Nutrient Loading and Transformations in the Columbia River Estuary Determined by High Resolution In Situ Sensors

We present results from a high temporal resolution observational study of nutrient loading and biogeochemical transformations in the Columbia River estuary, OR. During 2010, three autonomous nutrient sensors (Satlantic SUNA, SubChem Systems Inc. APNA, WET Labs Cycle-PO4), measured nitrate + nitrite, ammonium, ortho-phosphate, silicic acid, and nitrite across different seasons in the lower Columbia River and estuary. Large salinity ranges and conservative behavior of nutrients during daily flood-ebb mixing allowed for river and ocean end members to be calculated from estuary-based measurements. The results revealed the importance of the estuary as a “bioreactor” for inorganic nutrients despite relatively short water residence times. High concentrations of ammonium and phosphorus were identified in the estuary compared to the river during late summer, suggesting that elevated rates of nutrient remineralization were localized in the lower estuary and likely reflect organic matter accumulation and decomposition in the estuarine turbidity maximum and lateral bays. Furthermore, we have identified a synchronicity of large pulses of dissolved silicic acid and ammonium into the estuary during spring tide high turbidity periods, which was likely released from dissolution of benthic diatom-rich sediment within the lateral bays.