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The Wadden Sea is a tidal flat ecosystem at the land-sea interface of the southern North Sea with a high load of suspended matter and nutrients. Despite the general importance of microbial processes, the composition of bacterial communities in this ecosystem has been little studied. There- fore, we investigated the composition of the bacterial communities freely suspended (FL) in the bulk water, associated with suspended aggregates (AG) and with the oxic sediment surface (SE) over an annual cycle (April 1999 to June 2000) in the East Frisian Wadden Sea, by applying denaturing gra- dient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA gene fragments. Wind strength, sus- pended particulate matter (SPM), particulate organic carbon (POC), chlorophyll, phaeopigments, and the composition of the phytoplankton community were also studied. The dynamics of these parame- ters covaried to various extents, but not systematically, with that of bacterial numbers and community composition. A cluster analysis revealed that the DGGE banding patterns of the 3 communities grouped separately even though band overlaps among the communities occurred. Some identical or highly similar phylotypes were found in the FL- and AG-associated communities, and in the AG- and SE-associated communities, as shown by sequence analysis of excised bands. Prominent members of each community occurred over most of or during the entire study period. Dominant phylotypes in the FL-, AG- and SE-associated communities affiliated with α-Proteobacteria (mainly Roseobacter clade) and γ-Proteobacteria. Phylotypes affiliating with δ-Proteobacteria and Flavobacteria/Sphingobacteria of the Bacteroidetes phylum were detected occasionally in the AG- and SE-associated communities. During certain periods, 1 phylotype affiliating with Actinobacteria and 1 with β-Proteobacteria was detected in the FL-community. The prevalence of the rather stable composition of the communities over time, despite strong hydrodynamic forcing and pronounced seasonal changes in the phyto- plankton community, SPM and POC, indicated that the bacterial communities were adapted to the environmental conditions in this highly dynamic system. The band overlaps between the various communities, however, also reflected that exchange processes between the dissolved and particulate phase and the sediment occurred.

Composition of free-living, aggregate-associated and sediment surface-associated bacterial communities in the German Wadden Sea