Interdisciplinary research into the physical oceanographic, microbiological, and geochemical processes maintaining the supersaturation of methane in the surface ocean was carried out off southern and central California (1.9 x 10^5 km^2 area between 30 deg and 35.5 deg N). Data were obtained in the laboratory and at sea during twelve cruises over a period of 37 months. The supersaturation of methane in southern California waters (maximum <10^3 nM) and its variability across space and time were consistent with the proximity of hydrocarbon seeps in regions of physical mixing and stirring. In situ microbiological production of CH4 associated with net plankton was also observed and led to the first description of the enrichment, identification, and characterization of a marine methanogenic bacterium (kingdom Euryarchaeota: serologically typed to Methanococcoides methylutens) from the surface waters of the ocean. In vitro rates of methanogen doubling and CH4 production suggested that in situ methanogenesis may be a significant source of water column methane. The inherent uncertainties associated with extrapolating in vitro studies to natural waters, however, restricts their importance. A descriptive analysis of the observed variations in the concentration of CH4 over the temporal and spatial scales of this investigation, suggested that processes of a biological or physical nature were operating along localized isopycnals. A model of the distribution of CH4 in the upper water column showed that horizontal advection of seep-derived methane from the coast and the subsequent eddy diffusion across select isopycnal surfaces provided a satisfactory fit to the data. The dominant influence of physical oceanographic processes was further evidenced from analysis of three dimensional methane and hydrographic data that showed relatively high inventories of CH4 off Point Conception (34.5 deg N) were correlated with intensified upwelling phenomena, and anomalous patches of elevated CH4 were associated with offshore eddies of the prevailing California Current.

The flux of CH4 to the atmosphere was characterized on several spatial and temporal scales. Estimates of the dominant methane sink provided an important, although uncertain, term for exploring subsequent mass-balance considerations of the biogeochemical system. At the vertical scale of the ocean s mixed layer, a steady-state consideration of >100 vertical methane profiles together with corresponding sea-air flux estimates suggested that the concentration of CH4 in the mixed layer may be modeled simply as the balance between two physical processes: (1) the vertical eddy diffusive flux from the methanocline (located at the bottom of the mixed layer) and (2) the diffusive flux across the sea surface. At the horizontal scale of the Southern California Bight, estimates of the mass of CH4 vented to the atmosphere and estimates of the potential mass of methane transferred by the long-term mean Ekman transport of water away from the southern California coast, were concordant. At the planetary scale, extrapolation of the 1989-90 annual sea-air flux estimate for southern California (37 mg CH4 m^-2 yr^-1) to the global area of coastal waters, suggests that coastal waters may account for 25% of current estimates of the total oceanic input of CH4 to the atmosphere.