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Characteristics of Aquatic Biomes

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Characteristics of aquatic biomes

Like terrestrial biomes, aquatic biomes are influenced by a series of abiotic factors.

The aquatic medium-water- has different physical and chemical properties than air, however.
Even if the water in a pond or other body of water is perfectly clear (there are no suspended particles), water, on its own, absorbs light.

As one descends into a deep body of water, there will eventually be a depth which the sunlight cannot reach. While there are some abiotic and biotic factors in a terrestrial ecosystem that might obscure light (like fog, dust, or insect swarms), usually these are not permanent features of the environment.

Light in aquatic biomes

The importance of light in aquatic biomes is
central to the communities of organisms
found in both freshwater and marine
ecosystems. In freshwater systems,
stratification due to differences in density
is perhaps the most critical abiotic factor
and is related to the energy aspects of light.

The thermal properties of water (rates of heating and cooling) are significant to the function of marine systems and have major impacts on global climate and weather patterns. Marine systems are also influenced by large- scale physical water movements, such as currents; these are less important in most freshwater lakes.

The ocean is the largest marine biome. It is a continuous body of salt water that is relatively uniform in chemical composition; it is a weak solution of mineral salts and decayed biological matter.

Within the ocean, coral reefs are a second kind of marine biome. Estuaries, coastal areas where salt water and fresh water mix, form a third unique marine biome.

The intertidal zone, which is the zone between
high and low tide, is the oceanic region that is
closest to land. It can be rocky, muddy or a
sandy beach.

The intertidal zone is an extremely variable
environment because of tides. Therefore, living
things that thrive in the intertidal zone are
adapted to being dry for long periods of time. The shore of the intertidal zone is also repeatedly struck by waves, and the organisms found there are adapted to withstand damage from the pounding action of the waves.

The exoskeletons of shoreline crustaceans (such as the shore crab, Carcinus maenas) are tough and protect them from desiccation (drying out) and wave damage. Few algae and plants establish themselves in the constantly moving rocks, sand, or mud of this zone.

The neritic zone extends from the intertidal
zone to depths of about 200 m (or 650 ft)
at the edge of the continental shelf.

Since light can penetrate this depth,
photosynthesis can occur in the neritic zone.

The water here contains silt and is well-oxygenated, low in pressure, and stable in temperature. Phytoplankton and floating Sargassum (a type of free-floating marine seaweed) provide a habitat for the some sea life found in the neritic zone. Zooplankton, protists, small fishes, and shrimp are found in the neritic zone and are the base of the food chain for most of the world’s fisheries.

Beyond the neritic zone is the open ocean area known as the oceanic zone.

Abundant plankton serve as the base of the food chain for larger animals such as whales and dolphins. Nutrients are scarce and this is a relatively less productive part of the marine biome. When photosynthetic organisms and the protists and animals that feed on them die, their bodies fall to the bottom of the ocean where they remain; unlike freshwater lakes, the open ocean lacks a process for bringing the organic nutrients back up to the surface.

The majority of organisms in the aphotic zone include sea cucumbers (phylum Echinodermata) and other organisms that survive on the nutrients contained in the dead bodies of organisms in the photic zone.

All of the ocean’s open water is referred to as the pelagic realm (or zone). The benthic realm (or zone) extends along the ocean bottom from the shoreline to the deepest parts of the ocean floor.

Within the pelagic realm is the photic zone, which is the portion of the ocean that light can penetrate (200 m or 650 ft).


Beneath the pelagic zone is the benthic realm,
the deepwater region beyond the continental
shelf. The bottom of the benthic realm is
comprised of sand, silt, and dead organisms.

Temperature decreases, remaining above freezing,
as water depth increases. This is a nutrient-rich
portion of the ocean because of the dead
organisms that fall from the upper layers of the ocean.

Because of this high level of nutrients, a diversity of fungi, sponges, sea anemones, marine worms, sea stars, fishes, and bacteria exist.

The deepest part of the ocean is the abyssal zone, which is at depths of 4000 m or greater. The abyssal zone is very cold and has very high pressure, high oxygen content, and low nutrient content.

There are a variety of invertebrates and fishes found in this zone, but the abyssal zone does not have plants because of the lack of light.

Hydrothermal vents are found primarily in the abyssal zone; chemosynthetic bacteria utilize the hydrogen sulfide and other minerals emitted from the vents. These chemosynthetic bacteria use the hydrogen sulfide as an energy source and serve as the base of the food chain found in the abyssal zone.

Reefs are ocean ridges formed by marine invertebrates living in warm shallow waters within the photic zone of the ocean. They are found within 30° north and south of the equator.

The Great Barrier Reef is a well-known reef system. Other common coral reef systems are atolls, which are circular reef systems surrounding a former landmass that is now underwater.


Corals found in shallower waters (at a depth of approximately 60 m or about 200 ft) have a mutualistic relationship with photosynthetic unicellular algae. The relationship provides corals with the majority of the nutrition and the energy they require.

The waters in which these corals live are nutritionally poor and, without this mutualism, it would not be possible for large corals to grow.

Some corals living in deeper and colder water do not have a mutualistic relationship with algae; these corals attain energy and nutrients using stinging cells on their tentacles to capture prey. It is estimated that more than 4,000 fish species inhabit coral reefs. These fishes can feed on coral, the cryptofauna (invertebrates found within the calcium carbonate substrate of the coral reefs), or the seaweed and algae that are associated with the coral.

In addition, some fish species inhabit the boundaries of a coral reef; these species include predators, herbivores, or planktivores.

Halophytic plants are plants which are adapted to deal with the salinity resulting from saltwater on their roots or from sea spray.

In some halophytes, filters in the roots remove the salt from the water that the plant absorbs.

Other plants are able to pump oxygen into their roots.
Salinity Example

When mussels and clams (phylum Mollusca),
are exposed to low salinity, they stop feeding,
close their shells, and switch from aerobic
respiration (in which they use gills) to
anaerobic respiration
(a process that does not require oxygen).

When high tide returns to the estuary,
the salinity and oxygen content of the water
increases, and these animals open their shells,
begin feeding, and return to aerobic respiration.

Freshwater biomes include lakes and ponds (standing water) as well as rivers and streams (flowing water).

Humans rely on freshwater biomes to provide aquatic resources for drinking water, crop irrigation and industry.

These are referred to as ecosystem services.

Temperature is an important abiotic factor affecting living things found in lakes and ponds. In the summer, thermal stratification of lakes and ponds occurs when the upper layer of water is warmed by the sun and does not mix with deeper, cooler water. Light can penetrate within the photic zone of the lake or pond.

Phytoplankton (algae and cyanobacteria) are found here and carry out photosynthesis, providing the base of the food web of lakes and ponds. Zooplankton, such as rotifers and small crustaceans, consume these phytoplankton. At the bottom of lakes and ponds, bacteria in the aphotic zone break down dead organisms that sink to the bottom.

Nitrogen and phosphorus are important limiting nutrients in lakes and ponds. Because of this, they are determining factors in the amount of phytoplankton growth in lakes and ponds.

Rivers and streams are continuously moving bodies of water that carry large amounts of water from the source, or headwater, to a lake or ocean.

Abiotic features of rivers and streams vary along the length of the river or stream. The source water is usually cold, low in nutrients, and clear. The channel width is narrower than at any other place. Because of this, the current is often faster here than at any other point. Photosynthesis here is mostly attributed to algae that are growing on rocks; the swift current inhibits the growth of phytoplankton. Plants and animals have adapted to this fast-moving water. For instance, freshwater trout species (phylum Chordata) are an important predator.

As the river or stream flows away from the source, the width of the channel gradually widens and the current slows. Phytoplankton can be suspended in slow-moving water. Worms (phylum Annelida) and insects (phylum Arthropoda) can be found burrowing into the mud and the higher order predator vertebrates (phylum Chordata) include waterfowl, frogs, and fishes.

aphotic zone part of the ocean where no light penetrates

benthic realm (benthic zone) part of the ocean that extends along the ocean bottom from the shoreline to the deepest parts of the ocean floor

biome ecological community of plants, animals, and other organisms that is adapted to a characteristic set of environmental conditions

biotic living components of the environment

ecology study of interaction between living things and their environment

ecosystem services human benefits and services provided by natural ecosystems

emergent vegetation wetland plants that are rooted in the soil but have portions of leaves, stems, and flowers extending above the water’s surface

estuary biomes where a source of fresh water, such as a river, meets the ocean

intertidal zone part of the ocean that is closest to land; parts extend
above the water at low tide