Controls on temporal and spatial variations of phytoplankton pigment distribution in the Northern South China Sea

Abstract

The seasonal and spatial variations of phytoplankton pigment distribution and their interaction with environmental controlling parameters were investigated in the Northern South China Sea (NSCS) at 37 stations covering the coastal region, the continental shelf, the slope, and the deep-water basin during two summer and two winter cruises from 2010 to 2012. Strong spatial, interseasonal, and intraseasonal variations of pigment distribution were observed in the diverse biogeochemical regions, exhibiting extremely dynamic phytoplankton community structure in the NSCS. In addition to chlorophyll a, the major pigments observed included divinyl chlorophyll a (DV Chl a), total chlorophyll b (Chl b), zeaxanthin (Zea), 19′-hexanoyloxyfucoxanthin (Hex), 19′-but-fucoxanthin (But), fucoxanthin (Fuco), and prasinoxanthin (Pras). Overall, Fuco, Chl b, Zea, Hex, But, and Pras were the dominant pigments in the coastal and shelf region; DV Chl a, Chl b, Hex, But, Zea, and Fuco were the major pigments in the offshore water. After analyzing marker pigment correlations and cellular pigment concentrations, we conclude that Zea, Hex (with But), and Fuco can be used as specific marker pigments for Synechococcus, coccolithophores, and diatoms in the NSCS, respectively. We have also derived that prasinophytes accounted for most of the elevated Chl b observed in the coastal water and appeared to be an important pico-eukaryotic group (Pico) in the coastal region. With varying cellular pigment concentrations observed vertically and seasonally, it is essential to separate sampling stations to different biogeochemical domains to apply pigment to Chl a ratios to estimate phytoplankton community structure in the NSCS. We observed close association between soluble reactive phosphate and Chl a abundance in coastal, shelf, and deep-water regions during summer cruises. Major nutrient supply appears to be the main controlling factor on the temporal and spatial variations of major pigment distribution. The supply is mainly driven by terrestrial input in summer and water mixing strength in winter. During the summer periods, major nutrients are primarily supplied from riverine and groundwater discharge; during winter periods, strong winter monsoon intensifies sub-surface water mixing and results in nutrient and pigment elevation in the euphotic zone. Also driven by the monsoon, the intrusion of the low-temperature coastal current from Taiwan Strait to the coastal region of the NSCS may replace or alter the phytoplankton community in the coastal region in winter. Overall, the temporal and spatial variations of phytoplankton pigment distribution in the NSCS are regulated by the fluctuations of these two major environmental forcings.