Evidence for Seasonal Pulses of Riverine Phytoplankton as a Carbon Subsidy Driving Whole System Metabolism

A fraction of terrestrial carbon (C) inputs are exported from river networks, however global budgets rarely incorporate in-channel processing of C to account for retention as materials are transported to the coast. To evaluate the metabolic response of microbial communities as a function of carbon quantity and quality, linking microbial activity to ecosystem metabolism, we collected measurements in a sixth order, large Coastal Plain river, the Cahaba River, AL. We characterized carbon pools using lipid biomarkers, modeled ecosystem metabolism, and determined water-column microbial community composition by terminal restriction fragment length polymorphism. This work was timed with a seasonal increase in water column phytoplankton biomass. As phytoplankton biomass increased, the percentage of long-chain lipid biomarkers declined and the ratio of unsaturated to saturated biomarkers increased, both indicating higher algal contribution to carbon pools and a possible increase in carbon bioavailability. There was a corresponding shift in the heterotrophic microbial community composition, and increased whole ecosystem primary production and respiration. These results highlight important linkages between riverine production, microbial community composition, and large-scale carbon dynamics, suggesting that the importance of the functioning of large rivers in retaining C has been underestimated. These data will contribute to our current modeling efforts linking in-channel spatial heterogenity to biogeochemical cycling, particularly within the context of increasing climate change.