Net in situ growth rates of phytoplankton in an oligotrophic, tropical shelf ecosystem

Abstract

In situ growth rates of phytoplankton in tropical shelf waters of the Great Barrier Reef were measured with diffusion chambers. A broad disparity was observed between the growth responses of diatom species or assemblages (µmax = 2–5 doublings d−1) and assemblages of microflagellates, nonmotile ultraplankton, or phototrophic picoplankton (µmax < 2 doublings d−1). Several diatom species or groups consistently grew faster than the community Chl doubling rate (µmax = 1.6 doublings d−1). Most dominant species and groups, regardless of taxonomic affiliation, exhibited the potential for high relative growth rates (µ/µmax > 0.5) at ambient concentrations of dissolved inorganic N <0.2 µmol liter−1 and the time‐averaged diurnal irradiances in situ >50 µEinst m−2 s−1. If relative constancy in phytoplankton standing crop and composition is to be maintained, grazing rates must vary widely at the species level, but be broadly similar to growth rates over time scales of days. Although the full 40–80‐m water column in the central Great Barrier Reef lies the euphotic zone, phytoplankton growth rates near the bottom seem to be light limited.