Bloom or bust: long- and short-term drivers of cyanobacterial blooms on Muskegon Lake, Michigan – a Great Lakes estuary - Part 1

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Muskegon Lake, a drowned river mouth estuary that empties Michigan’s second largest watershed into Lake Michigan, historically suffers from cyanobacterial harmful algal blooms due to cultural eutrophication. This study provides both a historical take on cyanobacterial blooms in Muskegon Lake and a focus on the year 2019. While visible blooms still form periodically on the lake during the summer and fall, a look into historical data from 2003 to 2019 reveals a major decrease in cyanobacterial abundance over time along with shifts in community composition in response to changes in nutrients and temperature. Additionally, the long-term dataset from the Muskegon Lake Observatory buoy revealed 2019 to be an anomalous year in terms of weather. Biweekly sampling of three sites on the lake during 2019 captured how the aberrant weather affected the phytoplankton community. With an unusually cool spring and the highest precipitation accumulation on record, the phytoplankton community saw domination in all seasons by an unexpected group – the diatoms. Though multiple genera were found, cyanobacteria comprised less than 2% of the phytoplankton community in 2019 – the lowest on record for Muskegon Lake. Further, an in situ bioassay with bottom water and river water as treatments and near-surface water as a control allowed us to explore which nutrients stimulate algal growth, identify the sources of these nutrients (internal loading v. river influx), and determine whether this source changed seasonally. We conclude that nutrient forms and concentrations drive cyanobacterial abundance and composition in Muskegon Lake on a yearly basis, but large-scale climatic events impacting regional temperature and precipitation can override the system. These findings provide an alternate perspective on the impacts of a changing climate on the phytoplankton community in Great Lakes estuaries. Lessons learned from this model Great Lakes estuary should be applicable to similarly impacted freshwater estuaries and temperate lakes.

Presented by Jasmine Mancuso, Grand Valley State University, during the 2020 Michigan Inland Lakes Convention

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Bloom or bust: long- and short-term drivers of cyanobacterial blooms on Muskegon Lake, Michigan – a Great Lakes estuary - Part 1

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