Examining the relative effects of nutrient loads and invasive Dreissena mussels on Lake Michigan’s food web using an ecosystem model

Abstract

Lake Michigan has undergone ecosystem-wide changes over the past century due to changing nutrient loads and an influx of invasive species. Zebra (Dreissena polymorpha) and Quagga mussels (D. bugensis; collectively Dreissenid mussels) have been a particularly impactful invasive species, and currently account for the majority of benthic biomass in Lake Michigan. Dreissenid mussel filtration, along with declining nutrient loads following the 1972 Great Lakes Water Quality Agreement, has reduced primary productivity and caused oligotrophication in Lake Michigan’s offshore zone and a concurrent increase in nuisance algal blooms nearshore. However, the relative effects of these two factors on Lake Michigan’s food web remain a key knowledge gap. To quantify the relative effects of mussel grazing and nutrient loads on the Lake Michigan food web, I used the Lake Michigan Atlantis Model (LMAM) to predict biomass changes of Lake Michigan species under six model scenarios. The Atlantis model is a three-dimensional, spatially-explicit ecosystem model that takes into account water movement, seasonality, and food web interactions to dynamically predict biomass for functional groups at each trophic level over time. I calibrated the model using available agency food web data from 1994 to 2010, then ran 25 year simulations of 6 different scenarios including three nutrient loads (double baseline load, half baseline load, and baseline 1994 load) and two mussel scenarios (mussels present or absent). Model results indicated that mussel grazing has a much greater relative impact on the food web than changes in nutrient loads. Simulated mussel grazing on phytoplankton radiated up the food web to cause resource limitation for prey fish and piscivores. Effects on functional group biomass owing to changes in phosphorus loading were largely

masked by observed mussel effects. My findings contrast with other modeling studies that found nutrient loads also have a significant effect on productivity and biomass. These results suggest that management strategies that increase nutrient flow into Lake Michigan would have negligible positive effects on fish biomass.