Seasonal fluctuations in the biomass and metabolic activity of bacterioplankton and phytoplankton in a well-mixed estuary: the Ems-Dollard (Wadden Sea)

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Abstract
The biomass and metabolic activity of bacterioplankton were measured over 1 year in the Ems Dollard Estuary, a part of the Dutch-German Wadden Sea. Very productive phyzoplankton blooms, composed mainly of diatoms and the haptophycean alga Phaeocystis pouchetii, are a feature of the estuarine section studied. The production of bacterial populations, as measured using the [3H]thymidine method broadly followed the phytoplankton density during blooms in spring and late summer. There was no indication of a disproportionate increase in bacterial production during the bloom or decline of particular algal species. The rate at which 14C-labelled glucose, glutamate and leucine were incorporated by bacterial populations, measured as a metabolic potential, varied seasonally, but did not precisely follow the rate of [3H]thymidine incorporation. The utilization of the absorbed 14C substrates for respiration or for cellular synthesis was constant over a prolonged period of bacterial growth; apparent yields of 0.7, 0.4 and 0.8 were measured for glucose, glutamate and leucine, respectively. In the colder season most of the absorbed substrate was respired. The production by pelagic bacteria was 60 gC m−2 y−1, a value that amounted to ∼12% of the pelagic primary production. A preliminary experiment with a mixed culture of the abundantly occurring diatom species Thalassiosira excentrica and a marine spirillum, indicated that part of the algal exudates were rapidly converted by the bacteria, but another part resisted degradation. The incomplete bacterial degradation of algal exudates together with the short residence time of the estuarine phytoplankton may contribute to the apparently incomplete mineralisation of the primary production in the water.

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