Measuring nitrate fluxes to assess estuarine eutrophication

Summary form only given.The availability of nitrate sensors has enabled integration of these instruments into real-time profiling buoys and, when coupled with current meters, allows for calculation of nitrate fluxes into and out of estuaries. As the United States' estuaries are increasingly experiencing eutrophication this technological development is timely. We report on the use of nitrate sensors on our profiling buoy system as part of NANOOS, the Northwest Association of Networked Ocean Observing Systems, which is the Pacific Northwest regional association of the Integrated Ocean Observing System (IOOS), and in directed studies on causes for hypoxia in Hood Canal, a sub-basin of Puget Sound. Puget Sound is a large estuarine system in Washington State, USA. Although Puget Sound is about the same size as Chesapeake Bay in terms of spatial area, Puget Sound is a fjord-like network of glacially carved basins that average over 20 times the depth of Chesapeake Bay (7 vs. 150 m). The Puget Sound basins, their sills, and the complex system of rivers combine to create areas that range from highly physically mixed to very stable and persistently stratified. Nutrient dynamics are just as diverse, and vary on temporal scales as well, due to significant tidal exchange and seasonal variation (the Sound is located -48? N). In addition, both a human population of 4 million, which is currently growing at 50,000 people per year, along with strong climate influences make evaluation of this system challenging. In the main basin of Puget Sound, where the large urbanized centers of Seattle and Tacoma are located, the waters have long been known to mix well and therefore not be prone to eutrophication from human sources. However, many areas within the Sound experience strong stratification and nutrient limitation during summer. One such area is Hood Canal, a long, narrow, deep fjord within Puget Sound that has caught regional and national attention. Hood Canal is a glacially carved fjord. It has a sill depth of about 55 m and inside depths of 200 m, which restricts deep water ventilation and renders the inlet naturally hypoxic. Recent evidence shows that the intensity of hypoxia and the frequency offish kills have increased from the 1950s to the present decades. Primary productivity in Hood Canal is nitrogen limited during the summer growing season and it has been suggested that increased anthropogenic nitrogen inputs have resulted in more summer productivity and thus more carbon loading to the bottom waters. Currently, Hood Canal is the site of an intensive scientific study of the causes of the hypoxia (see the Hood Canal Dissolved Oxygen Program (HCDOP) Integrated Assessment and Modeling Study website: http://www.hoodcanal.washington.edu). Through funding from U.S. Navy-HCDOP and NOAA-IOOS, regional coastal ocean observing system assets of NANOOS (http://www.nanoos.org) have been deployed and play an important role in the study. To assess the contribution of human nutrient input to the increased hypoxia, we are utilizing Oceanic Remote Chemical-optical Analyzer (ORCA) buoys, autonomous water quality monitoring buoys, developed by the University of Washington. There are currently four ORCA buoys in Hood Canal, measuring profiles from the surface to the sea-bed (maximum station depth ranges 30 to 150 m) every two hours and transmitting data in near-real-time (within one hour) to the NANOOS and HCDOP websites. Measurements include temperature, salinity, oxygen, chlorophyll, PAR, currents, and nitrate. Strong gradients from surface to bottom are observed in nutrients, oxygen, and chlorophyll. Nitrate increases from undetectable values in surface waters to over 20 ?M at depth. Similarly, oxygen ranges from super-saturated to hypoxic, and a chlorophyll maximum is typically found subsurface at the nitricline. These ranges make estimation of fluxes difficult without a profiling buoy with continuous measurements. Using data from the NANOOS ORCA buoys' nitrate sensors and current meters along with river discharge records and other studies, we have constructed a nitrogen budget for the part of the inlet that is most subject to anthropogenic effects. The budget shows that, although the largest input to the euphotic zone is from oceanic nutrients brought in through natural estuarine circulation, the input of nitrogen from anthropogenic sources constitutes approximately 5-10% of the natural input. If all of this is used to create oxygen demand it is enough to lower the deep water oxygen content by 0.6-1.2 mg/l, which in certain years is enough to produce a fish kill.