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Effect of Lake Trophic State on Great Lakes Chlorophyll Distribution and Implications for Long-Term Trends

five people on deck of the Lake Guardian with the rosette sampler
Scientists aboard the R/V Lake Guardian use the rosette sampler to record measurements and take water samples from the lake surface to the bottom. Photo credit: Kristy Phillips

A recent study published in the journal Limnology and Oceanography highlights the importance of comparing how chlorophyll distributions vary in the Great Lakes, both across lakes and over time, to understand how changes to nutrients and water clarity may ultimately affect the lower food web.

This study is the first to use the complete long-term record (1996-2017) of water column profile data collected across all five Great Lakes by the EPA R/V Lake Guardian in a single analysis to investigate cross-lake patterns.

Read the full article: Deep chlorophyll maxima across a trophic state gradient: A case study in the Laurentian Great Lakes

Using an index of lake trophic state (how much biological production occurs in the lake) based on nutrient and chlorophyll concentrations in the spring, the authors were able to predict how chlorophyll is distributed in the water column during the summer. Chlorophyll pigment, present in phytoplankton cells, is a good proxy for phytoplankton biomass in lakes and oceans.

The study shows that lake trophic state is related to the formation of deep chlorophyll maxima (DCM), which are dense layers of chlorophyll that often form deep in the water column when a lake is stratified (separated into warm and cold layers), nutrients are depleted in the warm surface layer, and water clarity is high.

Under these conditions, the optimal place for phytoplankton growth is deeper in the water column, where more nutrients are available. DCM formation alters habitat and resource availability for higher trophic levels, including zooplankton, forage fishes, and large predators, changing the lake’s food web.

The characteristics of DCM can vary widely across lakes, including how deep they form, how thick they are, and how much phytoplankton biomass they represent. For example, DCM often form deeper and have lower phytoplankton biomass in Lake Superior, which is more oligotrophic (less total production), compared to Lake Ontario (see graphs below).  

two graphs showing water column profiles of chlorophyll-a concentration in Lake Ontario and Lake Superior
Example water column profiles of chlorophyll-a concentration (µg/L) with depth at offshore sites in Lake Ontario (left), where the DCM is relatively shallow and high in chlorophyll, and Lake Superior (right), where the DCM usually forms deeper in the water column. The shaded region of high chlorophyll-a concentration is the deep chlorophyll maximum (DCM). Overall, Lake Superior is more oligotrophic (less productive) and has higher water clarity than Lake Ontario, which can lead to differences in the shapes of DCM that form. These data were collected aboard the R/V Lake Guardian, using instrumentation on the rosette sampler.

Understanding how DCM may respond to changes in a lake’s trophic state will help scientists predict the food web impacts of ongoing long-term trophic changes in the Great Lakes, such as altered nutrient loads and increased dreissenid mussel populations.

For example, the authors found that DCM in Lake Michigan have been getting deeper over the past two decades, likely due to a combination of reduced nutrients and increased water clarity. Studying differences in DCM across the lakes, as well as over time, helps scientists anticipate how the lakes might respond to future ecosystem stressors and management actions. The study also relates patterns in the Great Lakes to long-described DCM phenomena in the open oceans, thus importantly connecting Great Lakes science to global discussions of how ecosystems in the world’s large water bodies function. 

The study is authored by Annie Scofield, who recently joined EPA’s Great Lakes National Program Office (GLNPO) as a Life Scientist. The paper is co-authored by Cornell researchers Lars Rudstam and Jim Watkins and GLNPO Physical Scientist Eric Osantowski. The Great Lakes Restoration Initiative funds GLNPO and its R/V Lake Guardian.