Building a collaborative monitoring strategy for a changing St. Louis River Estuary: a recommendation report

Abstract

As the St. Louis River Estuary (SLRE) approaches delisting as a Great Lakes Area of Concern (AOC), emerging stressors, particularly cyanobacteria blooms, raise new questions about the estuary’s longterm condition. To investigate these concerns, we conducted intensive water quality and phytoplankton monitoring across eight sites in 2023 and 2024, selected as “hotspots” based on their susceptibility to nutrient enrichment, hypoxia, and bloom formation. Our approach combined high-frequency routine sampling with targeted analyses to identify key drivers of cyanobacteria biovolume and assess spatial and temporal redundancies in monitoring design. We observed multiple small, ephemeral cyanobacteria blooms across both years, with diverse community composition, and documented a widespread lower estuary bloom in October 2023 dominated by Aphanizomenon flos-aquae. Random Forest Analysis identified low total nitrogen, elevated temperatures, low dissolved organic carbon, and high pH as the strongest predictors of cyanobacteria biovolume. Bloom development was further linked to late-summer drought conditions and calm wind conditions. Nonmetric multidimensional scaling analysis revealed spatial overlap in environmental and phytoplankton patterns, supporting a streamlined routine sampling design focused on fewer locations and parameters for regular monitoring. We recommend a shift from evenly timed, year-round sampling across all sites to a more flexible strategy that intensifies sampling during late summer at high-risk locations, with a refined set of core parameters. By providing foundational insights into phytoplankton dynamics and evaluating monitoring design, this work supports the development of a collaborative monitoring strategy for a changing SLRE.