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Limnology: Lake & River Ecosystems

Instructor: Dina El Chammas Gass

Dina has taught college Environmental Studies classes and has a master's degree in Environmental and Water Resource Engineering.

In this lesson you will understand the term limnology and what it encompasses. You'll also understand the meaning of the word 'ecosystem' and get an introduction to ecosystems in lakes and rivers.

Defining Limnology

Limnology is the study of surface waters that are located inland. Inland surface waters include lakes, ponds, springs, creeks, streams, rivers, estuaries and wetlands. Limnology encompasses both fresh and salty water bodies.

Limnology, however, does not include groundwater for obvious reasons; groundwater is not on the surface! It also does not study ecosystems in the ocean. That body of knowledge is called oceanography.

The field of limnology involves the physical, chemical, geological and ecological aspects of water bodies.

  • Physical aspects would include the integrity of river banks, the temperature of the water and the meandering pattern of flowing water bodies.
  • Chemical aspects include any water pollutants in the water like fertilizers or pesticides, the pH of the water and perhaps oxygen content.
  • Geological aspects would be concerned with the quality of the soil in river beds, whether spanning areas that require fine granular sediments exist and whether the soil on the banks is strong enough to stop bank vegetation from toppling over into the water body.

What is an Ecosystem?

The ecological aspect of limnology is the one we're most concerned with in this lesson. Ecosystems are communities of living organisms and the environment that they live in. When you study an ecosystem, you're concerned with the health of the living organisms, the health of their environment and the relationship between the two.

Aquatic living organisms include biotic (living) plants, animals and microorganisms. The environment they live in includes abiotic (nonliving) components like wind, soil particles, water, sunlight, oxygen, nutrients such as nitrogen and phosphorous, dissolved or suspended metals, and minerals. The physical, chemical and geological aspects of limnology are concerned with the environment in which aquatic living systems live.

Lake Ecosystems

Lakes are usually distinguished from other surface water bodies in that they don't flow, and they hold a volume of water deep enough so that sunlight can't penetrate to the very bottom. The fact that the water is stationary deems it appropriate to ecosystems that enjoy a sedentary lifestyle where the water is not flowing all the time. It's an ecosystem that's similar to a small Midwestern town as opposed to bustling New York City! Stationary aquatic ecosystems are called lentic ecosystems.

Lentic Microorganisms

At the bottom of this ecosystem food chain are bacteria. They can be suspended in water or form films on rocks and sand. They also form films on the roots of plants. Bacteria are crucial for decomposition and nutrient recycling. They are consumed by protozoa which in turn are consumed by zooplankton. Zooplankton are small animals that drift in water without the ability to swim.

The other 'plankton' in aquatic habitats is phytoplankton; microscopic plants that can perform photosynthesis. Phytoplankton and periphyton are the two categories of microscopic plants that are called algae. In flowing water, a lot of the oxygen gets into the water just by the mere action of flowing and tumbling. It's the best form of natural aeration. Lakes don't flow, so they rely very heavily on photosynthesis for oxygenation. Algae is the most abundant producer of oxygen in lakes.

Lentic Macroorganisms

Aquatic plants are also very important for oxygenating lakes and exist either as emergent, floating leaved, submersed, or free floating:

  • Emergent plants have their roots embedded in the soil at the depth of the lake but emerge at the surface.
  • Floating leaved plants have roots in the soil but float on the surface.
  • Submersed plants are entirely underwater.
  • Free floating plants float on the surface.

Waterlilies are an Example of Floating Leaved Plants
Waterlilies are an Example of Floating Leaved Plants

Furthermore, lakes are home to invertebrates, such as insects, crustaceans and mollusks. At the top of a lake's food chain are fish and other vertebrates, such as frogs, salamanders, snakes, turtles or alligators! They feast on zooplankton, phytoplankton, aquatic plants, invertebrates and each other.

River Ecosystems

Rivers, with their flowing waters, have lotic ecosystems. Some rivers flow slowly, but others have rapids, which are high flowing waters with ecosystems the equivalent of an aquatic New York City. These ecosystems are bustling with continuous movement on the surface but have a more laid back environment in the bottom.

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