Riverine Particulate Organic Matter Exported to the Adjacent Coastal Ocean: Implications for Marine Zooplankton in a Highly Productive Coastal Upwelling Area

Abstract Riverine organic matter strongly influences productivity and biogeochemical cycles in eastern boundary upwelling systems, enhancing microbial activity and carbon and nutrient cycling beyond natural upwelling inputs. To assess the impact of riverine organic matter from the Biobío River on particulate organic carbon (POC) and the carbon biomass of higher trophic levels (zooplankton) in the adjacent coastal upwelling area off Central Chile, we conducted a year‐long study (November 2009 to November 2010). Our study included analysis of land use, hydrographic data, nutrients, pigments, and POC and nitrogen, combined with geochemical tracers (i.e., carbon and nitrogen isotopes, and fatty acids). Results showed that nutrient and particulate organic matter (POM) fluxes, including phytoplankton biomass, peaked during periods of maximum rainfall and river discharge (winter and spring). A clear enrichment in δ 13 C‐POC from river to ocean during the upwelling season indicated that autochthonous production dominated the POM in the coastal zone. On average, terrestrial‐originated material contributed 25% to the marine POC during spring, increasing to 73% in winter, when freshwater discharge was highest. Fatty acid analysis revealed a higher contribution of terrestrial carbon near the river mouth, with zooplankton incorporating 30%–40% of this carbon, mainly during winter. These findings suggest that terrestrial carbon enters the food web via freshwater phytoplankton, terrestrial POC, or microbial loops. Given climate change impacts on hydrology and upwelling, further studies are needed to assess the long‐term effects of riverine carbon on coastal productivity and food webs in eastern boundary upwelling systems.