Twin Cities Seminar
Flow-related Dynamics in Suspended Algal Biomass and Its Contribution to Suspended Carbon and Nutrients in an Agricultural River Network of the Minnesota River Basin
Eutrophication, high rates of biological oxygen demand, and proliferation of toxic cyanobacteria are widespread issues in agricultural watersheds, and have recently received increased attention due to the contamination of downstream municipal water supplies. Understanding the ways in which hydrology intersects with the production and transport of aquatic carbon—particularly algal biomass—is important for the prioritization of management actions to mediate these water quality concerns. However, factors controlling phytoplankton dynamics in streams and rivers are still poorly understood relative to those in standing waters. Here, we evaluate relationships between stream flow, land cover, suspended algal biomass, particulate organic carbon and nutrients over multiple years for a highly agricultural watershed in southern Minnesota (the Le Sueur River Basin). Over time, the highest levels of suspended algal biomass across the river network were associated with intermediate-high flow conditions, rather than low-flow conditions, suggesting that many flow events may act to mobilize algae from benthic or other locally productive areas. Lakes and wetlands were also identified as sources of suspended algal biomass throughout the stream network. Overall, suspended algae accounted for approximately 20% of total suspended carbon in the river network, on average, but could its contribution could be considerably higher or lower depending on individual site characteristics and flow conditions. Nutrient-chlorophyll relationships suggested that suspended algae was largely phosphorus-limited. However, on two sampling dates (in early and late summer of 2015, respectively), suspended algal biomass was also positively related to total nitrogen, suggesting that the growth of suspended algal biomass in agricultural streams may also sometimes be nitrogen-limited. Finally, we estimated that, on average across all sites and dates, suspended algal biomass accounted for approximately 30% and 10% of PP and TP concentrations, respectively, indicating that suspended algal biomass plays an important role in the assimilation and transport of phosphorus in agricultural river networks.