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

Abstract

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^-, SO4^2-) 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.