Current and Historical Phytoplankton Productivity in Lake Michigan

Session: 32. - Long-Term Monitoring: Achievements, Challenges, and Solutions

Katelyn Bockwoldt, University of Wisconsin-Milwaukee, [email protected]
Harvey Bootsma, University of Wisconsin-Milwaukee, [email protected]

Abstract

Offshore primary production in Lake Michigan has been declining in the last several decades due to reductions in phosphorus loading and the invasion of dreissenid mussels. The objective of this study was to measure current phytoplankton production in Lake Michigan, determine its spatial variability, and quantify the contribution from different size classes. From May to November 2017, bi-weekly measurements of phytoplankton production were made near Milwaukee using size-fractionated (<2 um, 2-20 um, 20-200 um) samples from a pelagic site and non-size-fractionated samples from nearshore-offshore transects (15, 45, 75 m). Production was also measured during four whole-lake surveys in 2016 and 2017 at 2 offshore southern basin and 3-5 offshore northern basin sites. The results of this study suggest nearshore production is greater than offshore production, and picoplankton account for 50-60% of offshore primary production. There also appear to be north-south patterns in primary production, seston stoichiometry, and nutrient concentrations. This presentation will discuss the implications of spatial variability and size-specific primary production for higher trophic levels, and compare current estimates of phytoplankton production to historical estimates from 1973 to 2008. 

1. Keyword
Lake Michigan

2. Keyword
nutrients

3. Keyword
phytoplankton