Air-sea gas exchange is important not only in the open ocean but also in coastal marine environments, where it plays a major role in the biogeochemical budgets of many chemical species. Despite its pervasive influence, gas exchange remains a poorly quantified and characterized process. Gas exchange mechanisms include the creation of bubbles by surface waves, entrainment and subduction of these bubbles by waves and surface currents, and diffusive gas exchange. In estuaries, tidal fronts are extreme but widespread phenomena that contribute to the gas exchange. Within such fronts, the dissolution of gas bubbles can occur in conjunction with hydraulically driven plunging currents that subduct aerated water to depths >150 m where hydrostatic pressure forces them into solution. Such phenomena, while intrinsically interesting and important for the ventilation of coastal and estuarine waters in some areas, also provides an opportunity to study a system that is "strongly forced" in a unique and characteristic way, thereby giving large concentration signals that can be used to sensitively diagnose the processes involved. Since the suite of noble gases span an order of magnitude range of physical characteristics - in particular solubility and molecular diffusivity -bubble entrainment in such an environment should exhibit distinct responses that will allow quantitative separation of the different forcing mechanisms and permit quantitative inferences to be made about the rates and parameterization. Such knowledge should also eventually permit long-term monitoring of different gas exchange processes using moored instruments.
PIs from the Woods Hole Oceanographic Institution, including a recent Ph.D., will therefore conduct a pilot study of bubble entrainment and subduction of aerated water by sill flows in a semi-enclosed "model" estuary, the Saguenay Fjord, Quebec, Canada. In collaboration with investigators from Canada, they will determine the importance of bubble entrainment in tidal fronts for a coastal environment; assess the role of hydraulically controlled flow for delivering aerated water to intermediate depths; develop the use of dissolved gas measurements for an inverse approach to assess different air-sea gas exchange mechanisms; test new and promising off-the shelf technology in the oceanic environment; develop a plan for comprehensive study of the fjord including all gas exchange mechanisms relevant for the coastal environment, their seasonal cycles, and gas budgets. The planned measurements in May 2006 will comprise hydrography and dissolved gases in the outer basin of the estuary and Gulf of St. Lawrence and intense sampling of the tidal front and sill processes at the entrance of the estuary with echo sounder, ADCP, and towed CTD over several tidal cycles. They will be complemented with direct gas exchange measurements from a smaller vessel using a towed array of void fraction and temperature sensors as well as Acoustic Bubble Spectrometers, and will be interpreted with the help of a high-resolution 3D hydrostatic tidal model.
Some of the broader impacts of the proposed research include an improved parameterization of gas exchange for coastal ecosystems. In addition, new collaborative bridges will be built with Canadian colleagues.
