Many freshwater lakes have seen an increase in the frequency and intensity of harmful algal blooms (HABs) due to a combination of climate change and eutrophication. HABs may serve as a source of dissolved organic matter (DOM) in surface waters. Significant studies have evaluated the production of reactive intermediates (RIs) from different sources of DOM; however, no prior study has systematically evaluated the impact of HABs on the photochemical reactivity of freshwater lakes. In this study, over 250 open-water samples were collected from 111 lakes across New York by volunteers participating in the Citizens Statewide Lake Assessment Program (CSLAP). To complement these samples, eight shoreline bloom samples collected during HAB episodes, and four cyanobacterial strains supplied by UTEX were cultivated in HAB-free lake water under controlled laboratory conditions. The apparent quantum yields of RIs (Φapp, RI), including excited triplet states of DOM (3DOM*), singlet oxygen (1O2), and hydroxyl radicals (●OH) were measured. Higher median values of Φapp,3DOM*, Φapp,1O2, and Φapp,●OH were observed for blue-green chlorophyll a (BG-Chla)-containing samples than those for BG-Chla-free samples, indicating the potential contribution from cyanobacteria to changes in lake water photochemical reactivity. Laboratory culture experiments confirmed increased Φapp,3DOM*, Φapp,1O2, and Φapp,●OH during the growth of bloom samples or cyanobacteria in HAB-free lake water, further corroborating our observations from the CSLAP samples. Multivariate statistical analyses prioritized BG-Chla and nutrient parameters as the top predictors of Φapp,3DOM*, Φapp,1O2, and Φapp,●OH. Our findings highlight the impact of HABs on the photoreactivity of freshwater lakes and invite further studies to understand the aquatic photochemistry of HABs-impacted surface waters more holistically.

Photochemical characterization of surface waters from New York lakes: Probing the role of cyanobacteria

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