Characterization of Cyanobacterial Communities in Lakes Requires Consideration of Diurnal and Spatial Variation

Abstract

AbstractContinental-scale increases in aquatic system eutrophication are linked with increased cyanobacteria threats to recreational water use and drinking water resources globally. While some guidance regarding monitoring is available, it is largely reactive and insufficient for proactive risk mitigation and management, which necessarily requires an understanding of the composition and dynamics of cyanobacterial communities in the aquatic system. Their distribution is impacted by several factors, including water column mixing and buoyancy regulation responses to light availability that create oscillatory diurnal migration patterns within the water column, creating challenges in the ability to accurately describe and quantify cyanobacterial densities. These dynamic fluctuations are not typically reflected in monitoring protocols, which frequently focus on surface depths and either ignore sampling time or recommend large midday timeframes (e.g., 10AM-3PM), thereby precluding accurate characterization of cyanobacterial communities. While diurnal vertical migration of cyanobacteria has been reported in marine and eutrophic freshwater systems, reports in oligotrophic freshwater lakes are scant and characterization have focused on individual 24-hour periods neglecting to consider day-to-day variability. These dynamics must be better understood and reflected in water quality monitoring guidance to advance drinking water risk management and source water protection approaches. To evaluate the impact of diurnal migrations and water column stratification on cyanobacterial abundance, communities were characterized using a multi-time point sampling series across a 48-hour period in a shallow well-mixed lake interconnected to a thermally stratified lake in the Turkey Lakes Watershed (Ontario, Canada). Amplicon sequencing of the V4 region in the 16S rRNA gene was performed to characterize microbial community composition. Cyanobacteria were significantly represented in the microbial community in the midday and afternoon sampling times in the thermally stratified lake, but not in the well-mixed lake. Although the lakes in this study are interconnected, the cyanobacterial communities within them exhibited unique composition and distribution trends, thereby underscoring the importance of developing detailed sampling guidance to maximize the utility of cyanobacteria monitoring and better characterize and mitigate risk.Highlights▪ Water column stability impacts diurnal migrations of cyanobacteria▪ Gas vacuolate taxa are more abundant at surface, but also present at depth▪ Rain events can impact cyanobacteria distribution and impact detection▪ Cyanobacteria distribution can vary significantly between interconnected lakes▪ Cyanobacteria monitoring for risk management should incorporate time and depth