Water is the common link among the five biomes and it makes up the
largest part of the biosphere, covering nearly 75% of the Earth’s surface.
Aquatic regions house numerous species of plants and animals, both large and small.
In fact, this is where life began billions of years ago when amino acids first
started to come together. Without water, most life forms would be unable to sustain
themselves and the Earth would be a barren, desert-like place. Although water
temperatures can vary widely, aquatic areas tend to be more humid and the air
temperature on the cooler side.
The aquatic biome can be broken down into two basic regions, freshwater
(i.e, ponds and rivers) and marine (i.e, oceans and estuaries).
Freshwater Regions
Freshwater is defined as having a low salt concentration—usually less than
1%. Plants and animals in freshwater regions are adjusted to the low salt content
and would not be able to survive in areas of high salt concentration (i.e, ocean).
There are different types of freshwater regions: ponds and lakes, streams and
rivers, and wetlands. The following sections describe the characteristics of
these three freshwater zones.
Ponds and Lakes
These regions range in size from just a few square meters to thousands of square
kilometers. Scattered throughout the earth, several are remnants from the Pleistocene
glaciation. Many ponds are seasonal, lasting just a couple of months (such as
sessile pools) while lakes may exist for hundreds of years or more. Ponds and
lakes may have limited species diversity since they are often isolated from
one another and from other water sources like rivers and oceans. Lakes and ponds
are divided into three different “zones” which are usually determined
by depth and distance from the shoreline.
The topmost zone near the shore of a lake or pond is the littoral zone.
This zone is the warmest since it is shallow and can absorb more of the Sun’s
heat. It sustains a fairly diverse community, which can include several species
of algae (like diatoms), rooted and floating aquatic plants, grazing snails,
clams, insects, crustaceans, fishes, and amphibians. In the case of the insects,
such as dragonflies and midges, only the egg and larvae stages are found in
this zone. The vegetation and animals living in the littoral zone are food
for other creatures such as turtles, snakes, and ducks.
The near-surface open water surrounded by the littoral zone is the limnetic
zone. The limnetic zone is well-lighted (like the littoral zone) and is
dominated by plankton, both phytoplankton and zooplankton. Plankton are small
organisms that play a crucial role in the food chain. Without aquatic plankton,
there would be few living organisms in the world, and certainly no humans.
A variety of freshwater fish also occupy this zone.
Plankton have short life spans—when they die, they fall into the deep-water
part of the lake/pond, the profundal zone. This zone is much colder
and denser than the other two. Little light penetrates all the way through
the limnetic zone into the profundal zone. The fauna are heterotrophs, meaning
that they eat dead organisms and use oxygen for cellular respiration.
Temperature varies in ponds and lakes seasonally. During the summer, the
temperature can range from 4° C near the bottom to 22° C at the
top. During the winter, the temperature at the bottom can be 4° C while
the top is 0° C (ice). In between the two layers, there is a narrow zone
called the thermocline where the temperature of the water changes rapidly.
During the spring and fall seasons, there is a mixing of the top and bottom
layers, usually due to winds, which results in a uniform water temperature
of around 4° C. This mixing also circulates oxygen throughout the lake.
Of course there are many lakes and ponds that do not freeze during the winter,
thus the top layer would be a little warmer.
Streams and Rivers
These are bodies of flowing water moving in one direction. Streams and rivers
can be found everywhere—they get their starts at headwaters, which may
be springs, snowmelt or even lakes, and then travel all the way to their mouths,
usually another water channel or the ocean. The characteristics of a river
or stream change during the journey from the source to the mouth. The temperature
is cooler at the source than it is at the mouth. The water is also clearer,
has higher oxygen levels, and freshwater fish such as trout and heterotrophs
can be found there. Towards the middle part of the stream/river, the width
increases, as does species diversity—numerous aquatic green plants and
algae can be found. Toward the mouth of the river/stream, the water becomes
murky from all the sediments that it has picked up upstream, decreasing the
amount of light that can penetrate through the water. Since there is less
light, there is less diversity of flora, and because of the lower oxygen levels,
fish that require less oxygen, such as catfish and carp, can be found.
Wetlands
Wetlands are areas of standing water that support aquatic plants. Marshes,
swamps, and bogs are all considered wetlands. Plant species adapted to the
very moist and humid conditions are called hydrophytes. These include pond
lilies, cattails, sedges, tamarack, and black spruce. Marsh flora also include
such species as cypress and gum. Wetlands have the highest species diversity
of all ecosystems. Many species of amphibians, reptiles, birds (such as ducks
and waders), and furbearers can be found in the wetlands. Wetlands are not
considered freshwater ecosystems as there are some, such as salt marshes,
that have high salt concentrations—these support different species of
animals, such as shrimp, shellfish, and various grasses.
Visit our gallery of
wetlands images, which illustrate the amazing
diversity of wetland ecosystems.
Marine Regions
Marine regions cover about three-fourths of the Earth’s surface and include
oceans, coral reefs, and estuaries. Marine algae supply much of the world’s
oxygen supply and take in a huge amount of atmospheric carbon dioxide. The evaporation
of the seawater provides rainwater for the land.
Oceans
The largest of all the ecosystems, oceans are very large bodies of water that
dominate the Earth’s surface. Like ponds and lakes, the ocean regions are
separated into separate zones: intertidal, pelagic, abyssal, and benthic. All
four zones have a great diversity of species. Some say that the ocean contains
the richest diversity of species even though it contains fewer species than
there are on land.
The intertidal zone is where the ocean meets the land—sometimes
it is submerged and at other times exposed, as waves and tides come in and
out. Because of this, the communities are constantly changing. On rocky coasts,
the zone is stratified vertically. Where only the highest tides reach, there
are only a few species of algae and mollusks. In those areas usually submerged
during high tide, there is a more diverse array of algae and small animals,
such as herbivorous snails, crabs, sea stars, and small fishes. At the bottom
of the intertidal zone, which is only exposed during the lowest tides, many
invertebrates, fishes, and seaweed can be found. The intertidal zone on sandier
shores is not as stratified as in the rocky areas. Waves keep mud and sand
constantly moving, thus very few algae and plants can establish themselves—the
fauna include worms, clams, predatory crustaceans, crabs, and shorebirds.
The pelagic zone includes those waters further from the land, basically
the open ocean. The pelagic zone is generally cold though it is hard to give
a general temperature range since, just like ponds and lakes, there is thermal
stratification with a constant mixing of warm and cold ocean currents. The
flora in the pelagic zone include surface seaweeds. The fauna include many
species of fish and some mammals, such as whales and dolphins. Many feed on
the abundant plankton.
The benthic zone is the area below the pelagic zone, but does not
include the very deepest parts of the ocean (see abyssal zone below).
The bottom of the zone consists of sand, slit, and/or dead organisms. Here
temperature decreases as depth increases toward the abyssal zone, since light
cannot penetrate through the deeper water. Flora are represented primarily
by seaweed while the fauna, since it is very nutrient-rich, include all sorts
of bacteria, fungi, sponges, sea anemones, worms, sea stars, and fishes.
The deep ocean is the abyssal zone. The water in this region is very
cold (around 3° C), highly pressured, high in oxygen content, but low
in nutritional content. The abyssal zone supports many species of invertebrates
and fishes. Mid-ocean ridges (spreading zones between tectonic plates), often
with hydrothermal vents, are found in the abyssal zones along the ocean floors.
Chemosynthetic bacteria thrive near these vents because of the large amounts
of hydrogen sulfide and other minerals they emit. These bacteria are thus
the start of the food web as they are eaten by invertebrates and fishes.
Coral Reefs
Coral reefs are widely distributed in warm shallow waters. They can be found
as barriers along continents (e.g., the Great Barrier Reef off Australia),
fringing islands, and atolls. Naturally, the dominant organisms in coral reefs
are corals. Corals are interesting since they consist of both algae (zooanthellae)
and tissues of animal polyp. Since reef waters tend to be nutritionally poor,
corals obtain nutrients through the algae via photosynthesis and also by extending
tentacles to obtain plankton from the water. Besides corals, the fauna include
several species of microorganisms, invertebrates, fishes, sea urchins, octopuses,
and sea stars.
Estuaries
Estuaries are areas where freshwater streams or rivers merge with the ocean.
This mixing of waters with such different salt concentrations creates a very
interesting and unique ecosystem. Microflora like algae, and macroflora, such
as seaweeds, marsh grasses, and mangrove trees (only in the tropics), can
be found here. Estuaries support a diverse fauna, including a variety of worms,
oysters, crabs, and waterfowl.
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