Seasonal and size-dependent variations in the phytoplankton growth and microzooplankton grazing in the southern South China Sea under the influence of the East Asian monsoon

Abstract. To examine seasonal and size-dependent variations in the phytoplankton growth and microzooplankton grazing in oligotrophic tropical waters under the influence of seasonal reversing monsoon, dilution experiments were conducted during the summer of 2009 (21 May to 9 June) and winter 2010 (9 to 18 November) in the southern South China Sea (SSCS). The results showed that environmental variables, phytoplankton biomass, phytoplankton growth rate (μ), microzooplankton grazing rate (m), and correlationship (coupling) between the μ and m, rather than the microzooplankton grazing impact on phytoplankton (m/μ) significantly varied between the two seasons. Higher relative preference index (RPI) for the larger-sized (> 3 μm) phytoplankton than pico-phytoplankton (< 3 μm), indicating significant size-selective grazing by microzooplankton on the larger-sized phytoplankton, were also observed. The μ and m were significantly correlated with seawater salinity and temperature, and phytoplankton biomass, which indicated that salient seasonal variations in the phytoplankton growth and microzooplankton grazing in the SSCS were closely related to the environmental variables under the influence of the East Asian monsoon. We propose that intermittent arrivals of the northeast winter monsoon could lead to the low μ and m, and the decoupling between the μ and m in the SSCS, through influencing nutrient supply to the surface water, and inducing surface seawater salinity decrease. The low m/μ (< 50 % on average) indicates low remineralization of organic matter mediated by microzooplankton and mismatch between the μ and m, and thus probably accounts for part of the high vertical biogenic particle fluxes in the prevailing periods of the monsoons in the SSCS. The size-selective grazing suggests that microzooplankton grazing partially contributes to the pico-phytoplankton dominance in the oligotrophic tropical waters such as that of the SSCS.