BUILD UP AND DECLINE OF SUMMER PHYTOPLANKTON BIOMASS IN THE EASTERN WEDDELL SEA, ANTARCTICA

The seasonal development and decline of phytoplankton was investigated in the eastern Weddell Sea during summer and fall 1991. During the fi rst half of the study (15 Jan-13 Feb) in an area off Vestkapp, favoura ble irradiance/mixing regimes initiated net phytoplankton growth in ic e-free waters on the shelf and in stretches of open water over the par tially ice-covered deep ocean. Chl a concentrations in the upper water column were moderate (0.2-0.8 mu g l(-1)), but significantly above wi nter values. Later in the season (16 Feb-11 March), a phytoplankton bl oom with surface Chl a concentrations ranging from 1.6-2.3 mu g l(-1) was encountered in an area further to the east. We suggest that the up per water column must have been stratified in this region for time sca les of weeks to faciliate bloom development. Bacterial biomass and pro ductivity generally paralleled the seasonal development of the phytopl ankton. Nitrate concentrations in the upper mixed layer were substanti ally lower than would be expected from the existing phytoplankton stan ding stock, suggesting that heterotrophic consumption of organic matte r by bacteria and zooplankton removed a large fraction of the primary production. The shallow seasonal pycnocline was eventually eroded by t he passage of a storm, resulting in a homogeneous distribution of phyt oplankton biomass over the entire water column, followed by sedimentat ion and deposition of phytodetritus on the sea floor. After the storm induced destratification, bacterial productivity was particularly high , amounting to more than half of the primary production (range: 10%-12 0%) in the upper water column. Subsequently, phytoplankton biomass in the upper water column decreased to values < 1 mu g Chl al(-1). The co mbination of low incident irradiances and incessant deep mixing preven ted the phytoplankton biomass to increase again. During the last week of the investigation, extensive new-ice formation was observed. A majo r fraction of the residual surface plankton was incorporated into new sea ice, thus terminating the pelagic growth season of the phytoplankt on in the eastern Weddell Sea.