Xingyu Song

Latitudinal variability (6 degrees S-20 degrees N) of early summer phytoplankton species compositions and size-fractioned productivity from Java Sea to South China Sea

Abstract In order to study the effects of latitudinal change in physical and chemical environments on phytoplankton cells, we investigated the early summer phytoplankton biomass, species composition and size-fractioned productivity in surface water from the Java Sea to the South China Sea (from 6°S to 20°N) from 18 May to 27 May 2010. Chlorophyll a (Chl a) concentration decreased latitudinally from 0.18 (∼6°S) to 0.05 µg l–1 (∼10°N). The dominant species, the cyanobacteria Trichodesmium erythraeum, dinoflagellates (e.g. Gyrodinium dominans, Amphidinium carterae and Gonyaulax spp.) and diatoms (e.g. Thalassionema nitzschioides, Rhizosolenia spp. and Chaetoceros spp.) changed to only the dinoflagellate species. Also, the Chl a biomass increased to 0.14 µg l–1 towards the end of the survey (∼20°N) with the dinoflagellates as the most abundant group. Productivity of phytoplankton assemblages coincided with Chl a concentration, and decreased accordingly from 9.24 ± 0.71 to 2.87 ± 0.41 µg C l–1 day–1, then increased to 5.45±1.1 µg C l–1 day–1. Chl a concentration and productivity were significantly correlated (P<0.05) with microplankton cell abundance, as well as nutrient concentrations, which appeared to exert a strong influence over latitudinal variation in primary production.

Influence of mesoscale eddies on primary production in the South China Sea during spring inter-monsoon period

Mesoscale eddies have been suggested to have an impact on biological carbon fixation in the South China Sea (SCS). However, their overall contribution to primary production during the spring inter-monsoon period is still unknown. Based on large-scale biological and environmental in situ observations and synchronous remote sensing data, the distribution patterns of phytoplankton biomass and the primary production, and the role of mesoscale eddies in regulating primary production in different eddy-controlled waters were investigated. The results suggested that the surface chlorophyll a concentrations and water column integrated primary production (IPP) are significantly higher in cyclonic eddies and lower in the anticyclonic eddies as compared to that in non-eddy waters. Although eddies could affect various environmental factors, such as nutrients, temperature and light availability, nutrient supply is suggested to be the most important one through which mesoscale eddies regulated the distribution patterns of phytoplankton biomass and primary production. The estimated IPP in cyclonic and anticyclonic eddies are about 29.5% higher and 16.6% lower than the total average in the whole study area, respectively, indicating that the promotion effect of mesoscale cold eddies on the primary production was much stronger than the inhibition effect of the warm eddies per unit area. Overall, mesoscale eddies are crucial physical processes that affect the biological carbon fixation and the distribution pattern of primary production in the SCS open sea, especially during the spring inter-monsoon period.

Seasonal Community Structure of Mesozooplankton in the Daya Bay, South China Sea

Mesozooplankton are key components of coastal ecosystems, linking the microbial food web to the classic food chain. In this study, species composition and abundance of mesozooplankton is studied for the Daya Bay in April (spring) and October (fall), 2006. A total of 27 species of mesozooplankton were identified in spring and 58 species in fall. Dominant species were Oithona tenuis, Flaccisagitta enflata, Penilia avirostris and Centropages tenuiremis in spring, shifting to Microsetella norvegica, Oithona tenuis and Parvocalanus crassirostris in fall. Higher mesozooplankton abundance was found at Aotou Cove and Dapeng’ao Cove compared to other stations, indicating the influence of eutrophication on mesozooplankton community in the Daya Bay. The outbreak of Noctiluca scintillans bloom in spring reduced the species diversity and abundance of mesozooplankton.

Chlorophyll a increase induced by surface winds in the northern South China Sea

The response of chlorophyll a (Chl a) concentration to wind stress is analyzed in the South China Sea (SCS), using in-situ data of Chl a and remote sensing data (QuikScat-sea surface wind (SSW), AVHRR-sea surface temperature (SST), AVISO merged-sea level anomalies (SLA), SeaWiFS-derived Chl a and MODIS Terra-derived Chl a) in August/September/October 2004, 2006 and 2009. The variability of SSW, SST and SLA 7 d before in-situ Chl a sampling (including the work day of in-situ Chl a sampling) with the same latitude and longitude of the study area are investigated, and the correlation coefficients are calculated between these hydrographic factors and in-situ Chl a concentration. The results show that the Chl a-SSW correlation coefficients at upper layers (such as 0 m and 25 m) are more significant than those at deeper layers (such as 50, 75 and 100 m) 1–3 d before, which indicates that there is a time lag of strong surface winds stimulating phytoplankton bloom. By analyzing the relationship among the daily remote sensing derived (RSderived) SSW, SST, SLA and 3 d averaged SeaWiFS/MODIS-derived Chl a concentration in the northern SCS in September 2004 and 2009 respectively, it shows that the intensity and speed of surface winds could have great influence on extend of Chl a increase. If surface winds reach 4–5 m/s over, Chl a concentration would increase 1–3 d after the process of strong surface winds in open sea area of the northern SCS mainly during September.

Variations in silicate concentration affecting photosynthetic carbon fixation by spring phytoplankton assemblages in surface water of the Strait of Malacca

The Strait of Malacca (SoM), the world’s busiest sea-route, is increasingly polluted as the rapid development of world trades, affecting phytoplankton primary productivity therein. The variations of surface phytoplankton biomass, size-structure and carbon fixation were investigated across the SoM during the spring period (May 4 to 9, 2011). Chlorophyll a concentration increased from 0.12 µg/L at the northwest entrance of the SoM to a maximal 0.63 µg/L at narrowest section, and decreased to 0.10 µg/L at the southeast entrance. Photosynthetic carbon fixation by phytoplankton coincided well with Chl a biomass, and increased from 10.8 to 22.3 µg C/(L·d), then decreased to 9.21 µg C/(L·d); while the carbon fixation rate showed an inverse pattern to the changes of Chl a, and decreased from 87.1 to 35.5 µgC/(µgChla·d) and increased thereafter to 95.3 µg C/(µg Chl a·d). Picophytoplankton cells (<3 µm) contributed to more than 60% and 50% of the total Chl a and carbon fixation at both the entry waters; while the contributions of pico-cells decreased sharply to the minimum of 18.3% and 27.5% at the narrowest part of the SoM. In particular, our results showed that the silicate concentration positively regulated Chl a biomass and carbon fixation, reflecting that the higher silicate favoured the growth of phytoplankton and thus led to higher primary production in this strait.

A dinoflagellate Cochlodinium geminatum bloom in the Zhujiang (Pearl) River estuary in autumn 2009

A severe Cochlodinium geminatum red tide (>300 km2) was observed in the Zhujiang (Pearl) River estuary, South China Sea in autumn 2009. We evaluated the environmental conditions and phytoplankton community structure during the outbreak. The red tide water mass had significantly higher dissolved inorganic phosphate (DIP), ammonia, and temperature, but significantly lower nitrite, nitrate, dissolved inorganic nitrogen (DIN), and DIN/DIP relative to the non-red-tide zones. The phytoplankton assemblage was dominated by dinoflagellates and diatoms during the red tide. C. geminatum was the most abundant species, with a peak density of 4.13×107 cell/L, accounting for >65% of the total phytoplankton density. The DIN/DIP ratio was the most important predictor of species, accounting for 12.45% of the total variation in the phytoplankton community. Heavy phosphorus loading, low precipitation, and severe saline intrusion were likely responsible for the bloom of C. geminatum.