LYSIS-INDUCED DECLINE OF A PHAEOCYSTIS SPRING BLOOM AND COUPLING WITH THE MICROBIAL FOODWEB

We studied the development and decline of the 1990 phytoplankton spring bloom in the Marsdiep area of the North Sea (The Netherlands) with emphasis on the cause of the decline of the Phaeocystis bloom, the role of microbial organisms and the utilization of organic material produced by the algae. At the top of the bloom Phaeocystis was nitrogen limited. The bloom declined through cell lysis. Sinking of colonies and grazing were found to be relatively unimportant as loss factors. Biomass in the microbial foodweb (bacteria and protozooplankton) remained low during the bloom but increased sharply as the bloom started to decline indicating that organic carbon released by the phytoplankton was rapidly utilized in the microbial foodweb. Results suggest that dissolved organic carbon produced by phytoplankton through excretion and lysis was the main source of carbon for the microbial foodweb including copepods.