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Sechment bacterial production, abundance and P-glucosidase activity were studied in relation to carbohydrate concentrations in diatom-inhabited intertidal mudflats during the tidal cycle in the Ems-Dollard estuary in summer 1996 During the diurnally emerged period significant increases in water-and EDTA-extracted carbohydrates, bacterial production and abundance in the sediment surface layer (0 to 2 mm) were established. Exposure had no effect on algal biomass (chlorophyll a), doubling times of bacteria or P-glucosidase activity in the surface layer. Net changes in water-extracted carbohydrates together with net changes in total and CTC (5-cyano-2,3-ditolyl tetrazolium chloride)-stained bacteria accounted for 85% of the variations in net increase in bacterial production during the emerged period. During the submerged period significant decreases of water-extracted carbohydrate concentrations occurred. The changes in P-glucosidase activity and water-extracted carbohydrates could explain 93% of the net changes in bacterial production during the submerged period. Waterextracted carbohydrates derived mainly from live diatoms. The source of EDTA-extracted carbohydrates was less specific, but related to chlorophyll a and pheopigments. The results suggest that the net increases in water-extracted carbohydrates due to &atom activities are inbrectly coupled to benthic heterotrophic bacterial production. Bacteria do respond to tidal variations in organic compounds, and water-extracted carbohydrates are an important indicator.

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ABSTRACT. This study focuses on the effects of ultraviolet radiation (UVR) on bacterioplankton. The effect of different parts of the sunlight spectrum on the leucine and thymidine incorporation and on the induction of DNA damage in natural bacterial populations in the coastal Caribbean Sea off Curaqao were investigated. DNA photodamage in microorganisms and biodosimeters was quantified by the number of cyclobutane dimers (thymine dimers). Increasing DNA damage during the day was found when incubated in full surface solar radiation. When UVBR was excluded no DNA damage was observed, indicating that thymine dimers were only formed by UVB radiation. The amount of thymine dimers in the > 0 . 8 pm fraction was only one-third of the amount of induced thymine dimers in the <0.8 pm fraction, suggesting that phytoplankton is less sensitive to UV-induced DNA damage than bacterioplankton. Protein and DNA synthesis was inhibited to about 30% of the dark control during the day when exposed to surface solar radiation. In both protein and DNA synthesis a trend was found, with the highest inhibition under full solar radiation, lower inhibition when UVBR was shielded off and the lowest inhibition when UVAR (<375 nm) was also shielded off. The intracellular carbohydrate content of the phytoplankton incubated under full solar radiation was not significantly higher than the dark incubation, while the contents after incubation without UVBR were significantly lngher. The carbohydrate content in the samples incubated without UVBR and UVAR (<375 nm) was a little higher than with only UVBR shielded off. In summary, the results show that in the coastal Caribbean Sea UVBR is responsible for DNA damage in bacterio-and phytoplankton, while protein and DNA synthesis in bacterioplankton was inhibited by UVBR. UVAR and PAR and carbohydrate synthesis in phytoplankton by both UVBR and UVAR.

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