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.
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
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 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).
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.
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,
FIGURE 3.5 Oceanic circulation
The Pacific is also the deepest ocean, with
an average depth of over
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
FIGURE3.6 Structure ofthe oceans.
Approximately 80% of the solar_energy striking the ocean is absorbed in the firs
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
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 -
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
In addition to surface currents, there are deepwater
currents such as those produced as cooled, high-density water sinks at the
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
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 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
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
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
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 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
in Shallow Marine Waters:
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
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).
3.11 Coral reef at
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.
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).
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
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
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
FIGURE 3.14 Kelp forest structure.
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
Temperature limits the distribution of both
kelp and coral. Most kelp are limited to temperate shores, to those regions where
temperatures may fall below
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.
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.
Coral reefs and kelp beds occur where waters are well oxygenated.
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
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
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.
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
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.
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.
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.
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?
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
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
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.
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 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.
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).
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
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.
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).
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
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).
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.
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
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
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.
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.
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.
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
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,
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.
3.28 The meandering
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
FIGURE 3.29 Major rivers.
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.
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.
3.31 Headwater streams in. (a)forested
Great Smoky Mountains: and (b)
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
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
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).
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).
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.
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
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.
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.
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
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.
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
FIGURE 3.36 Distributions of some major lakes.
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
FIGURE 3.38 Seasonal changes in temperature in a temperate lake (data from Wetzel 1975).
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
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.
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.
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 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
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
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
3.40 Cumulative number of species introduced to the
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
Locally, zebra mussels have established
very dense populations within the
3.41 Two invaders of the
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.
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.
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
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.
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
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.
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.
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.
oceans cover about 360 million km2 and have an average depth of
3. Below about 600 to
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.
Biological interactions may also affect lake systems. How does the recent
history of the