Modeled contribution of shallow stream-derived microcystins to warm-monomictic reservoirs under fluctuating inflows

Abstract Harmful algal blooms are increasingly recognized as phenomena occurring across hydrologically connected waterways. Tributary inputs to receiving waterbodies are known to influence blooms in lakes by facilitating nutrient loading and/or cyanobacterial cells, but the contribution of stream-derived toxins to receiving waterbodies remains poorly understood. Here, we quantified the occurrence and potential contribution of stream cyanotoxins to receiving reservoirs in the southcentral US, a region characterized by warm-monomictic reservoirs and flashy inflows. Fifty percent of sampled tributaries had detectable levels of microcystins, with concentrations generally within the same order of magnitude as in-lake concentrations. To determine how much of the in-lake microcystins may be stream derived, we developed spatially explicit models incorporating toxin loading, advection, Eddy diffusion, and abiotic and biotic toxin decay using inflows generated from HAWQS. Model simulations suggest that shallow streams typically contribute < 5%, but up to 9.2%, of in-lake microcystins under prolonged loading scenarios, with upstream portions of reservoirs most affected. As even small concentrations of cyanotoxins may impact the relative activity of nitrogen cycling processes, stream-derived microcystins may aid in stabilizing blooms by disrupting denitrification.