Primary productivity and in situ quantum yields in the Ross Sea and Pacific Sector of the Antarctic Circumpolar Current

We determined the in situ primary productivity, particle absorption, pigment concentrations, and photosynthetic quantum yields during two Southern Ocean cruises in 1997 and 1998 in the Pacific sector of the Antarctic Circumpolar Current (ACC), and in the Ross Sea. Integrated primary productivity in the Ross Sea during the austral summer averaged 762 mg C m−2 d−1 (range: 413–1200 mg C m−2 d−1), and mean integrated chlorophyll a levels were 80.6 mg m−2 (range: 47.8–155 mg m−2). The mean (±standard deviation) quantum yield was 0.09 (±43%) mol C Ein−1 during summer in the Ross Sea. Less than 2% of the depth variability in quantum yield in the Ross Sea was attributable to light absorption by photoprotective pigments. There was an inverse relationship found between the value of the in situ maximum quantum yield (φ[max]) and the ratio of particulate organic carbon to nitrogen (POC:PON), indicating that variations in quantum yields were controlled largely by nutrient availability. POC:PON ratios near Redfield ratio of 6.6 were found to have in situ φ(max) values near the theoretical maximum of 0.12 mol C Ein−1. POC-to-particulate iron values were also found to have been lower in high φ(max) regions. Integrated primary productivities and chlorophyll a levels in the ACC during the early summer were similar to those in the Ross Sea. The mean integrated primary productivity was 734 mg C m−2 d−1 (range: 606–839 mg C m−2 d−1), and the mean integrated chlorophyll equaled 56.2 mg m−2 (range: 33.3–71.9 mg m−2). The mean (±standard deviation) quantum yield was 0.04 (±28%) mol C Ein−1 and was less than the mean value measured for the Ross Sea. As in the Ross Sea, light absorption by photoprotective pigments was responsible for <2% of the variability in quantum yield. There was no discernable relationship found between in situ φ(max) and elemental ratios of the particulates, although the sample size for the ACC was smaller (n=4) compared to the Ross Sea (n=10). Overall, the phytoplankton of the ACC required higher irradiance than the phytoplankton of the Ross Sea to saturate photochemistry.