On the factors controlling phytoplankton ice edge blooms in the marginal ice zone of the northwestern Weddell Sea during sea ice retreat 1988

Abstract

The factors controlling phytoplankton bloom development in the marginal ice zone of the northwestern Weddell Sea has been investigated during the EPOS expedition through measurements of physical and chemical processes and biological activities associated with the process of ice melting and their controlling variables. Among these, light limitation mediated by vertical stability and ice cover, trace metal deficiency and grazing pressure received attention. The combined observations and process studies show that bloom initiation was determined by the physical processes operating in the marginal ice zone at the time of ice melting. The additional effects of grazing pressure by protozoa and deep mixing appeared responsible for a rather moderate phytoplankton biomass (4mgChl.a m⁻³) also with relatively narrow geographical extent. The importance of each factor during the seasonal development of the ice edge phytoplankton bloom was delineated through a suite of simulation and with a coupled physical-biological model. This analysis clearly shows that wind mixing events – their duration, strenght and frequency – determine the distance of the sea ice associated phytoplankton bloom with respect to that of the ice edge as well as the occurrence in the ice-free area of secondary phytoplankton blooms during the summer period. The maximum ice edge phytoplankton concentration reached is on the other hand primarily determined by grazing pressure. Finally, the observed decline is due to the combination of grazing pressure and deep mixing events.