Sources and seasonal dynamics of nitrogen within the Tillamook Estuary, OR and its tributaries

Excess nitrogen loads into estuaries can lead to eutrophication and can have repercussions on water quality and ecosystem services. While strategies exist to reduce contributions of nitrogen from different human activities, identification of specific sources of nitrogen is crucial for developing effective remediation plans. Tillamook Estuary, OR receives nitrogen from a variety of sources including wastewater treatment facilities, septic systems and agricultural activities as well as the coastal ocean. To understand and address the dynamics of nitrogen entering the Tillamook Estuary, we sought to identify the dominant sources of nitrogen using isotopes of nitrate (δ15N & δ18O), isotopes of water (δD & δ18O), major ions, and other water quality metrics. Between July 2017 and September 2018, we collected water samples from five major tributaries above and below potential nitrogen sources and from nine sites spread throughout the estuary. Results suggest that seasonal hydrological changes are a significant driver of nitrogen within Tillamook Estuary. Nitrate concentrations and δ15N of the estuarine sites reflect the marine endmember during the dry season and reflect the signatures of the tributaries during the wet season as discharge increases. Differences during the dry season in the range and magnitude of nitrate isotopes between tributary sites above (e.g. Wilson River δ15N = -0.3 to 0.2 ‰, δ18O = -0.8 to 0.3 ‰), and below (e.g. Wilson River, δ15N = 3.4 to 6.3 ‰, δ18O = 0 to 2.15 ‰) potential nitrogen sources suggest contributions of nitrate from manure and human sources. Nitrate isotopes of nitrogen and oxygen provide insight toward identifying natural and anthropogenic sources along with biogeochemical processes that result in isotopic fractionation. This work demonstrates that multiple isotopes along with other geochemical indicators can provide additional information about source tracing within complex estuarine systems.

Impact/Purpose

Nitrate isotopes (δ15N & δ18O) are widely used to trace sources and biogeochemical cycling of nitrogen within watersheds. However, only few studies have successfully used nitrate isotopes to identify nitrogen sources in estuaries due to their complex and dynamic hydrological, chemical, and biological internal processes. Here we pair nitrate isotopes with water isotopes (δD and δ18O), major ions, and other water quality metrics to systematically characterize the spatial and temporal signatures of water collected within Tillamook Estuary, OR and its watershed. These signatures provide insight into sources and processes that contribute excess nitrogen into the Tillamook Estuary and will help to inform watershed-scale management strategies that aim to reduce nitrogen inputs into the estuary.

Citation

Rutila, E., C. Brown, J. Kaldy, S. Pacella, T Chris Mochon Collura, AND W. Rugh. Sources and seasonal dynamics of nitrogen within the Tillamook Estuary, OR and its tributaries. Coastal and Estuarine Research Federation, AL, Mobile, November 03 - 07, 2019.