Spatial and temporal variation of suspect screening derived cyanobacterial secondary metabolite mixtures during harmful algal bloom events in shallow agroecosystem lakes.

Freshwater harmful algal blooms (HABs) pose significant health risks to communities through drinking water sources and contact recreation. Cyanobacteria that drive HABs produce a complex array of secondary metabolites, notably cyanotoxins. The spatiotemporal variability of cyanometabolite mixtures during HAB events remains poorly understood, particularly in agroecosystems, where heavy nutrient loading can alter normal lake dynamics. Therefore, we applied high-resolution mass spectrometry-based suspect screening to characterize cyanometabolite mixture dynamics during active HAB events in two Eastern Iowa lakes as test sites, using time controlled sampling at one lake and locationally controlled sampling at the other. Using the CyanoMetDB to enable complex mixture comparisons alongside ELISA-based microcystin (MC) quantification, we tentatively identified 38 unique cyanometabolites, revealing substantial chemical diversity beyond traditional monitoring targets. Spatial analysis at one of the lakes exhibited dramatic variability across a 104-meter beach, where MC concentrations ranged from 0.65 µg L-1 (below advisory limits) to 36.86 µg L-1 (4.6-fold greater than the USEPA health advisory threshold), with metabolite mixtures exhibiting low similarity (cos(θ) = 0.38) for those same sites. Temporal monitoring at the other test lake demonstrated rapid temporal changes, with MC concentrations dropping from 19.98 to 8.64 µg L-1 within one hour; however, the cyanometabolite mixtures over the same time were similar (cos(θ) = 0.97). Collectively, these findings demonstrate that HAB chemical composition and toxicity fluctuate substantially over short distances and time scales. This first HAB metabolite suspect screening in agroecosystem lakes thus provides a useful initial framework for characterizing overlooked cyanometabolite diversity and tracking chemical dynamics, advancing efforts to better understand, monitor, and mitigate HAB-related exposure.