No abstract

[1] The influence of wind stress, small-scale waves, and surface films on air-sea gas exchange at low to moderate wind speeds (<10 m s À1 ) is examined. Coincident observations of wind stress, heat transfer velocity, surface wave slope, and surface film enrichments were made in coastal and offshore waters south of Cape Cod, New England, in July 1997 as part of the NSF-CoOP Coastal Air-Sea Chemical Fluxes study. Gas transfer velocities have been extrapolated from aqueous heat transfer velocities derived from infrared imagery and direct covariance and bulk heat flux estimates. Gas transfer velocity is found to follow a quadratic relationship with wind speed, which accounts for $75-77% of the variance but which overpredicts transfer velocity in the presence of surface films. The dependence on wind stress as represented by the friction velocity is also nonlinear, reflecting a wave field-dependent transition between limiting transport regimes. In contrast, the dependence on mean square slope computed for the wave number range of 40-800 rad m À1 is found to be linear and in agreement with results from previous laboratory wind wave studies. The slope spectrum of the small-scale waves and the gas transfer velocity are attenuated in the presence of surface films. Observations over largescale gradients of biological productivity and dissolved organic matter show that the reduction in slope and transfer velocity are more clearly correlated with surface film enrichments than with bulk organic matter concentrations. The mean square slope parameterization explains $89-95% of the observed variance in the data and does not overpredict transfer velocities where films are present. While the specific relationships between gas transfer velocity and wind speed or mean square slope vary slightly with the choice of Schmidt number exponent used to scale the heat transfer velocities to gas transfer velocities, the correlation of heat or gas transfer velocity with mean square slope is consistently better than with wind speed.

We investigated how regional differences in environmental parameters influenced enrichment of amino acids in the sea surface microlayer relative to underlying bulk seawater. Concentrations and compositions of dissolved free (DFAA), dissolved combined (DCAA), and particulate (PAA) amino acids were measured in the sea surface microlayer and corresponding subsurface waters along a transect from coastal Massachusetts to open ocean waters of the Sargasso Sea. We also measured total bacteria concentrations, the percent bacteria with damaged membranes, and concentrations of virus-like particles. Microlayer samples taken by two different techniques-screen sampler (thickness 200-400 m) and rotating drum (thickness 30-60 m)-were compared. On average, concentrations of bacteria and amino acids in both subsurface and microlayer water were higher in water masses with higher fluorescence, lower temperature and salinity, and historically higher chlorophyll. The microlayer was enriched with amino acids and bacteria and, in most instances, virus-like particles. Consistent microlayer features, such as nonuniform preferential enrichment with DFAA and PAA and a uniform difference in dissolved amino acid composition between microlayer and subsurface water, were unrelated to environmental parameters measured. Wind, humidity, and light also had no apparent effects on amino acid concentration, composition, or enrichment in the microlayer in this study.

Observations from a suite of platforms deployed in the coastal ocean are being combined with numerical models and simulations to investigate the processes that couple the atmosphere and ocean.

The effect of surface-active organic nmtter generated by seven common species of rnarine phytoplankton on gas ex•e rates under turbulent conditions at the sir-water interface was determined. Reductions in oxygen evasion rates ranging from 5 to 50% were observed relative to clean seawater controls. Relative oxygen exchange coefficients (expressed as R = Kw[sarnple]/Kw[control]) were shown to be sensitive to small changes in total dissolved c•rbohydrate at concentrations <1 mg C (carbon) L -• and to asymptotically decrease to & lower limit (R '-55-70%) at concentrations between 2 and 6 rag C L -1. A corresponding rel&tionship was observed in which R decreased with increasing relative surfactant •mounts derived from surface pressure-area measurements. However, gas exchange reductions were significant for plankton exudate samples displaying surface pressures •<1 mN rn -1. It thus seems that condensed monol•yer films axe not & prerequisite for reduced gas ex•e and that re•atlvely soluble surfactants derived from phytoplankton can strongly •ect the dissipation of near-surface turbulence and lead to changes in the Schmidt number dependency of K•. Based on detailed analyses of carbohydrate-containing surface-active exu•t• isolated by solid phase extraction from one of the species, Phaeodaci•l•n tricor•turn, it appears tkat small g!ucar•s and heteropolysaccharides associated with proteins and possibly lipids were responsible for the observed reductions in R. mN m -• (milliNewtons per meter) even when R was •50%. Since the bulk of r values measured in visible slicks using spreading oils at coastal and open ocean sites are of the order of 1 mN m -• or greater [Hunter and Liss, 1981], we can surmise that the development of coherent surface films is not a prerequisite for the reduction of interfacial gas exchange. Possibly, soluble surfactants, which are known to be effective in reducing gas exchange through rapid readsorption at the surface [Springer and Pigford, 1970], but which do not display concentration-dependent surface pressures, may have played a major role in retarding gas exchange in our experiments. Phytoplankton exudates and degradation products generally are believed to be major sources of marine surfactants [Zutic et al., 1981]. Typical excretion products of many marine phytoplankton species are complex heteropolysaccharides of high molecular weight that often are acidic due to the presence of varying combinations of uronic acids and carboxyl or sulfate ester groups [Allan et al., 1972; Ramus, 1972; Sinestad et al., 1974, 1975; Percival et al., 1980]. Highly soluble fi-glucan-type polysaccharides often are major components of cellular carbohydrate pools in diatoms [M•kelstad, 1974; M•kelstad et al., 1982] and frequently are found in abundance in natural marine waters during phytoplankton blooms [Sakugawa and Hands, 1985a]. While many of these compounds are quite soluble, they are likely to contain sufficient hydrophobic groups to be at least weakly surface-active [oeeenheer, 1985] and thus influence interfacial...