IMPACT OF PHYTOPLANKTON BLOOM MAGNITUDE ON A PELAGIC MICROBIAL FOOD-WEB

The impact of phytoplankton bloom magnitude on the microbial food web of the Dutch pelagic coastal zone (Southern Eight of the North Sea) wa s studied during spring 1993. Data were compared to a study carried ou t in 1992 at the same location. Light availability and temperature wer e 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(-1). Due to sinking of the diatoms no effect of the difference in bloom ma gnitude between the 2 years on the pelagic microbial food web was dete cted. A high onset level of nitrate (55 mu M) resulted in a dense bloo m of the colonial Phaeocystis, with maximum cell numbers of 1.8 x 10(8 ) cells l(-1). During the development of the bloom, the pH of the seaw ater increased from 7.9 to 8.7. The bloom of Phaeocystis declined thro ugh cell lysis (up to 0.3 d(-1)). Due to the enhanced biomass of Phaeo cystis the amounts of released algal cellular carbon were high (up to 400 pg C l(-1) d(-1)). Differences in Phaeocystis bloom magnitude were , as a consequence of algal cell lysis, reflected in bacterial product ion (up to 90 mu g C l(-1) d(-1) in 1993) and grazing activity of memb ers of the microbial food web. The elevated importance of the microbia l food web resulted in a high standing stock of microzooplankton (850 pg C l(-1)) and accumulation of regenerated inorganic nutrients. Calcu lations on both carbon and nitrogen budgets indicate that most of the Phaeocystis bloom was mineralized within the water column.