The recent decline in Lake Michigan productivity is often attributed to filter feeding by invasive quagga mussels, but some studies also implicate reductions in lakewide nutrient concentrations. We use a 3-D coupled hydrodynamic-biogeochemical model to evaluate the effect of changing nutrient concentrations and quagga mussel filtering on phytoplankton production and phytoplankton and zooplankton biomass. Sensitivity experiments are used to assess the net effect of each change separately and in unison. Quagga mussels are found to have the greatest impact during periods of isothermal mixing, while nutrients have the greatest impact during thermal stratification. Quagga mussels also act to enhance spatial heterogeneity, particularly between nearshore-offshore regions. This effect produces a reversal in the gradient of nearshore-offshore productivity: from relatively greater nearshore productivity in the prequagga lake to relatively lesser nearshore productivity after quaggas. The combined impact of both processes drives substantial reductions in phytoplankton and zooplankton biomass, as well as significant modifications to the seasonality of surface water pCO2, particularly in nearshore regions where mussel grazing continues year-round. These results support growing concern that considerable losses of phytoplankton and zooplankton will yield concurrent losses at higher trophic levels. Comparisons to observed productivity suggest that both quagga mussel filtration and lower lakewide total phosphorus are necessary to accurately simulate recent changes in primary productivity in Lake Michigan.
Bibliographical noteFunding Information:
The authors thank the UW Madison- Milwaukee Intercampus Grant Program, NASA (NNX/11AD59G), the UW-Madison’s Climate People and the Environmental Program at the Center for Climatic Research, and the Wisconsin Space Grant Consortium for funding this project. Part of this project was also completed while D.J.P. held a National Research Council Research Associateship award at the Pacific Marine Environmental Laboratory. All model analysis was performed using the MITgcm, which is freely available from http://mitgcm.org. Necessary code modifications required to configure MITgcm to Lake Michigan and incorporate quagga mussels, along with the computation scripts used to generate the manuscript figures can be found at https://github.com/darren-pil cher/MITgcm-Michigan. NARR Reanalysis data were obtained from the NOAA/OAR/ERSL PSD, Boulder, Colorado, USA, from the website http:// www.esrl.noaa.gov/psd. Haidi Chen from the University of Wisconsin- Madison provided lake ice forcing from the U.S. National Ice Center. Comments from two reviewers greatly improved the manuscript. We also thank Samantha Siedlecki for reading an earlier draft of this manuscript and providing helpful feedback. This is PMEL contribution number 4523.
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- Great Lakes
- biogeochemical modeling
- large lakes
- nutrient cycling