LIGHT AND PRODUCTIVITY OF ANTARCTIC PHYTOPLANKTON DURING AUSTRAL SUMMER IN AN ICE-EDGE REGION IN THE WEDDELL-SCOTIA SEA

The photosynthesis-irradiance relationships (P-I curves) of natural pl ankton samples were studied in the Weddell Sea ice-edge zone, between Elephant Island and South Orkney Islands, during the austral summer of 1988-89. Three water bodies were distinguished in the region: Belling shausen Sea waters modified after flowing through Drake Passage and Br ansfield Strait, Weddell Sea waters and Weddell Sea waters modified by melting. The stations situated in modified Bellingshausen waters show ed a net phytoplankton composition which was different from that of th e other two water bodies. Weddell Sea waters and Weddell Sea waters mo dified by melting of sea ice had the same net phytoplankton compositio n. In the area of modified Weddell Sea waters, there was an accumulati on of phytoplankton in the upper 40 m (>4 mg Chl m-3). (P(B)s and alph a were independent of depth, but varied between stations. The P-1 para meters exhibited an east-west gradient with the maximum values in the Elephant Island region, where large diatoms were absent. The light-sat uration parameter I(k) averaged 35 +/- 13 mumol m-2 s-1. The mean irra diance of the upper mixed layer was 34 +/- 13 mumol m-2 s-1, thus virt ually equal to I(k). These results are discussed in the context of cri tical depth (Z(c)), net compensation irradiance (I(n)) and depth of th e upper mixed layer (Z(uml)). We suggest that the phytoplankton in the region is adapted to maximize its carbon uptake and growth rate at th e mean irradiance of the upper mixed layer (I(uml)) in well-mixed zone s. We also suggest that the light-saturation parameter (I(k)) could be the irradiance at which photosynthesis approximately compensates the total losses of the community (I(n)).