Abstract
Hydrogen peroxide (H2O2) has been suggested to influence cyanobacterial community structure and toxicity. However, no study has investigated H2O2 concentrations in freshwaters relative to cyanobacterial blooms when sources and sinks of H2O2 may be highly variable. For example, photochemical production of H2O2 from chromophoric dissolved organic matter (CDOM) may vary over the course of the bloom with changing CDOM and UV light in the water column, while microbial sources and sinks of H2O2 may change with community biomass and composition. To assess relationships between H2O2 and harmful algal blooms dominated by toxic cyanobacteria in the western basin of Lake Erie, we measured H2O2 weekly at six stations from June – November, 2014 and 2015, with supporting physical, chemical, and biological water quality data. Nine additional stations across the western, eastern, and central basins of Lake Erie were sampled during August and October, 2015. CDOM sources were quantified from the fluorescence fraction of CDOM using parallel factor analysis (PARAFAC). CDOM concentration and source were significantly correlated with specific conductivity, demonstrating that discharge of terrestrially-derived CDOM from rivers can be tracked in the lake. Autochthonous sources of CDOM in the lake increased over the course of the blooms. Concentrations of H2O2 in Lake Erie ranged from 47 ± 16 nM to 1570 ± 16 nM (average of 371 ± 17 nM; n = 225), and were not correlated to CDOM concentration or source, UV light, or estimates of photochemical production of H2O2 by CDOM. Temporal patterns in H2O2 were more closely aligned with bloom dynamics in the lake. In 2014 and 2015, maximum concentrations of H2O2 were observed prior to peak water column respiration and chlorophyll a, coinciding with the onset of the widespread Microcystis blooms in late July. The spatial and temporal patterns in H2O2 concentrations suggested that production and decay of H2O2 from aquatic microorganisms can be greater than photochemical production of H2O2 from CDOM and abiotic decay pathways. Our study measured H2O2 concentrations in the range where physiological impacts on cyanobacteria have been reported, suggesting that H2O2 could influence the structure and function of cyanobacterial communities in Lake Erie.
Highlights
Because specific conductivity generally decreases with distance from the river mouths as water masses are mixed within the lake, the correlation between a305 and specific conductivity reflected the observation that chromophoric dissolved organic matter (CDOM) increased with proximity to the rivers in both 2014 and 2015 (Figure 5)
There was greater CDOM per unit specific conductivity in 2015 compared with 2014 (Figure 5A) concurrent with high Maumee River discharge associated with frequent storms in the region that led to the wettest June on record2
It was unlikely that photochemical production contributed substantially to the observed H2O2 production during these experiments given that photochemical production was calculated to be 3–10 times less than the observed production of H2O2 (1–10 nM h−1see Materials and Methods) The upper limit of photochemical production of H2O2 from CDOM during these experiments (36 nM h−1) was unlikely given that this estimate assumed that CDOM absorbs all UV light in the experiments; in contrast our results show that on average CDOM absorbed 70% of the UV light (Figure 9) in Lake Erie, and the ratio of CDOM to total UV absorbance was likely
Summary
Hydrogen peroxide (H2O2) is an oxidative stressor to aquatic microorganisms (Lesser, 2006; Drábková et al, 2007), and its ubiquitous presence in surface waters (Petasne and Zika, 1987; Cooper et al, 1989, 1994) has been proposed to influence the community composition and toxicity of cyanobacterial harmful algal blooms (CHABs; Qian et al, 2010, 2012; Dziallas and Grossart, 2011; Paerl and Otten, 2013). Leunert et al (2014) showed that a non-toxic strain of M.aeruginosa exhibited a physiological response to H2O2 concentrations as low as 50 nM, within the range of concentrations reported in surface waters (Cooper et al, 1994; Burns et al, 2012), while the toxic strain tolerated 10 times more H2O2. Others have shown a more variable response of M. aeruginosa to H2O2 by strain and by H2O2 concentration, with responses varying between toxic and non-toxic strains of M. aeruginosa and by H2O2 concentration (Dziallas and Grossart, 2011)
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