Coupled in situ Ammonium and Nitrate analyses of a tidally dominated estuary: New developments from the Elkhorn Slough Land/Ocean Biogeochemical Observatory network

For nearly nine years the Elkhorn Slough Land/Ocean Biogeochemical Observatory (LOBO) network of moorings has been delivering freely available hourly data to the web in near real time. Each mooring hosts a suite of instruments including an ISUS nitrate sensor. In addition to providing valuable information on ecosystem scale processes, the moorings serve as ideal test platforms for novel in situ chemical sensors & analyzers developed by the Monterey Bay Aquarium Research Institute. The recent addition of a newly developed in situ NH₄⁺ analyzer, the DigiScan-II, has provided additional insights into N cycling mechanisms within the slough. The analysis method estimates NH₄⁺ concentration via base conversion to NH₃ gas and diffusion across a membrane into an acid carrier stream with subsequent conductivity detection. Although this new NH₄⁺ analyzer is reagent based, it was developed to be relatively cheap, robust, and configurable for a range of deployment options and requires minimal, infrequent maintenance that is ultimately governed by battery life. The fundamental DigiScan-II platform can also be used for other analyses of interest, such as PO₄ or C_T (total inorganic carbon), by swapping the necessary reagents and components and by making minor code modifications. For deployment in Elkhorn Slough, the NH₄⁺ DigiScan-II was configured for mid-scale concentration detection with a linear calibration range of <0.2 to >30.0 μM NH₄⁺. The flux of different forms of bioavailable DIN through the system is driven by runoff inputs, tidal exchange, and biological processing. Large inputs of NO₃^- are sourced from the agriculturally influenced Old Salinas River (OSR), which enters the Slough near the estuary mouth and confluence with Monterey Bay. Rising ocean tides force this eutrophied water mass up into the slough where it is accessed by various biological communities during the course of the tidal period. Mass balance estimates suggest there is an imbalance between the amount of NO₃^- received by the slough and the amount that is released, with a net NO₃^- influx to the ecosystem. However, the slough ecosystem releases more NH₄⁺ than it receives, resulting in a net efflux of NH₄⁺ due to remineralization of organic matter (and potentially DNRA) in the slough. Although there is high variability among day to day flux estimates, the sign of the net flux term for NO₃^- and NH₄⁺ are constant on a monthly scale and seasonal trends in flux magnitude are apparent. Excess N inputs to the slough ecosystem calculated from the LOBO data can serve to estimate productivity when coupled with published estimates of sediment N flux. Published rates of denitrification in the system are few but generally very low. Using the LOBO nitrogen mass balance calculations and Redfield stoichiometry, we estimate that the nitrogen inputs from OSR and Monterey Bay support a net production of roughly 220 g C m^-2 yr^-1 in Elkhorn Slough, assuming no N is lost to denitrification. This estimate will be revised with detailed hydrological dynamics and available O₂ data to provide more accurate estimates of net ecosystem metabolism and the relative influence of allochthonous inputs on ecosystem productivity.