We study the dynamics of the planktonic ecosystem in the coastal upwelling zone within the California Current System using a three-dimensional, eddy-resolving cir-
The hypothesis that bacteria can cluster around phytoplankton cells in the turbulent mixed layer was tested with a model that simulates bacterial chemotaxis toward a neutrally buoyant phytoplankton cell exuding dissolved organic C. The model results indicate that bacteria can attain population densities orders of magnitude above background levels in microzones occupying ~0.1% of the fluid volume surrounding each phytoplankton cell. The simulation results indicate that at turbulence intensities expected in the upper mixed layer of the ocean (shear rates of -0.15 s ') bacteria initially approach phytoplankton through random swimming and relative fluid motions. Chemotactic response serves to prolong a bacteria's stay near the phytoplankter before it is carried away by random swimming and fluid motions. At these shear rates, up to 20% of the chemotactic bacteria population could be clustered around exuding phytoplankton cells, even though individual bacteria stay in a cluster less than a minute. For these conditions the time-averaged exudate exposure of the bacterial population could be 10 times higher than that of a nonchemotactic population. Exudate exposures in unsteady shearing were found to equal or exceed the corresponding steady shear values. Although unsteady bursts of turbulent mixing in the oceanic surface layer should disperse clusters, intervening calm periods are long enough to allow clusters to reform. The model indicates that bacterial clustering is unlikely to have a significant effect on phytoplankton nutrient uptake or on the fate of bacterial secondary production in the microbial food web.The observation that most marine bacterial isolates are motile (Baumann and Baumann 1978) has lead to speculation regarding the microscale spatial distribution of bacteria and the ecological implications of nonrandom interactions between phytoplankton and motile bacteria. Upon the discovery that some marine bacteria exhibit chemotactic behavior when exposed to algal exudate, Bell and Mitchell (1972) hypothesized that chemotactic bacteria actively congregate around phytoplankton cells to improve their exposure to the organic C exuded by phytoplankton.Chemotactic behavior has been observed repeatedly in marine and freshwater bacteria (e.g. Mesibov et al. 1973;Chet and Mitchell 1976;Dahlquist et al. 1976;Geesey and Morita 1979;Hazen et al. 1984), indicating that some bacteria have the capability of clustering around phytoplankton cells in motionless fluid.Indirect evidence that bacterial clustering can increase the dissolved organic C (DOC) exposure of a chemotactic bacteria population comes from several directions.Azam and Hodson (198 1) observed that glucose uptake ' Corresponding author. Current address: ENSR Consulting and Engineering, 35 Nagog Park, Acton, Massachusetts 0 1720. by marine bacteria remains unsaturated even when glucose concentrations are orders of magnitude above background levels. This capacity was attributed to a multiphasic uptake system that would be of little value to the bacteria un...
Simultaneous velocity, temperature, and conductivity data were taken in the convective flow emanating from a hydrothermal vent field located at 10°56′N, 103°41′W on the East Pacific Rise. The horizontal profiles obtained indicate that the flow field approaches an ideal plume in the temperature and velocity distribution. Such parameters as total heat flow and maximum plume height can be estimated using either the velocity or the temperature information. The results of these independent calculations are in close agreement, yielding a total heat flow from this vent site of 3.7 ± 0.8 MW and a maximum height of 150 ± 10 m. The nonlinear effects of large temperature variations on heat capacity and volume changes slightly alter the calculations applied to obtain these values.
A technique is developed for accounting for the contribution of free convection to the evaporation from a cooling pond. Established formulas for forced (wind‐driven) evaporation are corrected for convective effects and used in a formula for surface energy balance to estimate heat fluxes and surface temperatures of cooling ponds. The resulting expression for total heat loss agrees with observed cooling pond performance better than other formulas presently in use. Surface heat loss coefficients may be derived from the new heat loss formula for use in calculating temperature rises induced by heated discharges.
The extent of water movement across the surface sediment‐water interface of the salt marsh during tidal inundation is of great biochemical importance, because in large part it determines the degree of exchange of nutrients and metabolic products between marsh sediment and estuarine waters. The principles of flow in porous, elastic media lead to an analytical expression for infiltration during tidal inundation of the marsh surface; this analytical model is the basis for inferences of surface seepage using recording piezometers. The magnitudes of infiltration determined by this method in Great Sippewissett Marsh agree well with values obtained by direct active seepage meter techniques and are of the order of a few millimeters per inundation, substantially less than some previous estimates for this marsh. Upper limits on the magnitudes of biogeochemical fluxes of nitrogen, carbon, and sulfur compounds are implied by these data.
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