Photosynthetic parameters and primary production in the Bransfield Strait: relationships with mesoscale hydrographic structures

During January 1994, the photosynthetic response (P–E curves) of phytoplankton in the eastern part of the Bransfield Strait (Antarctica) was studied in relation to the mesoscale hydrographic structures in the area. The most important hydrographic features found in the study area were: (i) the Bransfield Strait front which separates surface Bellingshausen waters from surface Weddell Sea water in the northern part of the Strait; (ii) further to the North, the Weddell–Scotia Confluence north of Elephant Island; (iii) to the South, the ice-edge and the associated lenses of melting waters at the south-eastern part of the sampling area. These three structures were associated with zones of shallower mixing depth (<50 m) and contrast to those influenced by Weddell Sea waters in which the mixed layer depth was deeper than 150 m. The stronger stratification in these three areas was reflected in a higher chlorophyll (Chl) concentration at the surface waters (>1 mg Chl m−3). The photosynthetic response of the phytoplankton was also affected by the hydrographic structures. The phytoplankton of the well-mixed Weddell waters showed a lower light saturation parameter (Ek<100 μmol m−2 s−1) than the populations of the more stratified waters (Ek>100 μmol m−2 s−1). There were no differences in the light saturation parameters of phytoplankton samples from the different water bodies found in the region as demonstrated by a t-test for paired comparisons (0.88>P>0.46). The average Ek was 87±26 μmol m−2 s−1 and not significantly different (t-test NS, P=0.83) from the mean irradiance in the upper mixed layer (Zuml) without the Zuml≥150 m stations. The slope of the PmB vs. α relationship was 59±5 μmol m−2 s−1 (r2=0.66, P<0.001) and the mean irradiance of the upper mixed layer was 64±45 μmol m−2 s−1, suggesting that the phytoplankton was adapted to maximize its carbon uptake even in regions with high hydrographic variability. The mean integrated primary production of the water column was higher at the zones with the highest chlorophyll concentrations reaching values >1.5 g C m−2 d−1 between King George Island and Elephant Island.