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Abstract

Estuaries are characterized by a very high abundance of small aggregates enriched with bacteria, protozoa and sometimes metazoa compared to their abundance in the surrounding water. In this study, the microbial community and respiration rates on estuarine aggregates were analyzed by the combination of microscopy (bacteria, protozoa and metazoa enumeration), in situ hybridization and microsensor techniques. Aggregates were isolated and kept in darkness to prevent scavenging or production of new organic carbon. Bacterial abundance and respiration rates were stable during the first 2 d after collection. Bacterial numbers ranged from 0.84 x 10(6) to 23.7 x 10(6) cells aggregate (agg)(-1), Protozoa were dominated by nanoflagellates, which varied between 24 and 1497 cells agg(- 1). Total protozoan biovolume accounted on average for 4 to 13 % of the total estimated microbial biomass on aggregates, and the total grazing rate on attached bacteria was apparently low, Size-specific respiration rates were described by R = 5.86 x d(1.82), where R is measured in ng C agg(-1) h(-1) and the diameter (d) is measured in mm, The average respiration rate was 0.150 mug C agg(-1) d(-1), and the estimated average bacterial respiration rate was 22 fg C cell(-1) d(-1). Total respiration and cell-specific respiration decreased to 0.039 mug C agg(-1) d(-1) and 2.6 fg C cell(-1) d(-1), respectively, after 6 d, The bacterial community was dominated by members of beta-proteobacteria and the Cytophaga-Flavobacterium cluster, which together accounted for 54 % of all 4', 6-diamidino-2- phenylindole (DAPI) stained bacteria at the end of the study. The respiration rate on aggregates larger than 400 mum in the water column of the Elbe Estuary was 11.8 mug C l(-1) h(-1) as determined from the size-specific respiration rates, the aggregate abundance and size distribution. It accounted for 84 to 94 % of the estimated total respiration in the upper water column, The critical ambient O-2 concentration, at which the center of the aggregate interior turns anoxic, was found at similar to 16 muM.