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

[摘要] To examine seasonal and size-dependent variations in thephytoplankton growth and microzooplankton grazing in oligotrophic tropicalwaters under the influence of seasonal reversing monsoon, dilutionexperiments were conducted during the summer of 2009 (21 May to 9 June) andwinter 2010 (9 to 18 November) in the southern South China Sea (SSCS). Theresults showed that environmental variables, phytoplankton biomass,phytoplankton growth rate (μ), microzooplankton grazing rate (m), andcorrelationship (coupling) between the μ and m, rather than themicrozooplankton grazing impact on phytoplankton (m/μ) significantlyvaried between the two seasons. Higher relative preference index (RPI) for the larger-sized (> 3 μm) phytoplankton thanpico-phytoplankton (< 3 μm), indicating significantsize-selective grazing by microzooplankton on the larger-sized phytoplankton,were also observed. The μ and m were significantly correlated withseawater salinity and temperature, and phytoplankton biomass, which indicatedthat salient seasonal variations in the phytoplankton growth andmicrozooplankton grazing in the SSCS were closely related to theenvironmental variables under the influence of the East Asian monsoon. Wepropose that intermittent arrivals of the northeast winter monsoon could leadto the low μ and m, and the decoupling between the μ and m inthe SSCS, through influencing nutrient supply to the surface water, andinducing surface seawater salinity decrease. The low m/μ (< 50 %on average) indicates low remineralization of organic matter mediated bymicrozooplankton and mismatch between the μ and m, and thus probablyaccounts for part of the high vertical biogenic particle fluxes in theprevailing periods of the monsoons in the SSCS. The size-selective grazingsuggests that microzooplankton grazing partially contributes to thepico-phytoplankton dominance in the oligotrophic tropical waters such as thatof the SSCS.

[学科分类] 地球化学与岩石