Nitrate variability along the Oregon coast: Estuarine–coastal exchange

Coastal upwelling along the Eastern Pacific provides a major source of nutrients to nearby bays and estuaries during the summer months. To quantify the coastal ocean nitrogen input to Yaquina Bay, Oregon, nitrate concentrations were measured hourly from a moored sensor during summer upwelling in August 2000 outside the jetties to the estuary. Nitrate concentrations associated with coastal upwelling were generally high (up to 34 μmol l−1). The high-temporal resolution of the nitrate data clearly showed variations with a period of 12 h. The nitrate variations were tightly coupled with temperature variations, with warmer water corresponding to lower nitrate values (5–20 μmol l−1). Discretely-collected samples defined the estuarine conditions during the same period. The estuarine samples also varied from 5 to 20 μmol l−1 dissolved nitrate, suggesting that the lower nitrate values were associated with water ebbing from Yaquina estuary. Model calculations, used to estimate the amount of nitrate received by the estuary, indicate that the flux of nitrate into the bay averaged 12,900 kg day−1 during upwelling. The water chemistry at the nitrate sensor was a complex product of tidal forcing, wind-induced currents and biological utilization of nutrients. A discharge model was used to examine the ebbing tide entrainment hypothesis when ocean currents were steady. Where ocean currents change rapidly in a few hours, the plume trajectories, however, will meander horizontally, fractionate or become patchy. The model analysis supports the tidal Yaquina Bay outflow premise as a cause for nitrate and water property variations near the Yaquina Bay entrance jetties. High-temporal resolution nitrate analyses indicate that near shore coastal waters were influenced by nearby estuarine outflow as well as by coastal upwelling.