Fall 2017 eNewsletter

Featured Research

Quagga Mussels and Nutrients Linked to Lake Michigan Productivity Loss

Over the past 15 years, Lake Michigan experienced a rapid expansion of the invasive quagga mussel population. In contrast to zebra mussels, which had previously invaded nearshore areas of the lake, quagga mussels were able to proliferate in deeper water and in much greater numbers. Both quagga and zebra mussels consume large volumes of phytoplankton (microscopic algae) by filtering surrounding lake water, removing this important food source that forms the base of the entire Lake Michigan food web. Since quagga mussels have invaded, phytoplankton and the phosphorus concentrations that sustain them decreased while growth of nuisance algae on the lake bottom increased.

These changes have raised questions about the sustainability of the economically and culturally important fisheries in Lake Michigan, which rely on a food web supported by phytoplankton productivity. Fisheries managers are asking how quagga mussels, phosphorus loads, and weather impact the food web and fishery.

Chlorophyll a is a pigment specific to phytoplankton, which can be used as a measure of phytoplankton abundance and productivity. Prior to the quagga mussel invasion, there was a spring phytoplankton bloom that peaked in April. Rowe et al. (2015) combined maps of quagga mussel abundance with estimates of filter-feeding rate and lake depth to estimate the fraction of the water column that is cleared per day by filter feeding. The results showed an association between the location of mussel filter feeding intensity and decreased chlorophyll a in April, when the lake is vertically well-mixed (e.g. red arrows).

For the past two years, CIGLR Assistant Research Scientist Mark Rowe has been developing sophisticated models to understand the mussel-plankton association and help guide management decisions. In a 2015 paper, Dr. Rowe and others mapped the location of quagga mussels in Lake Michigan by applying a geostatistical model to lake bottom (benthic) survey data. This study showed, for the first time, a statistically significant association between locations of high mussel filter feeding intensity and reduced phytoplankton blooms observed in satellite imagery.

Building on this evidence of a direct link between quagga mussel filter feeding and reduced spring phytoplankton blooms, a more recent study by Rowe et al. (2017) applied a biophysical model to the problem. Rather than depending on statistical association, the biophysical model is a mathematical computer simulation of lake temperature, currents, and the lower food web (plankton). It is based on first principals such as conservation of mass, energy, and momentum, and a lower food web whose productivity is limited by phosphorus.

These new model simulations showed that quagga mussels can deplete phytoplankton in the overlying water during spring and fall, when strong vertical mixing carries surface water to the bottom and vice versa. But during summer and winter when this vertical mixing is weak, phytoplankton is separated from filter feeders living on the bottom.

A, B: Satellite-derived maps from two different sources (SeaWiFS [A], CPA [B]) show the combined effects of mussels and nutrient loads on phytoplankton abundance (chlorophyll a). C-E: Maps derived from biophysical model simulations are able to separate the effects of mussels and nutrient loads on phytoplankton abundance. These maps show that mussels deplete the spring phytoplankton bloom when nutrient loads are present (C, D), and that nutrient loads enhance nearshore spring phytoplankton blooms when mussels are present (C, E). See Biophysical Model Simulations of Lake Michigan here.

 

 

 

 

 

 

 

Manipulating the amount of nutrients available to phytoplankton in the computer simulation shed more light on the mussel-phytoplankton relationship. The model showed that changes to lake productivity averaged over the year were small when the amount of phosphorus stayed the same, despite the mussels filtering phytoplankton from the water. This result suggested that quagga mussels have reduced lake productivity both by filter feeding and by removing phosphorus from the water. Some of the phosphorus was stored in the tissue of living mussels, and some was likely transferred to the sediment. When the model simulated increased phosphorus from tributary inputs, lake productivity increased despite the presence of mussels. Although not simulated by the model, increased nutrient loads in the presence of mussels may lead to increased nuisance algae on the lake bottom.

With decreased offshore productivity in Lake Michigan, it has become increasingly important to understand how nearshore productivity supports the food web and fisheries. Biophysical models that simulate location-specific impacts of nutrient levels have the potential to be an important tool for informing lake management decisions.

About The Project
The project team included experts in ecology and lake hydrodynamics (Dr. Mark Rowe of CIGLR, and Drs. Eric Anderson, Henry Vanderploeg, Steven Pothoven, Ashley Elgin and Jia Wang of GLERL), and an expert in satellite remote sensing (Foad Yousef of UCLA). M.D. Rowe received funding from the National Research Council Research Associate program and Cooperative Institute for Limnology and Ecosystems Research (CILER) through the NOAA Cooperative Agreement with the University of Michigan.

Related Articles

  • Rowe, M.D., E.J. Anderson, H.A. Vanderploeg, S.A. Pothoven, A.K. Elgin, J. Wang and F. Yousef. 2017. Influence of invasive quagga mussels, phosphorus loads, and climate on spatial and temporal patterns of productivity in Lake Michigan: A biophysical modeling study. Limnology and Oceanography. (DOI:10.1002/lno.10595). (Scientific Article), Biophysical Model Simulations of Lake Michigan (Animations)
  • Rowe, M.D., E.J. Anderson, J. Wang and H.A. Vanderploeg. 2015. Modeling the effect of invasive quagga mussels on the spring phytoplankton bloom in Lake Michigan. Journal of Great Lakes Research. 41(Supplement 3):49-65. (DOI:10.1016/j.jglr.2014.12.018). (Scientific Article)
  • Bootsma, H.A., M.D. Rowe, C.N. Brooks and H.A. Vanderploeg. 2015. Commentary: The need for model development related to Cladophora and nutrient management in Lake Michigan. Journal of Great Lakes Research. 41(Supplement 3):7-15. (DOI:10.1016/j.jglr.2015.03.023). (Scientific Article)
  • Rowe, M.D., R.G. Kreis Jr. and D.M. Dolan. 2013. A reactive nitrogen budget for Lake Michigan. Journal of Great Lakes Research. 40(1):192-201. (DOI:10.1016/j.jglr.2013.11.005). (Scientific Article)
  • Rowe, M.D., D.R. Obenour, T.F. Nalepa, H.A. Vanderploeg, F. Yousef and W.C. Kerfoot. 2015. Mapping the spatial distribution of invasive dreissenid mussel biomass and filter-feeding impact in Lake Michigan. Freshwater Biology. 60(11):2270-2285. (DOI:10.1111/fwb.12653). (Scientific Article)