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We used fluorescence in situ hybridization to examine the spatial and temporal variation in the abundance of major bacterial groups in the Delaware Estuary. The abundance of alpha- and beta-proteobacteria and Actinobacteria varied systematically in the estuary and mirrored the pattern seen in lakes and oceans. Beta-proteobacteria and Actinobacteria were abundant in the Delaware River but were less so in the marine waters of the Delaware Bay. In contrast, alpha-proteobacteria, including the SAR11 clade, were most abundant in the Bay and rare in the Delaware River. Actinobacteria were active in assimilating thymidine and leucine and appeared to contribute substantially to bacterial production in the Delaware River. Among the several biogeochemical parameters we examined, only salinity accounted for a substantial portion of the variation in abundance of these bacterial groups. However, relative abundance of these groups often varied independently of salinity. Cytophaga-like bacteria were often abundant throughout the estuary, but they did not vary systematically over the estuarine gradient, unlike the other dominant bacterial groups, We hypothesize that this estuary-wide high abundance occurs because Cytophaga-like bacteria are very diverse, more so than other groups. Data on 16S rRNA sequences are consistent with this hypothesis. The consistent biogeographic patterns suggest that some bacterial groups, even at a broad phylogenetic level, operate as ecologically meaningful units for examining some processes, whereas the Cytophaga-like bacteria as now defined might be too diverse to be useful for ecological studies. [ABSTRACT FROM AUTHOR]

The contribution of major bacterial groups to the assimilation of extracellular polymeric substances (EPS) and glucose in the Delaware Estuary was assessed using microautoradiography and fluorescence in situ hybridization. Bacterial groups contributed to EPS and glucose assimilation in part according to their distribution in the estuary. Abundance of the phylogenetic groups explained 35% and 55% of the variation in EPS and glucose assimilation, respectively. Actinobacteria contributed 70% to glucose assimilation in freshwater, while Alphaproteobacteria assimilated 60% of this compound in saline water. In contrast, various bacterial groups dominated the assimilation of EPS. Actinobacteria and Betaproteobacteria contributed the most in the freshwater section, whereas Cytophaga-like bacteria and Alpha- and Gammaproteobacteria participated in EPS assimilation in the lower part of the estuary. In addition, we examined the fraction of bacteria in each group that assimilated glucose or EPS. Overall, the fraction of bacteria in all groups that assimilated glucose was higher than the fraction that assimilated EPS (15 to 30% versus 5 to 20%, respectively). We found no correlation between the relative abundance of a group in the estuary and the fraction of bacteria actively assimilating glucose or EPS; the more active groups were often less abundant. Our results imply that the bacterial community in the Delaware Estuary is not controlled solely by "bottom-up" factors such as dissolved organic matter.