Net growth rates of picocyanobacteria and nano-/microphytoplankton inhabiting shelf waters of the central (17°S) and southern (20°S) Great Barrier Reef

Growth rates of diffusion cultures of nano- and microphytoplankton from Great Barrier Reef shelf water, especially diatoms (mu (max) = 3.2 doublings d(-1)), exceeded those of picocyanobacteria (Prochlorococcus and Synechococcus) when DIN (NH4+ + No-2(-) + NO3-) concentrations were greater than or equal to0.05 muM. The picocyanobacteria (mu (max) = 1.1 and 1.6 doublings d(-1) for Prochlorococcus and Synechococcus respectively) achieved higher relative and absolute growth rates when time-averaged DIN concentrations were <0.05 muM. Most estimates (71 %) of in situ growth rates of nano-/microphytoplankton were less than or equal to 0.25 of mu (max) when DIN concentrations were <0.1 muM, while only 18 % of in situ picocyanobacteria growth rates were less than or equal to0.25 of mu (max) when DIN concentrations were <0.1 muM, the majority being greater than or equal to0.50 of mu (max) at such DIN concentrations. Thus growth rates of Synechococcus and Prochlorococcus populations did not appear to be significantly nutrient (nitrogen)-limited under ambient concentrations, and were of similar order to those measured in the equatorial Pacific Ocean, where NO3- concentrations (>2 muM) are far above growth-saturating levels, and in the oligotrophic North Pacific Gyre. In contrast, for those nano-/microphytoplankton for which in situ or simulated in situ growth rates were estimated, growth rates appeared to be nitrogen-limited at DIN concentrations <0.1 muM. The results provide further support for the hypothesis that picocyanobacteria dominate oligotrophic marine water columns because of their superior ability to grow at low nutrient concentrations.