Photosynthesis, primary production and phytoplankton growth rates in Gerlache and Bransfield Straits during Austral summer: cruise FRUELA 95

Bio-optical parameters, primary production, and phytoplankton growth rates were determined in the southern part of the Bellingshausen Sea, the western part of the Bransfield Strait, and the Gerlache Strait, during December 1995 and early January 1996. Three water bodies were present at the surface (<100 m); Bellingshausen warm waters (BWW), Bellingshausen cold waters (BCW), and transitional Weddell waters (TWW), which were separated by a thermal front. High chlorophyll concentrations were found in the upper mixed layer (UML) of the BCW waters (5.57±4.04 mg m−3), with intermediate values in BWW (2.85±1.24 mg m−3) and the lowest values in the TWW (1.53±0.94 mg m−3). The broadband photosynthetic parameters (PmaxB, αB and EKPAR) were highest in BWW and lowest in TWW, except for EKPAR, which did not show significant differences between water bodies. Spectral bio-optical parameters (aph, φmax and EKPUR) showed a similar distribution to chlorophyll concentrations except EKPUR, which was highest in BWW. A comparison between light-saturation parameters and water-column irradiance suggests that photosynthesis was not light-limited in the mixed layer. However, transitory situations exist where photosynthesis could be close to limitation at the bottom of the photic layer when high chlorophyll concentrations were present in the surface layers. The high maximum quantum yields (0.073±0.032 mol C (mol photons)−1) also suggest that iron limitation of photosynthesis should not be occurring in the region during the cruise. The mean primary production rates were 1.11±0.68 g C m−2 d−1 in the TWW, 2.28±0.98 g C m−2 d−1 in the BCW, and 2.68±0.94 g C−2 d−1 in the BWW. The Gerlache Strait and frontal zones were the most productive, with values analogous to those of upwelling areas. Carbon-specific growth rates in UML (0.43±0.16 d−1) were similar to those of temperate seas, neither nutrient- nor light-limited. Growth rates in the UML of BCW (0.54±0.20 d−1) were higher than those of TWW (0.37±0.08 d−1) and BWW (0.35±0.09 d−1). The results indicate that factors as microzooplankton grazing or sinking should control the standing stock of phytoplankton, since photosynthesis was not limited.