Photosynthesis-irradiance relationships and production associated with a warm-core ring shed from the Agulhas Retroflection south of Africa

Abstract

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 SubAntarctic Water. The l % 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 nng edge and outside the ring. Size-fractionated chl a demonstrated that >56 % of the phytoplankton were <20 pm in size within the warm-core ring while outside the ring this figure was about 30%. Picoplankton < 3 pm 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 ' , , , (max. production rate) and aB (initial slope of P-I curve) were 1.59 f 0.13 mg C (mg chl a ) -' h-' and 0.046 + 0 01 mg C (mg chl a)'' h-' (PE m'2 S-')-' Throughout, aB was higher at 100 m and Ik (irradiance of saturation) lower. Production integrated to 100 m was lowest inside the ring (287 mg C m-2 d-l), intermediate outside (404 mg C d-') and highest at the ring edge (453 mg C m-2 d-l). 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-luniting. 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 102 to 16.5 X 102 t C yr-', 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.

Short-term variability of photosynthetic parameters and particulate and dissolved primary production in the Alboran Sea (SW Mediterranean)

“…Decreasing P B m POC is frequently associated with increasing α (e.g. Dower & Lucas 1993, Basterretxea & Arístegui 2000, Morán et al 2001, whereas both parameters decreased similarly with depth in our experiments (Fig. 5).…”

“…2, using interpolation of photosynthetic parameters at each depth, as more appropriate than just taking 2 sets of constant values changed at a given depth of the water column. Apart from the observation that Z uml was shallower than the depth of the most superficial sample at Stns C2, A3 and B3 (Table 1), the fact that P-E parameters also varied with depth within the upper mixed layer, argued against the use of the uppermost values throughout all the upper mixed layer (Dower & Lucas 1993).…”

Phytoplankton distribution and nitrogen dynamics in the southwest indian subtropical gyre and Southern Ocean waters

“…Whether these were mesoscale eddies, filaments, or larger frontal structures, is not possible to determine from a single section, however such structures have been observed previously (Read and Pollard, 1993) and attributed to eddies (e.g., Pollard and Regier, 1992). Such eddies would result in substantial heat loss and convective overturning that entrains nutrients into surface waters (Dower and Lucas, 1993;Pollard et al, 2002), with the potential to enhance primary production wherever stability and stratification occurs. Despite high NO 3 concentrations (104-1074 mmol m −2 ), primary production is still based primarily on regenerated nutrients as revealed by low f-ratios (< 0.5) (Fig.…”

Exploitation of mesoscale oceanographic features by grey-headed albatross Thalassarche chrysostoma in the southern Indian Ocean

“…It has recently been demonstrated that eddies spawned off the major frontal systems are rich in zooplankton/micronekton standing stock (Craddock et al 1992, Wiebe & Youce 1992, Pakhomov et al 1994, Pakhomov & Perissinotto 1997. The eddies may persist for up to 6 mo (Lutjeharms & Gordon 1987, Lutjeharms & Valentine 1988 and exhibited elevated primary productivity at the edges (Dower & Lucas 1993, Froneman et al 1999. Eddies are therefore regarded as important vehicles in transporting unique or dense zooplankton communities, providing ideal feeding grounds for top predators (Sugimoto & Tameishi 1992, Barange et al 1998).…”