Close coupling between phytoplankton growth and microzooplankton grazing in the western South China Sea

Abstract

We conducted 28 dilution experiments during August‐September 2007 to investigate the coupling of growth and microzooplankton grazing rates among ultraphytoplankton populations and the phytoplankton community and their responses to habitat variability (open‐ocean oligotrophy, eddy‐induced upwelling, and the Mekong River plume) in the western South China Sea. At the community level, standing stocks, growth, and grazing rates were strongly and positively correlated, and were related to the higher abundance of larger phytoplankton cells (diatoms) at stations with elevated chlorophyll concentration. Phytoplankton growth rates were highest (>2 d−1) within an eastward offshore jet at 13°N and at a station influenced by the river plume. Among ultraphytoplankton populations, Prochlorococcus dominated the more oceanic and oligotrophic stations characterized by generally lower biomass and phytoplankton community growth, whereas Synechococcus became more important in mesotrophic areas (eddies, offshore jet, and river plume). The shift to Synechococcus dominance reflected, in part, its higher growth rates (0.87 ± 0.45 d−1) compared to Prochlorococcus (0.65 ± 0.29 d−1) or picophytoeukaryotes (0.54 ± 0.50 d−1). However, close coupling of microbial mortality rates via common predators is seen to play a major role in driving the dominance transition as a replacement of Prochlorococcus, rather than an overprinting of its steady‐state standing stock.