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The role played by bacteria in the decomposition of organic matter in the Elbe Estuary

At four tidal sites samples were taken over 21 to 36 hours along a transect from Hamburg to the outer boundary of the Elbe Estuary to study the share of bacteria in the decomposition of organic matter. The total decomposition rate was estimated by means of planktonic community respiration, assuming a respiration quotient of 0.85. The bacterial decomposition performance was estimated from the bacterial biomass production and bacterial growth efficiency. The samples that were taken in a high temporal resolution mirrored the wide variations induced by tidal currents. The mean values of the total decomposition rate in the upper limnic part of the estuary amounted to 10.2 mu g C l(-1) h(-1), with bacteria accounting for 82. Further downstream, the total decomposition decreased to about 2.7 mu g C l(-1) h(-1) with bacteria contributing only about 50 . The transect from Hamburg to the Island of Neuwerk, sampled with high spatial resolution, followed essentially the same regional pattern. The high rate of bacterial decomposition in the inner part of the estuary is due to the fact that the phytoplankton population, washed in from the shallower upstream part of the Elbe River, suffers from severe light limitation. The cause is the deeper water in the Hamburg harbour area combined with strong tidal mixing of the water column. Thus, the respiration of the phytoplankton is reduced and the total decomposition remains mostly restricted to the activity of bacteria and zooplankton, of which bacteria play the more prominent role in the degradation of organic matter in the Elbe Estuary.