PHOTOSYNTHESIS-IRRADIANCE RELATIONSHIPS AND PRODUCTION ASSOCIATED WITH A WARM-CORE RING SHED FROM THE AGULHAS RETROFLECTION SOUTH OF AFRICA

Synoptic estimates of phytoplankton biomass and production in the world's oceans are dependent upon ocean colour remote sensing and suitable algorithms developed from simultaneous measurements of photosynthetic parameters and optical properties in the water column. This study presents Photosynthesis-Irradiance (P-I) parameters estimated for a warm-core ring shed from the Agulhas Retroflection, in autumn, in the region of the Sub-Tropical Convergence south of Africa. Comparisons are made between values within the ring, at its edge and outside the ring in surrounding Sub-Antarctic Water. The 1 % light depth within the warm-core ring ranged from 38 to 92 m, 40 to 54 m at the edge and 40 to 75 m outside the ring. Chlorophyll a (chl a) concentrations in surface waters increased from ca 0.43 mg m-3 within the ring to about 1.0 mg m-3 at the ring edge and outside the ring. Size-fractionated chl a demonstrated that >56 % of the phytoplankton were <20 mum in size within the warm-core ring while outside the ring this figure was about 30 %. Picoplankton <3 mum made up between 8 and 9 % of the biomass at all sites. Considerable variability in photosynthetic parameters was found at all stations. Overall, the mean surface p(B)max (max. production rate) and alpha(B) (initial slope of P-I curve) were 1.59 +/- 0.13 mg C (mg chl a)-1 h-1 and 0.046 +/- 0.01 mg C (mg chl a)-1 h-1 (muE m-2 s-1)-1. Throughout, alpha(B) was higher at 100 m and I(k) (irradiance of saturation) lower. Production integrated to 100 m was lowest inside the ring (287 mg C m-2 d-1), intermediate outside (404 mg C m-2 d-1) and highest at the ring edge (453 mg C m-2 d-1). We argue that production within the warm-core ring is limited by convective instability while outside the ring photosynthesis is probably limited by light, since nutrients are non-limiting. The edge of the ring exhibits highest production because of stability conferred by the warm-core ring water and a more favourable light environment. In terms of defining 'bio-optical provinces' based on remote sensing and photosynthetic parameters, we have identified a hydrographic area south of Africa covering the Sub-Tropical Convergence (STC) where there are reliable Coastal Zone Colour Scanner data which provide an average annual chl a concentration estimate of 0.49 mg m-3. For this area we estimate total production integrated to 100 m to range from 10.5 X 10(2) to 16.5 x 10(2) t C km -2 yr-1, ca 0.5 to 0.8 % of the global oceanic production. This confirms the importance of the STC as a potential biogenic sink for atmospheric CO2.