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Aquatic Biomes
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Aquatic Biomes
Introduction
Biomes are major subdivisions of the ecological communities of Earth that contain species with similar
adaptations to particular physical environments. One major distinction among Earth’s biomes is the primary
medium in which organisms live—air in the case of terrestrial environments, and water in the case of aquatic
environments. Terrestrial organisms have easy access to oxygen, which constitutes about 21% of air, but need
special adaptations to obtain the liquid water that makes up the bulk of their bodies. For this reason availability of
liquid water, which is controlled by yearly patterns in atmospheric conditions of temperature and precipitation
(climate), distinguishes terrestrial biomes. In contrast, aquatic organisms have easy access to water but, because
water is relatively poor in dissolved oxygen (recall that oxygen diffuses slowly through water), need special
adaptations to obtain the oxygen required for aerobic metabolism. The biomes of aquatic places are distinguished
by adaptations of organisms to characteristics of their watery medium, such as depth, temperature, flow rate,
characteristics of the bottom, and what’s dissolved in it (such as salts or oxygen)—not by climate.
For this tutorial, we have organized descriptions and images of the major aquatic biomes into tabs that facilitate
between­biome comparison.
Select the buttons at the bottom of the screen to investigate features of the aquatic biomes.
Streams and Rivers
Description tab:
Streams and riverscontain flowing fresh (not salty) water that drains land masses. Small “headwater” streams
form at the top of a watershed (an area of land that is drained by a single system of connected water channels)
and flow downhill; depending on the steepness of the slope, they may be fast­ or slow­flowing. These small
streams merge as they flow downhill to form larger streams, and then rivers, which often (but not always) are
slower­flowing.
Physical characteristics tab:
Headwater streams have narrow channels, rocky stream­bottoms, and rapidly flowing, cold water that is
relatively nutrient­poor and oxygen­rich. These conditions favor organisms with life styles that require more
oxygen.
Downstream rivers have broad channels, soft, silty stream­bottoms, and more slowly flowing, warmer, and often
oxygen­poor water. Nutrient levels are high because the waters have accumulated dissolved nutrients and
organic material washed down from the upstream watershed.
Biological characteristics tab:
Organisms living in streams and rivers are adapted to a variety of types of stream­bottoms, water temperatures,
oxygen and nutrient levels. In all cases they must deal with moving water, which brings a constant flow of
nutrients or food, but at the same time threatens to carry them downstream into a different environment.
In headwater streams, organisms either attach to rocks to avoid being swept downstream, or they are strong
swimmers. Most photosynthetic organisms are algae that form thin films on the rocks; there are few tall, rooted
plants. Common animals are small aquatic insects that cling to rocks. Some scrape algae from the rocks. Others
spin nets to catch food particles that drift by. Still others are predators that catch insects they encounter as they
crawl on the rocks. Fish are strong swimmers, like trout, that catch insects that land on the water surface or
that drift in the water column.
The increased nutrient levels of larger rivers support a lush growth of photosynthetic organisms, including
filamentous algae and higher plants that are rooted in the slow water near the riverbank. Bottom sediments
support burrowing worms as well as sediment­feeding freshwater crustaceans and mollusks. Fish that eat algae
can make a living in these productive streams, alongside insect­eaters and fish­eaters.
Terrestrial organisms often visit streams and rivers to drink or feed.
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Wetlands
Description tab:
Wetlands include glades, swamps, and marshes—areas with water­saturated soil or very shallow standing fresh
water that supports emergent vegetation. Wetlands form in river floodplains and in areas where sediments have
filled shallow basins with underlying impermeable bedrock.
Physical characteristics tab:
Wetlands have still water and soft, fine­sediment substrates. High dissolved­nutrient and light levels support lush
plant growth and microbial activity, which depletes dissolved oxygen levels.
Biological characteristics tab:
In wetlands, high levels of nutrients and light support a large biomass of photosynthetic organisms. Still, shallow
water allows growth of rooted higher plants and algae, as well as floating plants, attached algae, and
phytoplankton. Rooted plants have adaptations for low soil oxygen levels.
Wetlands support diverse communities of invertebrate and vertebrate animals that burrow in the sediment, crawl
or perch on plants, or swim or wade in standing water. Relative to flowing waters, there are few attached filter­
feeders—animals must themselves move to obtain food because the water does not bring it to them. Some
animals eat dead organic matter; others eat plant tissues or scrape algae. Tiny freshwater crustaceans filter out
floating algae as they swim; fish eat everything from these tiny zooplankton, to algae, aquatic insects or worms,
snails, and other fish. As is true also of streams and rivers, the close juxtaposition of land and water supports
numerous animals, such as aquatic insects and amphibians, that have both aquatic and terrestrial life stages, as
well as terrestrial animals, such as wading birds, moose, or raccoons, that come to the wetlands to feed.
Ponds and Lakes
Description tab:
Ponds and lakes are bodies of standing fresh water that may support emergent vegetation at their edges or over
their entire area. Ponds are smaller and shallower than lakes, and can be subject to seasonal drying. Lakes range
considerably in size; Lake Baikal, which holds 23,600 km3 of water, is the deepest lake in the world and also the
largest in terms of water volume; Lake Superior holds half the water of Lake Baikal, but is the largest lake in
terms of area (82,414 km2).
Physical characteristics tab:
Standing bodies of fresh water vary in their salinity, nutrient content, oxygen levels, and permanence. Small,
shallow lakes and ponds tend to have high nutrient levels and low oxygen levels, whereas large lakes, especially
those in cold climates, tend to have lower nutrient levels and higher oxygen levels (although these levels are still
below those of streams and rivers). Lakes and ponds in dry climates tend to be saline because dissolved salts
brought in by streams are left behind when water evaporates. Small water bodies, especially those in dry
climates, often dry completely—they are ephemeral.
Physical conditions within lakes vary with water depth, distance from shore, and light penetration. Light levels
are high in the photic zone near the water surface and decrease, as temperature does, with depth. Oxygen levels
also decrease with water depth except during spring and fall in some temperate­zone lakes, when oxygen­poor
bottom waters mix with oxygen­rich surface waters. This seasonal mixing occurs because water is most dense at
4 °C. In summer, warm surface water floats on top of denser, cooler bottom water; and in winter, ice floats on
top of denser, warmer bottom water. In autumn, surface water cools more rapidly than deeper water and sinks,
bringing oxygen with it, until the surface cools below 4 °C. At this point the mixing ceases and cold water lies
above warm water. In spring, mixing occurs again as surface ice melts and sinks, until the summer temperature
profile is established.
Biological characteristics tab:
Organisms that live in shallow water near the shores of permanent lakes and ponds resemble those associated
with wetlands. Rooted, emergent vegetation rims lakes, whereas floating photosynthetic plants and single­celled
algae (phytoplankton) dominate the photic zone (the zone near the surface that is penetrated by sunlight) in
areas of deeper water. Burrowing and clinging animals are most abundant near shore, leaving the deeper waters
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to swimmers and floaters (zooplankton). Terrestrial species rarely venture beyond the shallow shorelines.
Saline and ephemeral lakes and ponds present particular challenges for aquatic organisms and therefore tend to
host a specialized set of organisms. Inhabitants of salt lakes, like brine shrimp, can tolerate salinities greater than
those of the world’s oceans. Inhabitants of ephemeral ponds have adaptations for tolerating or avoiding dry
conditions. Some, like amphibians, have both aquatic and terrestrial life stages; others, like fairy shrimp, have a
dormant life stage that endures until the pond fills once again with water. Plant communities around ephemeral
lakes or ponds often are zoned according to their moisture tolerance: the most drought­tolerant species occur in
the outermost, highest zone.
Salt Marshes
Description tab:
Salt marshes occur in temperate climates where rivers flow through gently­sloping sediment deposits into the
sea. These areas are alternately inundated when the tide rises and flows up the network of river channels, and
exposed when the tide falls.
Physical characteristics tab:
Salt marshes have high levels of dissolved nutrients deposited in the sediments flushed downstream by rivers.
They experience gradients in water salinity, from fresh water in upslope regions that are rarely inundated by high
tides, to intermediate levels of salinity at midslope parts of the marsh, to saline conditions in downslope regions
that are frequently washed by tides. Except at the highest elevations, marsh sediments are waterlogged and low
in oxygen from microbial activity. Marshes also experience fluctuations in water level from the ebb and flow of
tides.
Biological characteristics tab:
High nutrient levels support lush growth of grasses and sedges, herbaceous plants, and low­growing shrubs.
Shrubs generally dominate above high tide levels, and are replaced by more­and­more salt­tolerant herbaceous
species, and algae, at lower and lower zones of the marsh. The lowest reaches of the marsh are bare mud flats at
low tide, devoid of vegetation.
As one might expect given the transitional nature of salt marshes, animals include a diverse mix of terrestrial,
freshwater, and marine species. Burrowing, detritus­feeding worms and crustaceans, and filter­feeding mollusks
inhabit the mud flats and are fed upon by birds, fish, and mammals, including humans. Marine invertebrates and
fishes, as well as birds, often breed in salt marshes.
Mangrove Forests
Description tab:
In warm, tropical and subtropical regions, mangrove forests replace salt marshes at the mouths of rivers.
Physical characteristics tab:
Like temperate­zone salt marshes, mangrove forests experience high nutrient levels, gradients in salinity, and
tidal fluctuations in water level. However, they are even more productive than salt marshes because of warm
tropical temperatures. Intense microbial activity depletes oxygen in the water­logged sediments.
Biological characteristics tab:
Unlike salt marshes, mangrove forests are dominated by a few species of small trees and shrubs—mangroves—
that have specialized anatomical and physiological adaptations for coping with waterlogged soil and saline
conditions. Although relatively low in plant species diversity, mangrove forests are rich in animal life that includes
terrestrial vertebrates and invertebrates that live in the forest canopy, and aquatic invertebrates and vertebrates
that inhabit the water and sediments. Mangrove forests are important fish breeding areas.
Intertidal
Description tab:
Intertidal zones are portions of sandy or rocky coastlines that are alternately inundated and exposed by rising
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and falling tides.
Physical characteristics tab:
The marine intertidal experiences strong forces as ocean waves break upon continental shores. Alternating rising
and falling tides submerge and expose the intertidal, subjecting organisms there to periodic drying and to great
fluctuations in temperature. The length of dry, hot periods increases with height in the intertidal. Water is high in
dissolved oxygen and salt.
Biological characteristics tab:
All intertidal organisms have adaptations for withstanding the force of waves and moving water, periodic high
temperatures, and alternating submerged and dry conditions.
The rocky intertidal is dominated by attached algae and invertebrates such as mussels, anemones, and
barnacles; or by small fishes, snails, or starfish that can hide in crevices to avoid the full force of waves.
Organisms of the sandy intertidal avoid being swept out to sea by burrowing quickly into the sand whenever a
wave uncovers them. All intertidal organisms have mechanisms for avoiding desiccation, and for enduring high
temperatures, when they are exposed by low tide.
Because they are exposed to moving water, many intertidal animals (such as barnacles and anemones) are filter­
feeders that remove food from the water as it passes by. Others (such as snails and starfish) move about to feed
when submerged during high tide.
Intertidal communities often change rapidly with elevation in the intertidal because of spatial gradients in
physical conditions, forming visually conspicuous zones just as they do in salt marshes. Species that occur high in
the intertidal must endure longer periods of dry and hot conditions than those at low elevations.
Kelp Forests
Description tab:
Kelp “forests” occur below the intertidal in shallow coastal waters of temperate and cold latitudes.
Physical characteristics tab:
Below the intertidal zone, the force of waves is greatly reduced. As a consequence, sediments accumulate on the
continental shelf, and the rooted algae of kelp “forests” can grow tall toward the light without much risk of being
torn apart by strong physical forces.
Biological characteristics tab:
Kelp “forests” are dominated by large, leafy brown algae (kelp) that support a wide variety of marine life.
Herbivorous invertebrates such as sea urchins dine on kelp, and in turn are dined upon by vertebrates such as
sea otters, which in turn are eaten by sharks and predatory marine mammals. Kelp “forests” provide places for
fishes of all sorts to hide, breed, and feed.
Seagrass Beds
Description tab:
Seagrass beds are “meadows” dominated by completely submerged monocot higher plants found in very shallow,
sheltered waters of temperate and tropical marine oceans.
Physical characteristics tab:
Seagrass beds experience little disturbance from waves and have high light levels and soft sediment bottoms.
Dissolved oxygen levels are high, but not as high as in turbulent, shallow waters.
Biological characteristics tab:
Seagrass beds are dominated by species of four monocot plant families that are adapted to life submerged in salt
water. Temperate seagrass beds tend to be contain only a single species of seagrass, whereas multiple species
coexist in some tropical areas. Algae are the other major group of photosynthetic organisms in seagrass beds;
some grow on seagrass, others on the sediment.
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Seagrass beds are highly productive marine ecosystems that host a diverse community of animals in all phyla—
including turtles, fish, worms, mollusks, crustaceans, birds, and marine mammals. They are important breeding
grounds for marine vertebrates.
Coral Reefs
Description tab:
Most coral reefs occur in shallow, warm, clear, well­stirred marine waters, although some corals do occur in deep
or cold water. The reefs consist of colonies of invertebrate animals in the phylum Cnidaria (which includes
anemones, jellyfish, and corals) that form hard calcium carbonate skeletons for protection. The skeletons have
diverse forms and persist over long periods of time, giving reefs their structure.
Physical characteristics tab:
Corals are adapted to marine waters that are clear, well­lit, warm but not hot, low in nutrients, and relatively
high in oxygen from good water circulation.
Biological characteristics tab:
Coral reefs are sometimes called the “rainforests of the sea” because they are highly diverse and productive
ecosystems. Primary producers are algae, including skeleton­producing coralline algae and the single­celled algae
(zooxanthellae) that are symbiotic with corals. Zooxanthellae are critical to coral nutrition. Corals starve when
pollution reduces water clarity (and hence the photosynthetic rate of zooxanthellae), or high temperatures cause
them to expel their symbiotic algae—a phenomenon called “bleaching.”
The complex physical structure of coral reefs supports a diversity of vertebrate and invertebrate animals.
Sponges, barnacles, anemones, tunicates, and tubeworms attach to the hard surfaces. Fish, crustaceans,
starfish, mollusks, and worms find safe refuge in crevices. Brightly colored fishes scrape and glean food from the
reef, snatch food particles that float by, or prey on other fish in the water column.
How does such a productive and diverse ecosystem occur in nutrient­poor waters? This puzzle was recently
solved: sponges rapidly “sponge up” the dissolved organic matter that algae produce and pass it on to detritus­
eating animals. In this way, the energy fixed by primary producers is recycled locally rather than being washed
away from the reef ecosystem.
Open Ocean
Description tab:
The open ocean comprises the vast areas of Earth’s blue oceans that lie beyond the intertidal shores of islands
and continents.
Physical characteristics tab:
Except for areas in the immediate vicinity of rivers that drain large land areas, open ocean waters are relatively
clear and nutrient­poor. Oxygen levels are relatively high because microbial activity is nutrient­limited, and
because ocean waters are mixed by winds and currents. Other physical and chemical characteristics of the open
ocean change with depth. Light sufficient for photosynthesis penetrates only to a depth of approximately 200 m
—the photic zone. Temperature decreases and pressure increases with water depth. Most of the sea floor is
covered in soft sediments.
Biological characteristics tab:
Organisms in the open ocean are restricted to particular depth zones because physical conditions change so
dramatically with depth. Bottom­dwelling species are not exposed to strong water currents, but they must
endure very high pressures, darkness, and (except for those associated with hydrothermal vents), cold
temperatures. Some crawl on or burrow in bottom sediments; others swim. There are no photosynthetic
organisms; the only sources of chemical energy are organic detritus drifting down from surface, or the tissues of
detritus­feeders.
All organisms that live in the water column must swim or float. Photosynthetic organisms, restricted to the
photic zone, consist of floating single­celled algae and photosynthetic bacteria and archaea (phytoplankton). They
are fed upon by viruses and small floating animals (zooplankton) that include larval invertebrates and fish,
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crustaceans, worms, and jellyfish. Free­swimming fish, marine mammals, turtles, squids, and birds dine on these
zooplankton or on one another.
Hydrothermal Vents
Description tab:
Hydrothermal vents are cracks the deep ocean floor where tectonic plates are separating and being replaced by
magma from deep­sea volcanoes. Very hot, mineral­rich water issues from the cracks. As the hot water contacts
surrounding near­freezing sea water it cools, and minerals precipitate around the cracks, forming chimney­like
structures.
Physical characteristics tab:
Like other abyssal environments, hydrothermal vents are dark and still, and water pressure is high. Unlike other
abyssal waters, however, the water that issues from hydrothermal vents is very hot, acidic, and rich in minerals,
including methane and sulfur compounds such as hydrogen sulfide.
Biological characteristics tab:
Unlike most areas of the dark ocean abyss, life teems near hydrothermal vents. The reason is that vent waters
supply chemoautotrophic bacteria and archaea with hydrogen sulfide, which they can use as an energy source
instead of sunlight. Some of these microbes form free­living mats, whereas others form symbioses with worms or
mollusks, supplying the hosts with energy­rich carbon compounds in exchange for a steady supply of hydrogen
sulfide. A diverse group of mollusks, worms, crustaceans, fish, and Cnidaria subsist on the microbes directly or
indirectly.
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