Abundance, distribution and flow-cytometric characterization of picophytoprokaryote populations in central (17degreesS) and southern (20degreesS) shelf waters of the Great Barrier Reef

Synechococcus was more abundant and had a greater biomass than Prochlorococcus at most inshore and mid-shelf sites in the central (17°S) Great Barrier Reef (GBR), and at all shelf sites in the southern (20°S) GBR. Significant Prochlorococcus populations were confined to mid- and outer-shelf sites with mixed or partially stratified water columns of greater oceanic character in the central GBR, where depth-weighted average Synechococcus and Prochlorococcus abundances were better correlated with salinity, shelf depth and chlorophyll a concentration, than with concentrations of NH4+, NOx– (i.e. NO2– + NO3–), or PO43–. Vertical gradients of normalized mean cellular red and orangefluorescence of Synechococcus and Prochlorococcus populations imply that vertical mixing rates were sufficiently low to allow these populations to photoacclimate at depth at shelf locations in thecentral GBR, but too great for substantial photoacclimation to be observed at sites in the southern GBR. The presence of Prochlorococcus populations at inshore sites in the central GBR in the absence of extensive intrusion events suggests that Prochlorococcus populations were actively growing.