Contemporaneous measurements are reported for nitrification, denitrification, and net sedimentwater fluxes of NH,+ and N03-in the mesohaline region of Chesapeake Bay. Seasonal cycles over a 2-yr period were characterized by a midsummer maximum in NH, + efflux to the overlying water and a May peak in NO,-. removal from water by sediments. Coherent temporal patterns for nitrification and denitrification were observed, with relatively high values in spring and fall and virtual elimination of both processes in summer. Indirect measurements indicate that nitrification was limited by the shallow 0, penetration (< 1 mm) here compared to reports for other marine sediments (2-6 mm). In addition, a strong positive correlation between the two processes suggested that denitrification was generally controlled by nitrification. Comparisons of NO,-fluxes and net nitrification rates (nitrification minus N03-reduction to NH,+) revealed that measurements of denitrification with the acetylene block method systematically underestimated actual rates. Rates of N, loss in denitrification were similar to NH,+ recycling fluxes to the overlying water in spring and fall, but in summer negligible denitrification contributed to enhanced NH,+ recycling. These results suggest that inhibition of denitrification in eutrophic estuaries such as Chesapeake Bay may reinforce the effects of nutrient enrichment by allowing increased rates of NH,' recycling.

Seasonal O2 budgets were developed for the mesohaline region of Chesapeake Bay (USA), which experiences bottom water O2 depletion in summer. Rates of O2 production and consumption by the planktonic community and O2 consumption by the benthos were measured at 1 to 4 wk intervals from March to October at 2 stations. Under summer anoxic conditions, rates of sulfide diffusion from sediments were also measured directly with in situ chambers. Weekly observations of water column temperature, salinity and 02, combined with wind data and regression models, allowed calculation of air-sea gas exchange. Using these rates in mass-balance analyses for the upper and lower water column layers, we were able to compute net physical O2 transport across the pycnocline and longitudinally through the bottom layer. Mean monthly estimates of these net physical O2 transports were highly correlated to their respective O2 gradients. Slopes of these correlations provided estimates of the average spring-summer vertical dispersion coefficient (0.2 cm2 S-') and net gravitational water velocity (5 cm S-'), both of which correspond to previous reports. Vertically integrated planktonic respiration rates in the lower water column layer were compared to benthic O2 consumption from April to August. In general, planktonic processes dominated 0 2 consun~ption, comprising almost two-thirds of the total. Oxygen consumption associated with benthic processes, however, exceeded planktonic rates in early spring prior to vernal warming and in late August when large S2-fluxes resulted from release of accumulated pore water pools. By combining our respiration data with values from other coastal environments and plotting rates versus water-column depth, we find a general relation in which planktonic respiration exceeds benthic respiration for systems deeper than 5 m. Hence, for stratifled estuaries with bottom layers thicker than 5 m, seasonal O2 depletion is dnven primarily by planktonic respiration rather than benthic consumption of accumulated organic pools. A comparison of mean monthly rates for bottom respiration (plankton plus benthos) and net physical O2 replenishment here revealed that the 2 processes were highly correlated between March and October; both rates increased through July and declined thereafter. This strong correlation underscores a fundamental interdependence of biological O2 consumption and net physical transport, w h~c h is based on the O2 gradient by which the 2 processes are coupled. Consequently, relatively large reductions In respiratory O2 consumption (e.g. with decreased organic inputs) would lead to substantially smaller decreases in the extent of bottom water O2 depletion because of an inherent adjustment between the coupled biological and physical processes.

In a 28 mo mesocosm experiment, levels and patterns of productivity and respiration were observed for a range of nutrient additions selected to provide a gradation from conditions in lower Narragansett Bay, Rhode Island, USA, to maximum impact for a n urban estuary receiving present day levels of sewage effluents. For a 32-fold increase in nutrients, system apparent production increased by only a factor of 3 5. Seasonal patterns of autotrophy during the winter-spring diatom bloom a n d heterotrophy during summer and early fall occurred at all treatment levels. With the exception of the 8 X treatment, all treatments above 2 X had a greater respiratory demand in the water column than the benthos. The highest treatment mesocosm (32 X ) went briefly anoxic during the second summer of the experiment during a period when little productivity was occurring in the water column.

Rates of plankton community respiration were measured in surface and bottom waters during spring, summer and autumn in the mesohaline region of Chesapeake Bay, USA. Seasonal patterns of plankton respiration generally followed the annual temperature cycle, with peak rates in July and August of 40 to 70 pg 0,l-I h-', which are among the highest values reported in the literature. In addition, strong diel variations in surface water community respiration, with mid-day maxima, were coherent with cycles of irradiance and photosynthesis, suggesting the dependence of community respiration on recently produced organic matter. The range of diel variations in respiration during summer was almost half the annual range in mean rates. Size-fractionated incubations demonstrated the importance of picoplankton (< 3 pm) respiration compared to total rates On average, picoplankton accounted for most of community respiration in surface waters (56%) and dominated total rates in the summer pycnocline (89%). This size fraction was less important in the bottom waters during spring (23 %). Temperature per se as a control on plankton respiration was investigated by comparing rates at in situ temperatures against those measured in temperature-manipulated experiments in spnng and summer. Strong relations between respiration and temperature were observed in all cases; least-squares regresslons (exponential and linear) for experimental data were not significantly different between seasons, nor between the experin~ents and the seasonal rates measured at in s~t u temperatures. These results suggest the absence of significant physiological adaptation and/or selection for temperature optima over the annual cycle. Respiration by bacterioplankton (< 1 p) exhibited no oxygen dependence down to concentrations of 0.16 mg O2 l?'; however, rates for whole water (unfiltered) declined with decreasing oxygen below 0.8 mg O2 l-'. Evidently, diffusion limited oxygen consumption by proto-and metazoans as well as bacteria associated with detrital particles and organic flocs.

Concentrations of organic carbon and rates of dissin~ilative sulfate reduction in surface sediments of marine mesocosms were examined along an experimental eutrophication gradient. Phytoplankton biomass increased due to addition of inorganic nutrients (N. P, Si). This increase was especially pronounced during the winter-spring diatom blooms, which increased in magnitude and duration along the nutrient gradient. Net system production in winter and spring resulted in carbon deposition and accumulation in surface sediments (maximum net accumulation 17 m01 C m-'). Benthic remineralization of cdrbon exceeded depositional supply during summer and fall. Sediment carbon concentrations approached background levels in December and February, suggesting very little annual accun~ulation of sediment carbon Sediment oxygen consumption and suifate reduction rates both increased as a result of carbon sedimentation. Sulfate reduction rates in organic enriched sedunents were an order of magnitude higher than control and were correlated with temperature and carbon concentrations (r2 = 0.85). Anaerobic respiration rates in unenriched sediments were related only to seasonal patterns of temperature (r2 = 0.70). Anaerobic metabolism was the dominant metabolic pathway in control and treated sediments, with 50 to 7 0 % of annual carbon remineralization due to sulfate reduction.