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Limnology: The Science of Inland Waters

Limnology is a field of science that helps aquaculture managers understand what's going on in Alaska's inland waters.

by | March 15, 2022

Filtering Leisure Lake water for carbon, nitrogen, and phosphorus in the Alaska Department of Fish and Game lab in Soldotna. CIAA

Aquaculture managers in Alaska are often preoccupied with an interdisciplinary branch of science called limnology. The Alaska Department of Fish and Game maintains a network of labs that process limnology samples for the Cook Inlet Aquaculture Association (CIAA) and other regional hatchery programs. In other words, limnology and aquaculture go hand in hand. Learn how the test results can tell a detailed story about a broad area of salmon habitat.

What Is Limnology?

A researcher measures the light incidence of water on Shell Lake. CIAA

Limnology is the study of inland waters. While oceanographers turn their attention to the earth’s vast seas, limnologists focus on lakes, marshes, rivers, and streams. Limnologists also follow how water flows through underground aquifers and artificial reservoirs, as well as how the atmosphere can influence inland waters.

According to the Association for the Sciences of Limnology and Oceanography, limnology combines the sciences of biology, chemistry, and physics. Limnologists primarily concern themselves with the following:

  • How plants and animals adapt to a changing environment
  • Growth through the various life cycles
  • Nutrients in an ecosystem
  • The productivity of certain habitats

Limnology helps society understand and conserve freshwater resources — for drinking water, recreation, industrial use, irrigation, and as a home to much of the fish we depend upon.

The Story Inside a Water Sample

Zooplankton in a limnologist’s water sample. CIAA

A limnology analysis usually includes measurements and samples from a particular body of water. In the field, researchers record water temperature, the amount of dissolved oxygen, and light incidence to measure how clear the water is. They note how sediments scatter light passing through the water sample. And the researchers take a couple of gallons of water back to the lab to filter.

At the limnology lab, technicians analyze water samples for certain chemicals, such as nitrogen, phosphorus, silicon, as well as color and plant-based pigments. They also study the growth of zooplankton — copepods and other tiny animals that serve as some of the salmon’s primary sources of food. Copepods are tiny crustaceans, many no larger than a speck of dust. They are maybe the most abundant kind of animal in the world.

All this data tells fisheries managers about the environment where salmon hatch and grow. For instance, a chemical imbalance could disrupt microscopic plant growth and mean less food for zooplankton.  A drop in zooplankton populations could mean there will be less food for young salmon to eat in a particular lake.  A drop in zooplanktion populations could also signal that there may be too many immature salmon in a lake and adjustments to the stocking program are needed.

Collecting and analyzing limnology samples over a number of years allows limnologists to tell a story about what is happening in the lake. By using these techniques, researchers can track changes and productivity over time with consistent sampling. Every water body is different. The goal is to be sure salmon enhancement matches the water body.

What CIAA Learns From Limnology

Former CIAA Executive Director Gary Fandrei pulls up a zooplankton tow. The water captured in the cup at the bottom of the net contains zooplankton and phytoplankton. CIAA

CIAA uses limnology to determine the best place to release salmon fry to lakes. If the chemical balance of a lake’s water is just right, and it has a rich supply of zooplankton, the lake is probably a good candidate for an enhanced run. 

Limnology also helps CIAA know where to apply liquid fertilizer and how much should be used. This fertilizer boosts the growth of phytoplankton, which feed zooplankton, which in turn feed juvenile salmon. These efforts boost the lake’s productivity, a measure of how large of a salmon population that lake can support.

In 2021, CIAA collected limnology samples at Bear, Hidden, Kirscher, and Leisure lakes. In 2022, the association will also sample Upper Paint and Selukpuk lakes as part of CIAA’s Paint River project, which compares results over many years.

Biologists would like to find out how productive Upper Paint and Selukpuk lakes are for juvenile salmon rearing and collecting limnology samples will provide valuable information. They will also compare samples to those collected in these lakes over 30 years ago to look for any changes to the lakes’ environments that have occurred.

How You Can Observe Limnology

Take a glass of water from a natural body of water, like a creek or lake. Look closely at the particles floating in the water. Many of these tiny objects may be phytoplankton or zooplankton which juvenile salmon feed upon. You can also learn more about CIAA habitat research projects on our website.

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