Picophytoplankton biomass, community structure and productivity in the Great Astrolabe Lagoon, Fiji

Phytoplankton biomass, community structure and productivity of the Great As trolabe lagoon and surrounding ocean were studied using measurements of chl orophyll concentration and carbon uptake. The contribution of picophytoplan kton to biomass, productivity and community structure was estimated by size fractionation, C-14-incubation and flow cytometry analysis. Picoplankton r ed fluorescence was demonstrated to be a proxy for chlorophyll < 3 mu m. Co nsequently, the percentage contribution to chi a < 3 mu m from each picopla nkton group could be calculated using regression estimated values of psi(i) (fg chi a per unit of red fluorescence). In the lagoon, average chlorophyl l concentration was 0.8 mgm(-3) with 45% of phytoplankton <3 mu m Primary p roduction reached 1.3 gCm(-2) day(-1) with 53% due to phytoplankton <3 mu m . Synechococcus was the most abundant group at all stations, followed by Pr ochlorococcus and picoeukaryotes. At all stations, Prochlorococcus represen ted less than 4% of the chi a < 3 mu m, Synechococcus between 85 and 95%, a nd Picoeukaryotes between 5 and 10%. In the upper 40 m of surrounding ocean ic waters, phytoplankton biomass was dominated by the >3 mu m size fraction . In deeper water, the <1 mu m size fraction dominated. Prochlorococcus was the most abundant picoplankton group and their contributions to the chloro phyll a <3 mu m were close to that of the picoeukaryotes (50% each).