Deciphering the origin of riverine phytoplankton using in situ chlorophyll sensors

Riverine algal groups with distinct life histories can generate unique patterns of structural and functional behavior. As such, novel methods to discriminate between these groups can improve the understanding of river ecosystem processes. We examined benthic vs. planktonic contributions to suspended algal biomass by monitoring suspended chlorophyll concentration and turbidity during 48 storm events at 2 locations with contrasting hydraulic storage associated with low-head dams. Upstream from the dams, chlorophyll hysteresis showed concentrating effects and counterclockwise rotation, suggesting stormflow concentrated algae from benthic sources. When autotrophic conditions (P/R > 1) preceded storms, chlorophyll hysteresis switched to more proximal benthic sources (faster mobilization). Downstream of the dams, hysteresis showed greater dilution effects and more proximal sources of planktonic algae than at the upstream site, in contrast to high similarly in turbidity hysteresis between sites. Our study supports the analysis of chlorophyll and turbidity hysteresis to infer sources and transport of suspended algal biomass.