Oxygen production and comsumption rates were determined for the various communities (plankton, sediment, and seagrass-plus-epiphytes) associated with 2 adjacent seagrass communities, one dominated by Zostera marina and the other by Ruppia maritima. Annual estimates of gross production summed for all communities were high compared to other reports (ca 1600 gC m-' and 1100 gC m-* in the 2. marina and the R. maritima dominated communities, respectively). Plankton and sediment communities accounted for ca 45 and 36 % in the 2 areas, respectively. Seasonal patterns of production for the 2 areas differed. In the 2. marina area, plankton and sediment production dominated during summer, whereas in the R. maritima area production of seagrass (plus epiphytes) dominated throughout the study. Temporal trends for oxygen production and consumption of the plankton and sediment components corresponded closely and suggested direct in situ utilization. However, a lag observed between production and consumption in the seagrass-plus-epiphyte component indicated indirect utilization mediated by microbially dominated decomposition and remineralization processes.The ratio of annual gross production to dark respiration (P:R) exceeded 1.0 in both seagrass communities and for all components, suggesting net export and/or burial of carbon in these systems.

Benthic nutrient and oxygen fluxes were measured during summer in the nearshore zone of the Georgia Bight, USA. Ammonium, nitrite, nitrate and phosphate were released by the sediments to the overlying water, while dissolved organic nitrogen was taken up by sediments. Oxygen uptake was equivalent to 1.1 g Cm-* d-'. Based on other data available for this area the benthos accounted for the remineralization of approximately 55 % of net primary production and 25 % of total community (benthic and pelagic) metabolism. To balance a simple carbon budget for the nearshore region during summer an outside source of organic matter, most likely originating in adjacent estuaries, is required.The benthos supplied 16 % of the N and 53 % of the P required by phytoplankton. Pore water nutrient concentrations suggested substantial denitrification (estimated at 4.9 mg Nm-2 d by mass balance) in sediments which may have caused the low N : P benthic release ratios we observed. The results support the hypothesis that benthic processes in shallow marine areas control the availability of N and P for water column production.

This paper presents an in-depth insight into the role of periphyton in aquatic food webs and as a critical metabolic component of aquatic ecosystems. Energy and organic matter fluxes in general, growth regulation and habitat variables, community metabolism and links between periphyton and higher trophic levels are discussed. Recycling is also discussed as the key to nutrient fluxes and maximizing productivity of periphyton. Finally, modulation of periphyton by light availability and by mortality and losses, and the potential effects of global changes in climate and related environmental conditions are also covered.

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Bacterioplankton abundance and production were examined over the course of a destratification event in the lower James River, Virginia, USA. Goals of the study were to determine if destratification would influence temporal patterns of bacterioplankton parameters and relationships between bacterioplankton and other biological and abiological parameters. Mean bacterial abundance grouped over stations did not change over the course of the study, and were characterized by much smaller coefficients of variation than all other planktonic constituents. However, bacterial production measured by 3H-thymidine (3 H-~d r) incorporation decreased significantly from a stratified (324 pg C I-' d-l) to a destratified (187 pg C I-' d-l) hydrography. The importance of bacterial-autotrophic coupling was also suggested from oxygen metabolism experiments, which indicated substrate limitation of bacteria, and the existence of a rapidly utilized photosynthetically produced substrate. Correlative relationships between bacterial parameters with chlorophyll a were significant during stratified hydrography, but diminished or became non-significant during destratified hydrography. Estimates of microzooplankton grazing rates upon bacteria decreased significantly during the onset of destratification. During the stratified hydrography, bacterial parameters displayed highly significant negative correlations to ammonium, however these relationships disappeared during the destratified hydrography. Results of this study indlcate that destratification changes the trophic interactions of bacterla within the microbial loop, however these changes are not necessarily reflected by temporal patterns of bacterial abundance.