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

Abstract

Growth rates of diffusion cultures of nano- and microphytoplankton from Great Barrier Reef shelf water, especially diatoms (μmax = 3.2 doublings d-1), exceeded those of picocyanobacteria (Prochlorococcus and Synechococcus) when DIN (NH4+ + NO2- + NO3-) concentrations were ≥0.05 μM. The picocyanobacteria (μmax = 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 μM. Most estimates (71 %) of in situ growth rates of nano-/microphytoplankton were ≤0.25 of μmax when DIN concentrations were <0.1 μM, while only 18 % of in situ picocyanobacteria growth rates were ≤0.25 of μmax when DIN concentrations were <0.1 μM, the majority being ≥0.50 of μmax 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 μM) 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 μM. 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.