According to a recently published study, invasive quagga mussels in Lake Michigan, paired with decreasing nutrient availability, could explain an observed decline in food chain productivity in Lake Michigan.
Currently, Lake Michigan is in the midst of a decade-long decline in food chain productivity, or the lake’s ability to produce biomass at the lowest levels of the food chain. But the reason behind this drop-off has, until now, remained unknown.
The study, published in the Journal of Geophysical Research and co-authored by University of Wisconsin–Madison graduate Darren Pilcher, who received his doctorate studying Lake Michigan, used an ocean circulation model that was adapted for Lake Michigan’s physical properties, food chain and chemical composition.
“With one of these large circulation models we can actually do these simulations or experiments where we change the nutrient concentrations and say, ‘well what impact does that have on productivity?’” Pilcer, now a research scientist at the Joint Institute for the Study of the Atmosphere and Ocean at the University of Washington in Seattle, says.
“We can just do that and not have any mussels and say, ‘this is the relative effect of declining nutrients,’ and then we can keep the nutrients the same and add quagga mussels.”
Quagga mussels are an invasive species in Lake Michigan, and have been growing in number on the lake bottom since around 2005 or 2006 when they were introduced by shippers using the St. Lawrence Seaway. The mussels’ diet consists of phytoplankton, the smallest organisms in Lake Michigan’s food chain, which use photosynthesis to produce its energy.
Phytoplankton are eaten by zooplankton, which are in turn eaten by larger organisms and fish. Decreases in phytoplankton populations are felt up the food chain, affecting even the largest fish populations in the lake, including commercially important species like salmon and whitefish.
Because Lake Michigan is so large, during the spring and fall its waters overturn, similar to processes that happen in oceans. The phytoplankton are carried down with the surface water to the hungry mussels at the bottom, which can filter anywhere from one to five liters of water per day. As the mussels gobble up phytoplankton, they reduce the amount left for fish and other organisms, effectively starving them of their food.
According to Pilcher, high levels of quagga mussel grazing on phytoplankton partially explains why food chain productivity is down. Another facet of the decline in food chain productivity comes from the summer process in the lake.
In the summer, when the lake is no longer mixing, phytoplankton sit at the surface, above the mussels deep down in the lake. But the phytoplankton residing in the nearshore regions of the lake are not safe, and can still be consumed by mussels.
These nearshore regions, where phytoplankton populations are down and mussels do well, have seen an increase in algae called cladophora. The algae takes advantage of nutrients in the lake that are not being consumed by the phytoplankton, and grow at high rates in the phytoplankton absence.
Therefore, according to Pilcher, fish that eat mussels or algae in nearshore regions of the lake could fair better than the salmon or whitefish that rely on zooplankton in the deeper offshore regions.
“The big concern here of why this matters is because a lot of the fish that we care about in the great lakes are ultimately tied to productivity at the base of the food chain,” Pilcher says.
Although the new study sheds some light on what might factors might be causing this decline in lake productivity, Pilcher is still asking questions about the ultimate impact on the lake ecosystem.
“So what kind of impact does that have on fish species that we really care about for commercial fisheries but also for recreational fishing?”
Header image: Norwegian Atlantic salmon, Jeff Miller / UW Madison