Phytoplankton production in Lakes Michigan and Huron: Spatial, seasonal, and historical patterns

Session: Great Lakes Primary Production: Methods, Results, and Management Implications (2)

Katie Bockwoldt, ORISE - US EPA GLNPO, [email protected]
Harvey Bootsma, University of Wisconsin-Milwaukee, [email protected]

Abstract

Although several recent studies have estimated phytoplankton production in Lakes Michigan and Huron using remote sensing, direct measurements of primary production have been limited. In spring 2016 and summer 2017, we conducted whole-lake surveys on Lakes Michigan and Huron to assess the spatial variability of phytoplankton production. Spring production was relatively uniform across both lakes, but production in Lake Huron was 40% lower than in Lake Michigan. During the summer, production was higher in the northern basins of each lake. From May to November 2017 in Lake Michigan, we measured production bi-weekly at a 75 m standard station near Milwaukee. Areal production peaked with the warmest temperatures in August/September and DCL production accounted for less than 20% of total water column production. Seston C:P, photosynthetic parameters, and growth estimates of DCL and epilimnetic phytoplankton in Lake Michigan do not appear to have changed since the mussel invasion despite decreasing TP concentrations, suggesting the decrease in phytoplankton production in Lake Michigan is due primarily to biomass loss to mussel grazing rather than increased nutrient limitation.