FACTORS CONTROLLING PHYTOPLANKTON ICE-EDGE BLOOMS IN THE MARGINAL ICE-ZONE OF THE NORTHWESTERN WEDDELL SEA DURING SEA-ICE RETREAT 1988 - FIELD OBSERVATIONS AND MATHEMATICAL-MODELING

The factors controlling phytoplankton bloom development in the margina l ice zone of the northwestern Weddell Sea were investigated during th e EPOS (Leg 2) expedition (1988). Measurements were made of physical a nd chemical processes and biological activities associated with the pr ocess of ice-melting and their controlling variables particularly ligh t limitation mediated by vertical stability and ice-cover, trace metal deficiency and grazing pressure. The combined observations and proces s studies show that the initiation of the phytoplankton bloom, dominat ed by nanoplanktonic species, was determined by the physical processes operating in the marginal ice zone at the time of ice melting. The ad ditional effects of grazing pressure by protozoa and deep mixing appea red responsible for a rather moderate phytoplankton biomass (4 mg Chl a m - 3) with a relatively narrow geographical extent (100 -1 50 km). The role of trace constituents, in particular iron, was minor. The imp ortance of each factor during the seasonal development of the ice-edge phytoplankton bloom was studied through modelling of reasonable scena rios of meteorological and biological forcing, making use of a one-dim ensional coupled physical-biological model. The analysis of simulation s clearly shows that wind mixing events - their duration, strength and frequency - determines both the distance from the ice-edge of the sea ice associated phytoplankton bloom and the occurrence in the ice-free area of secondary phytoplankton blooms during the summer period. The magnitude and extent of the ice-edge bloom is determined by the combin ed action of meteorological conditions and grazing pressure. In the ab sence of grazers, a maximum ice-edge bloom of 7.5 mg Chl a m -3 is pre dicted under averaged wind conditions of 8 m s - 1. Extreme constant w ind scenarios (4-14 m s-1) combined with realistic grazing pressure pr edict maximum ice-edge phytoplankton concentrations varying from 11.5 to 2 mg Chl a m-3. Persistent violent wind conditions (greater-than-or -equal-to 14 m s-1) are shown to prevent blooms from developing even d uring the brightest period of the year.