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.

ABSTRACT. The aerobic degradation of phytoplankton debris collected in Dutch coastal waters on 2 days in 1991 (15 April and 8 May), representing 2 physiological stages of a phytoplankton spring bloom dominated by Phaeocystis sp., was studied in batch culture experiments. The bacterial production and the concentrations of particulate organic carbon (POC) and dissolved organic carbon (DOC) were monitored over a period of 102 d. Bacterial numbers and biomass were followed for 35 d. All experiments showed a rapid metabolic response of bacteria and a sharp decrease in the concentration of POC and DOC during the first days of the experiments. Thereafter, bacterial production rates remained constant, and POC and DOC decreased slowly. Apparently, the phytoplankton debris consisted of a lablle, rapidly degradable fraction and a refractory, slowly degradable fraction. The labile fraction comprised approximately 50% of the debris, and was degraded with a bacterial carbon conversion efficiency of between 10 and 20%. There were no indications that antibiotic compounds present in the algal debris inhibited the degradation. Acrylate, a proposed antibiotic compound which was present in the algal debris, was rapidly degraded in a control experiment. The percentage of the material that had been degraded after 102 d was highest in the experiment with material collected in May. It was concluded that during the early phase of the bloom, more refractory compounds are produced