Aquatic Biomes

Water is the common link between biomes. It is the largest part of the biosphere, covering nearly 75% of the Earth’s surface. Aquatic regions house numerous species of plants and animals. 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.

Freshwater is defined as having a low salt concentration—usually less than 1%. There are different types of freshwater regions: standing (ponds and lakes), flowing (streams and rivers), and wetlands. The following sections describe the characteristics of these three freshwater zones.

Standing Water - Ponds and Lakes
These regions range in size from just a few square meters to thousands of square kilometers. Many ponds are seasonal, lasting just a couple of months, 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 and the biotic community is diverse including several species of algae (like diatoms), rooted and floating aquatic plants, grazing snails, clams, insects, crustaceans, fishes, and amphibians. 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 and dominated by plankton, phytoplankton and zooplankton. A variety of freshwater fish also occupy this zone. 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 (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 temperatureof 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.

Flowing Water - 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 contains freshwater fish such as trout. 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 are areas of standing water that support aquatic plants. Marshes, swamps, and bogs are all considered wetlands. Wetlands have the highest species diversity of all ecosystems. Plant species adapted to the very moist and humid conditions including pond lilies, cattails, sedges, tamarack, black spruce. cypress and gum. Many species of amphibians, reptiles, birds (such as ducks and waders), and furbearers can be found in the wetlands.

Estuaries are areas where freshwater streams or rivers merge with the ocean. This mixing of waters with such different salt concentrations creates a very unique ecosystem. Algae, 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.

Marine Regions
Marine regions cover about three-fourths of the Earth’s surface and include open ocean, coral reefs, benthic, and intertidal zone. 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. The largest of all the ecosystems, oceans are very large bodies of water that dominate the Earth’s surface.

The ocean is separated into separate zones. All four zones have a great diversity of species.

edited from information on UCMP The World's Biomes website