Phytoplankton size structure in the western South China Sea under the influence of a ‘jet-eddy system’

Abstract

Abstract A northeastward jet in the western South China Sea (SCS) usually induces phytoplankton blooms during summertime. This jet is often sandwiched by a cyclonic eddy in the north and an anticyclonic eddy in the south. Using in situ and satellite data, the present study analyzes the combined impact of the northeastward jet and two eddies on the phytoplankton size structure (PSS) from August to September 2014. Generally, picophytoplankton is the major size fraction in surface water, contributing 73% of the total chlorophyll a concentration. The data showed that a high chlorophyll a belt (av. 0.29 ± 0.18 μg L−1) with a large percentage of microphytoplankton (av. 14%) appeared in the northeastward jet. Meanwhile, similar chlorophyll a concentrations were observed in the cyclonic (av. 0.072 ± 0.019 μg L−1) and anticyclonic eddies (av. 0.087 ± 0.02 μg L−1), but microphytoplankton contributed 6.7% more in the anticyclonic eddy. Below the surface, however, the dominant size of phytoplankton switched from pico to nano and micro with increasing depth. In contrast to the observations at the surface, the jet and anticyclonic eddy presented a lower microphytoplankton contribution than the cyclonic eddy. Horizontally, advection of coastal upwelling water by the northeastward jet enhanced the growth of phytoplankton and influenced the surface PSS. Meanwhile, divergence/convergence in cyclonic/anticyclonic eddy interaction with the northeastward jet formed the high chlorophyll a belt at the edge of the eddy and increased the microphytoplankton contribution through water mass transport and mixing. Nutrient supply and weakening of the light intensity below the surface layer synergistically influenced the concentration and size structure of phytoplankton in the cyclonic/anticyclonic eddies at different depths. Finally, this study proposed a ‘jet-eddy system’ to explain summer spatial characteristics of PSS in the western SCS. Source water (riverine and coastal upwelling water) that feeds the ‘jet-eddy system’ is another key factor affecting phytoplankton biomass and its size structure.