Freshwater Biomes

Freshwater biomes occur in water that contains little or no salt. Freshwater biomes include standing water and running water biomes.

Standing Freshwater Biomes

• Standing freshwater biomes include ponds and lakes.
• Ponds are generally smaller than lakes and shallow enough for sunlight to reach all the way to the bottom.
• In lakes, at least some of the water is too deep for sunlight to penetrate.
• As a result, like the ocean, lakes can be divided into zones based on availability of sunlight for producers.

• The littoral zone is the water closest to shore. The water in the littoral zone is generally shallow enough for sunlight to penetrate, allowing photosynthesis. Producers in this zone include both phytoplankton and plants that float in the water. They provide food, oxygen, and habitat to other aquatic organisms. The littoral zone generally has high productivity and high biodiversity.
• The limnetic zoneis the top layer of lake water away from shore. This zone covers much of the lake’s surface, but it is only as deep as sunlight can penetrate. This is a maximum of 200 meters. If the water is muddy or cloudy, sunlight cannot penetrate as deeply. Photosynthesis occurs in this zone, and the primary producers are phytoplankton, which float suspended in the water. Zooplankton and nekton are also found in this zone. The limnetic zone is generally lower in productivity and biodiversity than the littoral zone.
• The profundal zone is the deep water near the bottom of a lake where no sunlight penetrates. Photosynthesis cannot take place, so there are no producers in this zone. Consumers eat food that drifts down from above, or they eat other organisms in the profundal zone. Decomposers break down dead organisms that drift down through the water. This zone has low biodiversity.
• The benthic zone is the bottom of a lake. Near the shore, where water is shallow, the bottom of the lake receives sunlight, and plants can grow in sediments there. Organisms such as crayfish, snails, and insects also live in and around the plants near shore. The plants provide shelter from predatory fish as well as food and oxygen. In deeper water, where the bottom of the lake is completely dark, there are no producers. Most organisms that live here are decomposers.

Turnover process

• The surface water of a lake is heated by sunlight and becomes warmer than water near the bottom.
• Because warm water is less dense that cold water, it remains on the surface.
• When dead organisms sink to the bottom of a lake, they are broken down by decomposers that release the nutrients from the dead organism.
• As a result, nutrients accumulate at the lake’s bottom.
• In spring and fall in temperate climates, the surface water of a lake reaches the same temperature as the deeper water.
• This gives the different water layers the same density, allowing them to intermix.
• This process, called turnover, brings nutrients from the bottom of the lake to the surface, where producers can use them.

Classification of lakes based on Nutrient levels

Lakes can be categorized on the basis of their overall nutrient levels, as shown in Table 1. Oligotrophic lakes have low nutrient levels, so they also have low productivity. With few producers (or other aquatic organisms), the water remains clear and little oxygen is used up to support life. Biodiversity is low.

Table 1: Trophic Classification of Freshwater Lakes

Acid rain and eutrophication

• Acid rain is another cause of low productivity in lakes.
• Acid rain falling into a lake causes the lake water to become too acidic for many species to tolerate.
• This results in a decline in the number and diversity of lake organisms.
• This has happened to many lakes throughout the northeastern United States and in urban lakes across the globe.
• The water in the lakes is very clear because it is virtually devoid of life.
• Lakes with high nutrient levels have higher productivity, cloudier water, lower oxygen levels, and higher biomass and biodiversity.
• Very high nutrient levels in lakes are generally caused by contamination with fertilizer or sewage.
• The high concentration of nutrients may cause a massive increase in phytoplankton, called a phytoplankton bloom.
• The bloom blocks sunlight from submerged plants and other producers and negatively impacts most organisms in the lake.

Running Freshwater Biomes

Running freshwater biomes include streams and rivers. Streams are generally smaller than rivers. Streams may start with surface runoff, snowmelt from a glacier, or water seeping out of the ground from a spring. If the land is not flat, the water runs downhill. The water joins other streams and then rivers as it flows over the land. Eventually, the water empties into a pond, lake, or the ocean.

Plant and Animal Life in freshwater biomes

• Some species living in rivers that empty into the ocean may live in freshwater during some stages of their life cycle and in salt water during other stages.
• For example, salmon are born and develop in freshwater rivers and then move downstream to the ocean, where they live as adults.
• In contrast, some eels are born and develop in the ocean and then move into freshwater rivers to live as adults.
• Compared with standing water, running water is better able to dissolve oxygen needed by producers and other aquatic organisms.
• When a river rushes over a waterfall, most of the water is exposed to the air, allowing it to dissolve a great deal of oxygen.
• Flowing water also provides a continuous supply of nutrients.
• Some nutrients come from the decomposition of dead aquatic organisms.
• Other nutrients come from the decomposition of dead terrestrial organisms, and other organic debris such as leaves, that fall into the water.
• Algae are the main producers in running freshwater biomes. If water flows slowly, algae can float suspended in the water, and huge populations may form, like the phytoplankton bloom.
• If water flows rapidly, algae must attach themselves to rocks or plants to avoid being washed away and generally cannot form very large populations.
• Plants are also important producers in most running water biomes.
• Some plants, such as mosses, cling to rocks.
• Other plants, such as duckweed, float in the water.
• If nutrient levels are high, floating plants may form a thick mat on the surface of the water.
• Still other plants grow in sediments on the bottoms of streams and rivers.
• Many of these plants have long narrow leaves that offer little resistance to the current.
• In addition to serving as a food source, plants in running water provide aquatic animals with protection from the current and places to hide from predators.
• Consumers in running water include both invertebrate and vertebrate animals.
• The most common invertebrates are insects.
• Others include snails, clams, and crayfish.
• Some invertebrates live on the water surface, others float suspended in the water, and still others cling to rocks on the bottom.
• All rely on the current to bring them food and dissolved oxygen.
• The invertebrates are important consumers as well as prey to the many vertebrates in running water.
• Vertebrate species include fish, amphibians, reptiles, birds, and mammals. However, only fish live in the water all the time.
• Other vertebrates spend part of their time on land.
• The movement of running water poses a challenge to aquatic organisms, which have adapted in various ways.
• Some organisms have hooks or threadlike filaments to anchor themselves to rocks or plants in the water.
• Other organisms, including fish, have fins and streamlined bodies that allow them to swim against the current.
• The interface between running freshwater and land is called a riparian zone. It includes the vegetation that grows along the edge of a river and the animals that consume or take shelter in the vegetation. Riparian zones are very important natural areas for several reasons:
  • They filter pollution from surface runoff before it enters a river.
  • They help keep river water clear by trapping sediments.
  • They protect river banks from erosion by running water.
  • They help regulate the temperature of river water by providing shade.