Collaborative Research: Estuarine metabolism and gas exchange determined from dissolved oxygen time series: method development, field evaluation, and application to historical data

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Estuaries play a key role in the global cycling of elements because they are hot spots of biological activity and chemical transformations that lie at the interface between land, ocean, and atmosphere. As materials are transported from land to ocean, estuaries profoundly transform or filter these materials through various processes, chief among which are photosynthesis and respiration, collectively referred to as metabolism, and the exchange of gases (such as carbon dioxide and oxygen) between the estuary and the atmosphere. However, these processes are poorly constrained due to lack of sufficient data, high variability in space and time, inconsistent methodology, and lack of a unified model. This research will advance understanding of these processes through a novel combination of field campaigns, model development, and historical data analysis. The data currently available to quantify these processes, which are continuous measurements of dissolved oxygen, are complicated by tidal currents. Hence a main objective of the research is to evaluate and improve methods that remove the influence of tidal currents, which is called advection. This objective will be achieved by direct measurements of advection in two contrasting estuaries in contrasting seasons. Products of this research will be made available to high school teachers, college students, and research professionals. Easy-to-use software based on the open-source R programming language will be developed to analyze the dissolved oxygen data. A Research Experience for Teachers will engage a high school teacher in estuarine metabolism and gas exchange research. Curricular materials for high school and college employing the software will be developed with education specialists. The software will be integrated into the current software suite that is used to analyze dissolved oxygen data from a national data base. A postdoctoral researcher will be involved in all aspects of the broader impacts and gain experience in pedagogy and outreach. The first two objectives of the proposed research are to evaluate and improve advection-removal techniques and gas transfer parameterizations in four one-month field campaigns in contrasting estuaries (Hudson River and Apalachicola Bay) and seasons (spring and late summer). A control volume approach will be adopted, providing a rare opportunity to completely constrain the dissolved oxygen budget at an estuarine location. Achieving these two objectives will allow the following hypotheses to be tested: (H1) advection-removal errors increase as the correlation between sun angle and tidal height increases and (H2) turbidity and fetch measurably influence the gas transfer velocity. The third objective is to develop a new dissolved-oxygen data assimilation method, Estuarine BAyesian Single-station Estimation (EBASE), for simultaneously determining gross primary production, ecosystem respiration, net ecosystem production, and gas exchange. EBASE will combine the advection-removal technique (and its determined errors estimated in the first objective) with Bayesian metabolism techniques recently developed in limnology. EBASE will also retrieve model parameters, such as the initial slope of the photosynthesis-irradiance curve, the temperature dependence of respiration, and the fetch dependence of the gas transfer velocity. The fourth objective is to apply EBASE to the field campaign data and to 16 long-running (at least 14 years), high-quality dissolved oxygen time series within the U.S. National Estuarine Research Reserve System (NERRS), which also contains critical continuous measurements of temperature, salinity, turbidity, wind speed, wind direction, and surface irradiance as well as monthly measurements of chlorophyll and nutrients. EBASE will be applied to one-month segments of the NERRS data, facilitating an analysis of the drivers of seasonal, interannual, and cross-system variability in estuarine metabolism and gas exchange. This analysis will allow further testing of H2 as well as testing of metabolism-related hypotheses: (H3a) the initial slope of the photosynthesis-irradiance curve increases with nutrients and chlorophyll and decreases with turbidity and (H3b) the slope of the respiration-temperature relationship increases with salinity (a proxy of dissolved organic matter) and chlorophyll (a proxy of bacterial abundance).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.