Phytoplankton trends in the Laurentian Great Lakes (2001–2021) reveal significant changes in phytoplankton abundance and taxonomic composition

We present Laurentian Great Lakes phytoplankton trends from 2001 to 2021 in spring and summer. Trend analysis identified significant changes in phytoplankton abundance and taxonomic composition. This included the loss of phytoplankton biovolume, especially diatoms, in lakes Huron and Michigan following the quagga mussel invasion, an increase in cyanobacteria especially in the central basin of Lake Erie, and an increase in phytoplankton biovolume in Lake Superior. Within functional groups, we observed increases in the absolute and relative abundances of flagellated organisms, largely owing to increases in single-celled cryptophytes and dinoflagellates in the spring and colonial chrysophytes in the summer, and a rise in lightly silicified rhizosolenoid diatoms during summer. Random forest analysis identified potential drivers and mechanisms for phytoplankton changes alongside concurrently sampled invertebrate and water quality parameters. The quagga mussel invasion was an important driver of major phytoplankton shifts, but climate change-related stressors affecting stratification are also likely drivers of changes. We observed strong relationships between phytoplankton and microzooplankton, with rotifers and copepod nauplii showing positive relationships with many phytoplankton groups, though whether these are top-down or bottom-up associations is not clear. This study recognizes continuing reorganization of the Great Lakes phytoplankton community that is likely to have long-term impacts on food webs.