Simultaneous mooring-based measurements of seawater CO, and 0, of’f Cape Hatteras, North Carolina

We deployed CO, and 0, sensors on the U.S. continental shelf off Cape Hatteras, North Carolina, during late summer 1994. A continuous 32-d gas record was obtained at 20 m in 25 m of water, below the thermocline for most of the period. Analysis of the correlation between CO, and 0, indicates that biological and advective processes dominated the gas variability, with small or insignificant fluxes due to air-sea exchange, vertical eddy diffusion, and carbonate dissolution or formation. The observed 0, : CO, correlation was 1.39, within the range predicted for the photosynthetic quotient. Photosynthesis and respiration appeared to be tightly coupled, resulting in no net community production in these waters during the late summer. It is evident from these results that the combination of mooring-based CO, and 0, measurements will be a powerful tool for studying the marine carbon cycle. The ability to predict future levels of atmospheric CO, under different energy use and climate change scenarios is dependent on our understanding of the processes that control the partial pressure of CO, (pC0,) in surface ocean waters. Ocean pC0, has large temporal and spatial variability, which has been difficult to characterize from ships (e.g. Watson et al. 1991; Bates et al. 1996). Physical and biological oceanographers often rely upon moored instrumentation for studying complex ocean processes (e.g. Dickey et al. 1993). Unfortunately, most parameters of biogeochemical interest are not readily measured on moorings. However, recent technological breakthroughs have made it possible to measure seawater pC0, on moorings (DeGrandpre 1993; DeGrandpre et al. 1995a; Friederich et al. 1995). In this study we have used our Submersible Autonomous Moored Instruments for seawater CO, (DeGrandpre 1993; DeGrandpre et al. 1995a), also known as SAMI-CO,, to measure ocean pC0,. We have combined the SAMI-CO+ with commercially available 0, electrodes to record the first simultaneous time series of CO, and 0, on an ocean mooring. The instruments were deployed on the U.S. Middle Atlantic Bight (MAB) -16 km off Cape Hatteras, North Carolina. Shelf water is thought to flow off the Cape Hatteras shelf into the North Atlantic (Biscaye et al. 1994), possibly exporting significant amounts of carbon (in various forms) to the interior ocean (Blair et al. 1995). I Corresponding author, now at: Department of Chemistry, University of Montana, Missoula, Montana 59812. Acknowledgments We gratefully acknowledge Steve Smith for technical assistance and Jeff Kinder and Richard Kohrman for deploying and retrieving the mooring. Mooring gear was generously provided by Len Pietrefesa. Helpful discussions with Steve Lentz and Jim Churchill arc also acknowledged. This research was supported. by the DOE Ocean Margins Program (grant DE FG02-92ER61437). Therefore, the focus of this study was to determine CO, sources and sinks within MAB waters prior to movement of the water off the shelf. An important additional objective was to provide a rigorous field test of the moored instrumentation. The field study consisted of a 46-d deployment in 25 m of water beginning in late July 1994. SAMI-CO+ were moored at 5 and 20 m and each was accompanied by two 0, sensors. The sensor depths were chosen so that the sensors would reside within the surface and bottom mixed layers. We obtained 3-d and 32-d time series from the 5-m and 20-m SAMI-CO+, respectively. Because the time series from 5-m depth was abbreviated owing to damage of the reagent bags by wave action, only the 32-d time-series from the 20-m SAMI-CO, and 0, sensors is presented here. Analysis of the 0, : CO, correlation at 20 m indicates that biological and advective processes dominated the gas variability, with small or insignificant fluxes due to air-sea exchange, vertical eddy diffusion, and carbonate dissolution or formation. The observed 0, : CO, ratio was I .39, within the range predicted for the photosynthetic quotient (PQ). Photosynthesis and respiration seemed to be tightly coupled, resulting in no net community production in these waters during the late summer.