Network-Scale Patterns and Drivers of Microcystins in a Heavily Impacted Watershed

Abstract

The most abundant and problematic of the cyanobacterial toxins, microcystins (MCs), degrade water quality, are lethal to livestock at high concentrations, and target the liver in humans. Most research has focused on lakes and reservoirs, while less is known about MCs in rivers, and few studieshave integrated toxin threats across entire drainage networks. Here, we quantified landscape patterns of MCs and phytoplankton abundance in one of Canada’s most heavily impacted watersheds. This catchment exhibits intensive agriculture and livestock production, with runoff controlled through a hydrologically modified network of regulated rivers, storage reservoirs, and a natural wetland used for slaughterhouse effluent treatment. While the headwater wetland complex exhibited greatly elevated concentrations of MCs (> 300 lg L-1 ), there was little evidence of MCs export to the river drainage network in years with either good or poor hydrological connectivity. Generalized linear models showed that wetland MC concentrations scaled positively with phytoplankton abundance, nitrogen concentrations, salinity, and water temperature. In contrast, MC concentrations in rivers correlated with sampling month and only weakly with salinity, with no apparent connection to river flows. Correlation of MCs with cyanobacteria-specific pigment concentrations was weaker than that for total phytoplankton, suggesting knowledge of cyanobacterial community biomass added little to the prediction of toxin patterns. A continental meta-analysis showed that MC concentrations in the effluent-receiving wetland were high but not anomalous for lentic ecosystems, whereas rivers were greatly understudied (< 1% of total observations). Our findings underscore the distinct environmental controls on MCs in lentic versus lotic ecosystems and emphasize the need for habitat-specific management strategies.