Journal cover Journal topic
Biogeosciences An interactive open-access journal of the European Geosciences Union
Journal topic
Volume 12, issue 22
Biogeosciences, 12, 6809–6822, 2015
https://doi.org/10.5194/bg-12-6809-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.
Biogeosciences, 12, 6809–6822, 2015
https://doi.org/10.5194/bg-12-6809-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 27 Nov 2015

Research article | 27 Nov 2015

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

L. Zhou1,2, Y. Tan1,2, L. Huang1,2, Z. Hu1,2,3, and Z. Ke1,2 L. Zhou et al.
  • 1CAS Key Laboratory of Tropical Marine Bio-resources and Ecology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
  • 2Guangdong Provincial Key Laboratory of Applied Marine Biology, Guangzhou 510301, China
  • 3University of Chinese Academy of Sciences, Beijing 100049, China

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.

Publications Copernicus
Download
Short summary
We observed that phytoplankton biomass and growth rate (μ), microzooplankton grazing rate (m), and coupling (correlation) between the μ and m significantly varied between the summer and winter, and microzooplankton selectively grazed more on the larger-sized phytoplankton, and a low grazing impact on phytoplankton (m/μ < 50%) in the SSCS. The salient seasonal variations in μ and m, and their coupling were closely related to environmental variables under the influence of the East Asian monsoon.
We observed that phytoplankton biomass and growth rate (μ), microzooplankton grazing rate (m),...
Citation