Preliminary Spatial and Temporal Patterns of Dissolved Organic and Inorganic Carbon Delivery to Tillamook Estuary, Oregon (USA).

Understanding inputs of allochthonous dissolved organic and inorganic carbon (DOC/DIC) are critical to evaluating estuarine trophic status; especially in complex systems with multiple watershed sources. We sampled DOC, DIC and other water quality parameters 10 times in the five rivers (Miami, Kilchis, Wilson, Trask, Tillamook) flowing into Tillamook Estuary, OR between Sept 2017-2018. Concurrently, we also measured DIC, DOC and water quality parameters at 9 sites throughout Tillamook Estuary. Four rivers were sampled at two locations; one above the forest/agriculture land-use transition to measure inputs from the forested watershed. The other downstream from agricultural lands but close to the estuary to capture agriculture inputs. The Miami watershed lacked extensive agriculture and was only sampled at one location. Riverine concentrations of DOC were low compared to other systems. The Miami, Wilson and Kilchis rivers exhibited little or no seasonal pattern in DOC concentrations, while DIC concentrations were lower during the dry season. The Trask and Tillamook rivers had downstream enrichment of DOC throughout the year, while DIC concentrations were lower during the dry season but also enriched downstream. Downstream enrichment appears to be associated with agricultural activities in the watershed. DIC/DOC delivery rates for each river were estimated using river flow rates. Estuarine mixing diagrams did not exhibit clear sink/source patterns within the bay, carbon stable isotope analysis may help to clarify source/sink relationships in rivers and estuary.