Some bio-optical characteristics of phytoplankton in the NW Indian Ocean

Light absorption by phytoplankton and the parameters of the photosynthesis-light curve were measured in the NW Indian Ocean in January to February 1993. The light absorption characteristics of phytoplankton show some regional differences, probably reflecting changes in the phytoplankton population: cyanobacteria (Synechococcus spp.) were present in all 3 regions and dominated the phytoplankton biomass in the Somali Current area; prochlorophytes were mainly confined to samples from the Somali Current area; and larger algal species, including diatoms, were found in the Red Sea and Gulf of Aden. The initial slope alpha(B) and the assimilation number P-m(B) of the photosynthesis-light curve had dynamic ranges of 3.7 and 3.9, respectively, and appeared to be correlated with environmental co-variables such as nutrient concentrations and ambient light levels as well as phytoplankton absorption in the blue and red parts of the spectrum. Even though the sampling was confined to the mixed layer, there was some evidence of photoadaptation, suggesting that the rate of vertical mixing in the mixed layer was rather slow, compared with photoadaptation rates. The quantum yield of photosynthesis varied over a factor of 6, and was correlated with relative changes in absorption in the blue and red parts of the spectrum. The implications of these results for computation of primary production using photosynthesis-light models are discussed, with special reference to applications of remotely sensed data on ocean colour.