The Use of Stable Nitrogen Isotopes in Macroalgae to Evaluate Watershed Level Anthropogenic Nitrogen Inputs to Hood Canal, Washington

Nitrogen is a limiting nutrient in marine waters and anthropogenic additions of nitrogen may lead to eutrophication and hypoxia. Hood Canal has experienced eutrophication and fish kills due to hypoxia since the 1920s.  Perpetual enrichment can lead to changes in the dominant species of lower trophic levels, which may have bottom up effects on the entire ecosystem. Anthropogenic sources of nitrogen include farm wastes, inorganic fertilizers, and human sewage. These sources of nitrogen have different isotopic signatures (δ15N), which can be used to trace anthropogenic inputs of nitrogen. This study evaluated stable nitrogen isotopic compositions of multiple marine algae species.  This study used Washington State’s Watershed Characterization analysis to look at connections between land use metrics and isotopic composition of algae. This research suggests that although the largest source of nitrogen to Hood Canal is the Pacific Ocean, focusing on managing development and providing forest cover would reduce anthropogenic nitrogen inputs.  Improvement to wasterwater treatment would also improve water quality.  Reduction in anthropogenic nitrogen inputs may be enough to reduce fish kills in the area.  Future work should focus on temporal tracking of nitrogen and evaluating changes in isotopic signatures to determine the influence of anthropogenic nitrogen during eutrophication events.