ABSTRACT-The impact of phytoplankton bloom magnitude on the microbial food web of the Dutch pelagic coastal zone (Southern Bight of the North Sea) was studied during spring 1993. Data were compared to a study carried out in 1992 at the same location. Light availability and temperature were the key factors for the timing of the diatom spring bloom. Specific algal cell lysis rates during the diatom bloom were always <0.1 d-' Due to sinking of the diatoms no effect of the difference in bloom magnitude between the 2 years on the pelagic microbial food web was detected. A high onset level of nitrate (55 PM) resulted in a dense bloom of the colonial Phaeocystis, with maximum cell numbers of 1.8 X 1 0 ' cells I-'. During the development of the bloom, the pH of the seawater increased from 7.9 to 8.7. The bloom of Phaeocystis declined through cell lysis (up to 0.3 d-'). Due to the enhanced biomass of Phaeocystls the amounts of released algal cellular carbon were high (up to 400 pg C 1-' d-'). Differences in Phaeocystjs bloom magnitude were, as a consequence of algal cell lysls, reflected in bacterial production (up to 90 pg C 1" d-' in 1993) and grazing activity of members of the m~crobial food web. The elevated importance of the microbial food web resulted in a high standing stock of microzooplankton (850 pg C I-') and accumulation of regenerated inorganic nutrients. Calculations on both carbon and nitrogen budgets indicate that most of the Phaeocystis bloom was mineralized within the water column.

Water samples were collected at a fringing coral reef in overlying water, in bottom water between corals and in crevices under coral colonies, and analyzed for nutrient concentrations, bacterial numbers and production. We found decreasing bacterial densities from overlying water through bottom water into crevices (range 9 to 2 X 105 ml-l). Bacterial specific growth was enhanced in reef crevices (range 0.005 to 0.04 h-'). Although bactenal growth was enhanced, bacterial numbers were reduced, showing a transfer of bacterial biomass into the reef. The differences in bacterial numbers and grnwth between water types depended on water movement and bottom relief Nutrients were enhanced in reef crevices as a result of mineralization. Mineralization of bacterla removed by filterfeeders could contribute 11 and 21 '70 to the increase in N and P, respectively, in coral reef crevices.