Does microzooplankton grazing contribute to the pico-phytoplankton dominance in subtropical and tropical oligotrophic waters?

Microzooplankton grazing has been proposed as an important factor regulating the phytoplankton size composition, particularly in oligotrophic waters, but it has not been well examined. To examine the role of microzooplankton grazing in the pico-phytoplankton dominance in subtropical and tropical oligotrophic waters, dilution experiments with size-fractionated chl-a were conducted in the South China Sea (SCS) in August–September 2008 and May 2009. Total phytoplankton growth rate (µ) was 0.34–1.98 d−1(0.96 ± 0.49 d−1). The corresponding microzooplankton grazing rate (m) was 0.30–1.99 d−1 (0.83 ± 0.54 d−1). The microzooplankton grazing impact (m/µ) on total phytoplankton was 83.5% ± 76.3%. The growth rates of pico-phytoplankton (<3 µm) (1.02 ± 0.55 d−1) were higher than those of nano- (3–20 µm) (0.95 ± 0.58 d−1) and micro-cells (20–200 µm) (0.60 ± 0.71 d−1), while the microzooplankton grazing rates on pico-phytoplankton (0.77 ± 0.58 d−1) were lower than those on nano-cells (1.00 ± 0.79 d−1), resulting in the higher grazing impact on larger cells. Increase of the pico-cell proportion and decrease of the nano- and micro-cell proportions after the incubation coincided well with the varied grazing impacts. Our results indicate that the high microzooplankton grazing impact keeps the phytoplankton standing stock at a low level, and the selective grazing of microzooplankton on nano-phytoplnakton as well as the low growth rates of micro-phytoplankton contributes to the pico-phytoplankton dominance in subtropical and tropical oligotrophic waters of the SCS. Variation in the phytoplankton-size-dependent grazing impact in different studies has been discussed.