Stable isotopes reveal an imprint of harmful algal blooms on Lake Erie

In Lake Erie, harmful algal blooms have become a recurrent problem during the summer-through-fall growing season. We investigated if the annual disturbance of harmful algal blooms in Lake Erie relates to distinct differences in carbon and nitrogen stable isotope composition (δ13C and δ15N values). Guided by near real-time satellite imagery, we tracked the blooms in western and central Lake Erie during July and August 2019 for the collection of in-bloom and out-of-bloom suspended particulate organic matter. The δ15Npom values did not differ between in-bloom and out-of-bloom samples; however, δ13Cpom values were significantly higher in the in-bloom compared to out-of-bloom samples. We used principal component analysis to investigate how water quality relates to biogeochemical indicators in in-bloom and out-of-bloom samples. In-bloom samples were only found in the western basin. They contained lower concentrations of ions (Na+, Cl-, SO42-) and inorganic nitrogen (NO2-, NO3-, NH4+), and higher concentrations of total nitrogen, total phosphorus, and organic particles (TSS, VSS, DOC). After uncovering patterns in in-bloom and out-of-bloom samples in the western portion of Lake Erie, we created isoscapes to compare particulate organic matter samples collected throughout the lake during July-September. The δ15Npom values varied spatially across the three basins, and in the central and eastern basins, δ15Npom values were higher in September relative to July. We found δ13Cpom values displayed a longitudinal spatial gradient with higher values in the western basin and lower values towards the eastern basin with no temporal separation based on sampling month. δ15Npom and δ13Cpom values provide a baseline for future isotope studies in Lake Erie, and observed variation in δ13Cpom reveals a harmful algal bloom-produced change in the carbon biogeochemistry of Lake Erie.