Assessing algal bloom conditions between two river plumes in western Lake Erie

Lake Erie experiences annual harmful algal blooms (HABs), particularly in the western basin, due to factors including anthropogenic and climatic drivers. Western Lake Erie (WLE) is subject to high-load, low-volume inputs from the Maumee River and high-volume, nutrient-poor inputs from the Detroit River. We assessed water quality and phytoplankton along a ∼ 31-km transect between the two inflows, sampling monthly May–October in 2021 and 2022. Coupled with satellite imagery analysis, findings suggest that the transect grouped into distinct zones characterized by either system or the mixing zone between them. Temperature and specific conductivity increased and photosynthetically active radiation attenuated more rapidly nearer the Maumee River, particularly during active-bloom months. We found less temporal variability in water quality parameters nearer the Detroit River and less spatial variability during pre-bloom than active- and post-bloom months. Total phosphorus consistently increased nearer the Maumee River, and soluble reactive phosphorus remained low. Inorganic nitrogen concentrations were typically higher in May–June than July–October. Ammonium consistently decreased towards the Maumee River and nitrate did so during bloom periods, indicating the Detroit River could supply inorganic nitrogen to the bloom. Suspended materials and chlorophyll concentration increased nearer the Maumee River during the active-bloom months. Phytoplankton community composition and abundance differed spatially and temporally, with important differences between years that echo bloom dynamics, including being more widespread but less concentrated in 2021 than 2022. It is imperative to understand how annual variability in climatic and non-climatic drivers affect WLE’s most influential sources of inflow and the transition zone between them.