EFFECTS OF GRAZING, SEDIMENTATION AND PHYTOPLANKTON CELL-LYSIS ON THESTRUCTURE OF A COASTAL PELAGIC FOOD-WEB

The influence of grazing, sedimentation and phytoplankton cell lysis o n the dynamics of a coastal pelagic ecosystem in the Southern Bight of the North Sea was studied during spring/summer 1992. Diatoms in the > 8 mu m size fraction dominated during early spring, due to size-differ ential control by microzooplankton. This diatom spring bloom became si licate depleted and declined by sedimentation. A Phaeocystis bloom dev eloped in early summer. Phytoplankton cell lysis was the major loss fa ctor for Phaeocystis, accounting for 75% of the decline of the bloom. Bacterial production was positively correlated with phytoplankton cell lysis, and bacterial carbon demand could be supplied by cell lysis. T his illustrates the importance of phytoplankton cell lysis in providin g energy for the microbial loop. A new method (nicotine addition techn ique) was used to estimate mesozooplankton grazing on microzooplankton . Mesozooplankton appeared to prefer microzooplankton as a food source , though there occasionally was substantial grazing on phytoplankton. We conclude that grazing and sedimentation, as well as cell lysis, are structuring mechanisms for algal bloom dynamics.