Spring thermal bar dynamics and impacts on plankton biomass and composition in southeast Lake Michigan

Abstract

This study identified formation time, movement and disappearance of spring thermal bars in southeast Lake Michigan off Muskegon from 2010 to 2019, and analyzed 2019 and 2015 zooplankton distribution and composition in response to thermal bar dynamics and environmental characteristics. Thermal bar location was identified using available Sea Surface Temperature satellite data. Analysis of zooplankton biomass and composition distribution with environmental characteristics used data from NOAA and Great Lakes Environmental Laboratories research programs. Analysis used generalized linear models. Results showed that thermal bars formed mainly in April, persisted one to four weeks, and formation and movements influenced by air temperature occurred later with cooler air. Location relative to shore was positive and significantly influenced by runoff and air temperature 7 days prior to the thermal bar identification, but not significantly influenced by air temperature on the day bars were identified. Movement rate towards offshore, ranging 0 to 3.53 cm/s, increased significantly with air temperature 10 days prior to the end of thermal bar movements and with distance to shore but was not significantly related to river runoff discharge. Zooplankton biomass significantly varied among zones relative to thermal bars along the Muskegon transect and was higher in zones inshore from thermal bars. Chlorophyll a (CHL) biomass was highest nearshore from thermal bars with highest colored dissolved organic matter (CDOM) and water temperature. Zooplankton biomass distribution was consistent with CHL, water temperature and CDOM distribution. Zooplankton composition also varied among areas relative to thermal bars. In 2015, Leptodiaptomus sicilis copepod was most abundant in nearshore areas in April and near thermal bars with preferred cooler temperatures in May. In 2019, L. sicilis and L. ashlandi were most abundant near thermal bars in April, but decreased in May as cyclopoid copepod Diacyclops thomasi became more abundant.