Abstract
Cyanobacteria in the genus Microcystis are dominant components of many harmful algal blooms worldwide. Their pelagic–benthic life cycle helps them survive periods of adverse conditions and contributes greatly to their ecological success. Many studies on Microcystis overwintering have focused on benthic colonies and suggest that sediment serves as the major inoculum for subsequent summer blooms. However, the contemporaneous overwintering pelagic population may be important as well but is understudied. In this study, we investigated near-surface and near-bottom pelagic population dynamics of both microcystin-producing Microcystis and total Microcystis over six weeks in winter at Dog Lake (South Frontenac, ON, Canada). We quantified relative Microcystis concentrations using real-time PCR. Our results showed that the spatiotemporal distribution of overwintering pelagic Microcystis was depth dependent. The abundance of near-bottom pelagic Microcystis declined with increased depth with no influence of depth on near-surface Microcystis abundance. In the shallow region of the lake (<10 m), most pelagic Microcystis was found near the lake bottom (>90%). However, the proportion of near-surface Microcystis rose sharply to over 60% as the depth increased to approximately 18 m. The depth-dependent distribution pattern was found to be similar in both microcystin-producing Microcystis and total Microcystis. Our results suggest the top of the water column may be a more significant contributor of Microcystis recruitment inoculum than previously thought and merits more attention in early CHAB characterization and remediation.
Highlights
Cyanobacterial harmful algal blooms (CHABs) are an increasingly common consequence of eutrophication in freshwater systems
Gilmour Point has a history of regular annual CHABs, with a large bloom observed in the summer of 2018, which provided an excellent opportunity for sampling
Concentration of the Microcystis 16S rRNA gene ranged from 5.62 × 105 to 4.53 × 108 copies per liter of water sampled near the bottom of the water column, with a mean of
Summary
Cyanobacterial harmful algal blooms (CHABs) are an increasingly common consequence of eutrophication in freshwater systems. Defined as a rapid increase in localized cyanobacterial biomass [1,2], CHABs can have significant negative impacts on aquatic ecosystems, reducing water quality [3], creating asphyxiating hypoxic ‘dead zones’ [4], limit nutrient turnover [5], and producing potent cyanotoxins [6,7,8] Their impact is increasing worldwide, promoted by nutrient loading [9,10], rising atmospheric CO2 concentrations [11] and rising temperatures [12,13,14,15]. Reducing inputs may not be feasible in developing countries due to population pressures and limited budgets
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