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GLNPO's Zooplankton Monitoring Program: A Primer

Scientists onboard the R/V Lake Guardian rinse a zooplankton net

(Oct. 26, 2018) One of the core missions for U.S. EPA’s Great Lakes National Program Office is monitoring the health of the Great Lakes. The zooplankton monitoring program described below is just one of many monitoring programs conducted by GLNPO that are funded by the GLRI.

What are zooplankton?

Zooplankton are small, free-floating aquatic microorganisms including crustaceans, rotifers, open water insect larvae and aquatic mites. The zooplankton community is composed of both primary consumers, which eat free-floating algae, and secondary consumers, which feed on other zooplankton.  Zooplankton are animals, albeit very small ones, while phytoplankton are microscopic algae.  Zooplankton are one step above phytoplankton in the food chain, i.e., zooplankton consume phytoplankton and in turn are consumed by larger predators such as small fish.

Why does GLNPO monitor zooplankton?

Zooplankton are a vital component of the Great Lakes food web and the Great Lakes fishery. These organisms serve as an intermediary species in the food chain, between algae and fish. All Great Lakes fish feed solely on zooplankton at some point in their life cycle. Decreasing zooplankton populations in the Great Lakes can limit the capacity of the Lakes to support prey fish, which can translate into less food for top-level predator fish. Also, changes in zooplankton populations and diversity can indicate water quality changes in the lakes. Non-native zooplankton can also be introduced into the Great Lakes and potentially alter the Great Lakes ecosystem. Monitoring zooplankton is therefore another way to track the introduction of new invasive species.

What can zooplankton tell us about the condition of Great Lakes water?

Information on the kinds of zooplankton that are found in the water, and the abundance of certain species relative to one another, serves as a measure of biological conditions within the lakes. Zooplankton are good indicators of change in nutrient pollution over time because they respond quickly to changes in nutrient input to the waterbody.  Further, zooplankton data helps fishery and water quality managers determine how changes in lower food web communities may impact the fisheries and water quality in the Great Lakes.

A calanoid copepod (Family Diaptomidae) from the Great Lakes.

Why is GLNPO’s zooplankton monitoring program unique?

GLNPO’s Great Lakes Biology Monitoring Program is unique amongst Great Lakes monitoring programs; it is the only monitoring program that samples water quality and the lower food web of all five lakes on an annual basis. GLNPO’s zooplankton data provides a consistent time series of zooplankton abundance and biomass across all five Great Lakes taken from a single survey. The EPA research vessel Lake Guardian is used by GLNPO and current cooperator Cornell University to collect zooplankton samples at 72 fixed open water locations in the Great Lakes annually during both the spring (late March, after ice break-up) and summer (August) seasons. The spring survey monitors lake health during a time of low biological activity, when the lakes are well-mixed. The summer survey monitors lake health during the season of high biological activity, when the lakes are stratified.   

What are the resulting data trends?

Since the early 2000s, summer zooplankton communities have declined in numbers and biomass in Lakes Huron, Michigan and Ontario. The zooplankton communities in these lakes have become increasingly similar to Lake Superior.  Populations of calanoid copepods, considered indicative of low nutrient levels, have increased. At the same time, populations of cladoceran zooplankton, commonly called water fleas, have declined. Cladocerans are easily caught prey for many fish species in the lakes, so this decline means less zooplankton available for fish in these Lakes.  Zooplankton density and biomass in Lake Erie is the highest for all of the Great Lakes owing to Lake Erie’s high productivity, and the community is dominated by cladocerans and cyclopoid copepods.  

Where to access the GLNPO zooplankton data

A downloadable file is now available at EPA's Central Data Excange, which contains GLNPO’s abundance and biomass data for the Great Lakes crustacean zooplankton community. The data are organized by major taxonomic group, for all five lakes from 1997, the first year in which deep (100 m) zooplankton tows were analyzed by GLNPO’s program, through 2016.

EPA's Great Lakes Zooplankton Monitoring Program