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

During January 1994, the photosynthetic response (P-E curves) of phytoplank ton in the eastern part of the Bransfield Strait (Antarctica) was studied i n relation to the mesoscale hydrographic structures in the area. The most i mportant hydrographic features found in the study area were: (i) the Bransf ield Strait front which separates surface Bellingshausen waters from surfac e 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 th e South, the ice-edge and the associated lenses of melting waters at the so uth-eastern part of the sampling area. These three structures were associat ed with zones of shallower mixing depth (< 50 m) and contrast to those infl uenced 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 b y the hydrographic structures. The phytoplankton of the well-mixed Weddell waters showed a lower light saturation parameter (E-k < 100 mu mol m(-2) s( -1)) than the populations of the more stratified waters (E-k > 100 mu mol m (-2) s(-1)). There were no differences in the light saturation parameters o f 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 E-k was 87 +/- 26 mu mol m(-2) s(-1) and not significantly differen t (t-test NS, P = 0.83) from the mean irradiance in the upper mixed layer ( Z(uml)) without the Z(uml) greater than or equal to 150 m stations. The slo pe of the P-m(B) VS. alpha relationship was 59 +/- 5 mu mol m(-2) s(-1) (r( 2) = 0.66, P < 0.001) and the mean irradiance of the upper mixed layer was 64 +/- 45 mu mol m(-2) s(-1), suggesting that the phytoplankton was adapted to maximize its carbon uptake even in regions with high hydrographic varia bility. The mean integrated primary production of the water column was high er 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.