Seasonal Variations Of Phytoplankton Biomarkers In Surface Seawater From The Northern South China Sea And Their Potential As Indicators Of Biomass/Community Structure

Biomarkers have been widely used to reconstruct phytoplankton productivity, however, thishypothesis has not been validated with modern survey in the northern South China Sea（SCS）. Inthis study we collceted four seasons’ surface particulate matters in the northern SCS to analyze thephytoplankton biomarkers （brassicasterol for diatoms, dinosterol for dinoflagellates and alkenonesfor coccolithophores） to investigate the biomarker-based phytoplankton biomass/communitystructure changes and its affecting factors.The sum of the three biomarkers’ contents reavels clear spatial distribution and seasonalvariations in the northerrn SCS surface water. High biomass values appear at the Pearl Riverestuary and Guangdong coastal areas and low values in the basin area with a seaward decreasingtrend. To compare the results of four seasons, the biomass in summer is the highest than the otherthree seasons with an average concentration of70ng·L^-1, followed by winter(66.4ng·L^-1),autumn(60.5ng·L^-1), and the lowest is in spring(45.8ng·L^-1). During the summer, high valuesoccur at the whole coastal area of Guangdong, while in the other three seasons the high valuesappear at the Pearl River estuary and coast of western Guangdong, which can be attributed to thesummer monsoon blowing the diluted water to the southeast.The distribution pattern of phytoplankton in the northern SCS is mainly controlled by nutrientssupply. The coastal current, upwelling and Pearl River diluted water bring abundant nutrients tothe surface water resulting in a dense region of the phytoplankton biomass. On the contrary, thelevel of surface water nutrient concentration in the basin is relatively low, which are brought aboutby the following reasons. Firstly, water stratification in open sea brings about nutrients in thebottom water can not be transported to surface. Secondly, there are two anticyclonic eddies innortheast of Hainan island and southeast of HongKong all the year round which lead to the lowernutrient level than the surrounding waters, as a result, the phytoplankton biomass performs low values in the basin.The spatial distribution of biomarker ratios indicates that diatoms are the dominant species inthe northern SCS,with proportion ranges from62.4%to79.3%, and there are no apparent regionaland seasonal variations. Dinoflagellates and coccolithophores perform the seaward increase trendswith high values in the open sea and low values in the coastal zone. For the temporal distribution,the concentration of dinoflagellates is higher in spring and summer and the concentration ofcoccolithophores is higher in autumn and winter.The distribution pattern for the community structure is due to the competition for nutrientsamong these algaes. Diatoms absorb nutrients quickly in nutrient-rich and balanced seawater.Dinoflagellates have low phosphate absoption constant, so they could thrive in phosphoruslimitation waters off the coast. Meanwhile, coccolithophores widely distributed throughout SCS,which have smaller cellular volume and lower absoption constant than diatoms for both the nitrateand phosphate. Thus, coccolithophores can utilize relatively low nutrient in the basin, and reachpeak values in the open ocean sites, which is opposite to the biomass distribution.Overall, these results based on the biomarker proxies in the particulate matters of surfaceseawater are consistent with previous studies of phytoplankton cell, remote sensing and pigments,which provides support for the use of biomarker to reconstruct phytoplankton productivity andcommunity structure in the northern SCS.