PHYTOPLANKTON COMMUNITY PHENOLOGY AND SPATIAL VARIABILITY WITHIN A HAB-AFFECTED LAKE SUPERIOR ESTUARY

In aquatic ecosystems, cyanobacterial harmful algal blooms (cHABs) have become increasingly frequent and are manifesting in unexpected locations. The St. Louis River Estuary (SLRE) of Lake Superior is one such location, with multiple unprecedented cHABs documented since 2021. Thus, we posed several previously unanswered questions: (1) how do spatial differences in phytoplankton abundance and community composition relate to the estuarine water quality gradient; and (2) how does phytoplankton community phenology vary among SLRE locations, only some of which have experienced cHABs? We implemented a high-frequency oversampling approach from fall 2022 through fall 2024 at 8 spatially heterogeneous SLRE stations. We used multi-dimensional ordination to characterize water quality and track phytoplankton community succession and used constrained ordination in conjunction with random forest analysis to examine relationships between phytoplankton and water quality along the SLRE gradient. Findings include: (1) stations in the lower SLRE had high chlorophyll and dissolved inorganic nitrogen (DIN); (2) diazotrophic cyanobacterial taxa dominated 2023 lower SLRE cHABs, despite persistently elevated DIN; (3) phytoplankton biovolume was controlled strongly by bottom-up regulation (eg., nitrate limitation) in the upper SLRE; and (4) phytoplankton succession showed strong weather-driven differences between the winters of 2023 and 2024. Our findings will help project end-users better understand cHAB threats as they work to develop a foundational long-term SLRE monitoring program.

Presentation Preference: Oral

Primary Presenter: Peter Birschbach, Natural Resources Research Institute at University of MN Duluth (birsc016@d.umn.edu)

Authors:

Peter Birschbach, Natural Resources Research Institute and University of MN Duluth  (birsc016@d.umn.edu)

Christopher Filstrup, Natural Resources Research Institute  (filstrup@d.umn.edu)

Euan Reavie, Natural Resources Research Institute  (ereavie@d.umn.edu)

Hannah Ramage, Lake Superior National Estuarine Research Reserve  (hannah.ramage@wisc.edu)