Seasonal dynamics of nitrogen in Tillamook Bay

We characterized seasonal trends in dissolved inorganic nitrogen (N) of waters within Tillamook Bay, OR to better understand the major sources and cycling of N within the estuary. While N is an essential nutrient for ecosystem function, excess N loads into estuaries can lead to eutrophication, hypoxia, and/or decreases in biodiversity. Understanding the seasonal dynamics of N into Tillamook Bay is necessary to create focused nutrient remediation plan. Tillamook Bay is located within a developed watershed that is hydrologically influenced by both ocean tidal cycles and by discharge from five major tributaries. To understand and address the dynamics of N entering the Tillamook Bay grab samples of water at various depths were collected at 9 sites across the estuary during 8 sampling events between July 2017 and September 2018. Water samples were analyzed for concentrations of the dissolved inorganic N species nitrate (NO3-), nitrite (NO2-), and ammonium (NH4+). Nitrate was the dominant species of N found throughout the estuary with concentrations ranging from a low of 0.6 µM NO3--N in July 2017 to a high of 67.7 µM NO3--N in February 2018. Results suggest that seasonal hydrological changes can act as a strong driver of N concentrations within Tillamook Bay, with discharge from nearby rivers contributing more to total inorganic N inputs during the rainy season (October through May) and ocean tides contributing more to total inorganic N inputs during the dry season (June through September). While the seasonal changes in concentrations of different inorganic N species alone cannot provide evidence about the specific sources of N to Tillamook Bay, the results of this study provide a useful first assessment. Future studies will benefit from coupling this seasonal N data with other chemical measurements (e.g. stable isotopes of N and O), providing a more robust method for tracking the dominant source(s) of inorganic N inputs to Tillamook Bay.

Impact/Purpose

Nitrogen loading into estuaries can result in water quality problems such as harmful algal blooms and decreased oxygen in the water, which can then negatively impact aquatic organisms. Because estuaries are hydrologically, biologically, and chemically complex, identifying the major sources of nitrogen into them can be difficult. This project examines seasonal changes of nitrogen in water throughout Tillamook Bay over the course of a year to better understand the major seasonal patterns of nitrogen within the estuary. The data suggests that hydrological controls from river discharge in the winter and ocean upwelling in the summer are important drivers within Tillamook Bay and will help to inform analyses in future work that seeks to identify sources of nitrogen into the estuary.

Citation

Rutila, E., C. Brown, S. Pacella, J. Kaldy, AND T Chris Mochon Collura. Seasonal dynamics of nitrogen in Tillamook Bay. Tillamook Estuary Science Symposium, Tillamook, Oregon, September 25, 2019.