Flow-related dynamics in suspended algal biomass and its contribution to suspended particulate matter in an agricultural river network of the Minnesota River Basin, USA

Abstract

Factors controlling phytoplankton dynamics in lotic systems remain poorly understood relative to those in standing waters, especially in smaller and mid-size streams. Here, we evaluate relationships between stream flow, suspended algal biomass, and particulate organic carbon over multiple years for a mid-size river network draining an intensively managed agricultural landscape in the Midwestern USA. As expected, we found that mid-size reaches (4th–6th order) yielded higher chlorophyll concentrations than smaller reaches (1st–3rd order); however, all reach types exhibited chlorophyll concentrations that could be considered eutrophic. Suspended algae accounted for approximately 20% of total suspended carbon in the river network, on average. Over time, the highest levels of suspended algal biomass across all sites were associated with intermediate–high flow conditions (above median discharge but below ~25% exceedance probabilities). Lakes and wetlands were also sources of suspended algal biomass to the stream network, although substantial phytoplankton production appeared to occur in-channel apart from lentic inputs. Our findings highlight the importance of flow as a regulator of suspended algal biomass, and suggest that moderate flow events act to mobilize algae from benthic habitats or other refugia.