Physical Control of Atmospheric Carbon Dioxide Flux in Estuaries

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The overarching goal of this project is to develop a comprehensive understanding of how physical and biogeochemical processes interact in estuaries to modulate atmospheric carbon dioxide (CO2) exchange. Measurements of the partial pressure of CO2 (pCO2) and dissolved oxygen (DO) in the Hudson River estuary from a moored array and from ship-based surveys, will be used to resolve variability in time and in the along- and across-estuary directions. These measurements will include both the surface and sub-surface distribution of dissolved gases, and their distribution will be related to variations in vertical density stratification and estuarine circulation. Direct covariance atmospheric CO2 flux and water column turbulence measurements will be made from a fixed platform that spans the air-sea interface at a location where near surface turbulence is likely impacted by wind, waves, and tides, and is significantly modified by variations in vertical density stratification. These data will provide a quantitative model for the gas transfer velocity, which will be used to estimate atmospheric fluxes from the spatially resolved measurements of surface pCO2. It is hypothesized that the high outgassing of CO2 commonly inferred in the upper regions of many estuaries is strongly controlled by the underlying estuarine circulation. The measurements will address two long-standing research needs that contribute to the large uncertainties in estuarine CO2 emissions: 1) spatial and temporal heterogeneity in surface pCO2 values, and 2) poorly constrained gas transfer velocities. The research addresses these two fundamental uncertainties, both of which are strongly modulated by physical processes, and a new conceptual model for gas exchange that is hypothesized to be applicable to a wide range of estuaries will be tested. Moored instrumentation will quantify the importance of temporal variability at a range of time scales, not resolved in most previous studies. Direct covariance atmospheric CO2 flux measurements combined with observations of water column turbulence, waves and vertical density stratification will rigorously quantify the relationship between turbulence in the aqueous surface boundary layer and surface gas exchange. This addresses a fundamental interdisciplinary problem of significant societal importance and will significantly improve estimates of CO2 emissions from estuaries. This project will provide interdisciplinary training for a graduate student, who will be involved in all aspects of the project. Results from this research will be communicated to the public and scientific audiences, and to interested stakeholders through public seminars hosted by organizations that focus on Hudson River scientific and environmental issues and through presentations at national meetings. Several community college students will gain hands on experience building and testing new scientific sensors, which will be deployed in the Hudson River. Data from these sensors will be displayed at the Center for the Urban River at Beczak and used to develop educational materials for visitors to this center on the Hudson River.This project will test the conceptual model that there is an estuarine gas exchange maximum (EGM), whose location is controlled primarily by the underlying estuarine dynamics. Analogous to the estuarine turbidity maximum (ETM), the location of the EGM is hypothesized to be controlled by the convergence in the landward estuarine circulation near the limit of salt and occurs because vertical density stratification prevents the respiratory demand of sub-pycnocline waters in from exchanging with the atmosphere. This dependence on stratification will likely result in large asymmetries in atmospheric flux between spring and neap tides, which may fundamentally control the along-estuary location of the EGM. Significant across-estuary variability also is expected, potentially driven by lateral upwelling or strong lateral gradients in stratification. In the light-limited Hudson River estuary, fortnightly variations in stratification and mixing likely influence phytoplankton dynamics, which also may contribute to the spatial and temporal variations in atmospheric exchange.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.