stratification and mixing vary with latitude. Lake flora and fauna largely reflect geographic location and nutrient content.

Potential threats to all these aquatic systems include over-exploitation of populations and waste dumping. Freshwater environments are particularly vulnerable to the introduction of exotic species. The nature of fish assemblages is being used to assess the "biological integrity" of freshwater communities. The application of this Index of Biological Integrity depends on detailed knowledge of the natural history of regional fish faunas.

REVIEW  QUESTIONS

1. Review the distribution of water among the major reservoirs of the hydrologic cycle. What are the major sources of freshwater? Explain why according to some projections availability of freshwater may limit human populations and activity.

2. The oceans cover about 360 million km2 and have an average depth of about 4,000 m. What proportion of this aquatic system receives sufficient light to support photosynthesis? Make the liberal assumption that the euphoric zone extends to a depth of 200 m.

3. Below about 600 to 1,000 m in the oceans there is no sunlight. However, many of the fish and invertebrates at these depths have eyes. In contrast, fish living in caves are often blind. What selective forces could maintain eyes in populations of deep-sea fish? (Hint: Many species of deep-sea invertebrates are bioluminescent.)

4. Darwin (1842) was the first to propose that fringing reefs, barrier reefs, and atolls are different stages in a developmental sequence that begins with a fringing reef and ends with an atoll. Outline how this process might work. How would you test your ideas?

5. How does feeding by urchins, which prey on young corals, improve establishment by young corals? Use a diagram outlining interactions between urchins, corals, and algae to help in the development of your explanation.

6. How might a history of exposure to wide environmental fluctuation affect the physiologicaI tolerances of intertidal species compared to close relatives in subtidal and oceanic environments? How might salinity tolerance vary among organisms living at different levels within the intertidal?

7. How might oxygen concentration of interstitial water be related to the grain size of the sand or mud sediment? How might the oxygen concentrations of tide pools in sheltered bays compare to those on the shores of exposed headlands?

8. According to the river continuum model, the organisms inhabiting headwater streams in temperate forest regions depend mainly upon organic material coming into the stream from the surrounding forests. According to the model, photosynthesis within the stream is only important in the downstream reaches of these stream systems. Explain. How would you go about testing the predictions of the river continuum model?

9. How could you test the generalization that lake primary production and the composition of the biota living in lakes are strongly influenced by the availability of nutrients such as nitrogen and phosphorus? Assume that you have unlimited resources and that you have access to several experimental lakes.

10. Biological interactions may also affect lake systems. How does the recent history of the Great Lakes suggest that the kinds of species that inhabit a lake influence the nature of the lake environment and the composition of the biological community?