The study investigates the variations in the activities of bacteria in the Dutch Wadden Sea and the possible factors that may determine these variations. A series of laboratory experiments was performed which tried to assess the applicability of several techniques of measuring bacterial activities in the field. From the results of the experiments, some techniques were used in field samplings on a diel and a six-month period. Total microbial biomass was derived from ATP concentrations. Bacterial standing stock was estimated by acridine orange direct counts. Active bacterial biomass was determined by the nalidixic acid-yeast extract enrichment inhibition technique. Fluctuations in bacterial biomass were observed to follow inversely the tidal cycle, minimum at high water and maximum at low tide. Long-term variations seemed to be closely linked to temperature and substrate levels. Biomass is minimum in winter and peaks at periods of high temperature and nutrient concentrations. In winter, bacterial carbon is the major component of microbial biomass but its importance declines in spring and is lowest when total microbial carbon is highest. The total number of bacteria is dependent on the number of active cells in spring. Spatial variations are observed in bacterial biomass but not in the total microbial biomass and ETS respiratory activity. These variations may be true only in certain parts of the Wadden Sea and are possibly dependent on the hydrographic interactions between the North Sea and the Wadden Sea.
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