Effects of surface current patterns on spatial variations of phytoplankton community and environmental factors in Sunda shelf

• Phytoplankton had the highest biodiversity and abundance in Sunda Strait.
• Surface currents significantly influenced spatial distribution of phytoplankton.
• A few species determined the variation of micro-phytoplankton community.
• Phytoplankton abundance mainly correlated with salinity, nitrate and silicate.

Phytoplankton community and environmental factors were investigated in the surface water from eastern Indian Ocean (EIO) to southern South China Sea (SCS) during April–May monsoon transition period both in 2010 and 2011. Our results indicated that the surface current patterns were different in the two cruises. Sea surface salinity (SSS) was lower in Java Sea (JS), but its salinity front obviously moved to the middle of Sunda Strait when surface waters flowed from JS to EIO in May of 2010. Nutrient concentrations were generally higher in Sunda Strait and JS. Silicate concentrations were significantly lower in EIO suggesting a possibility of silicate limitation for the growth of phytoplankton, which were less than 1 µmol L−1 in the offshore stations of EIO in May of 2010. Phytoplankton abundance and biodiversity were significant higher in Sunda strait and its adjacent stations, with larger cell-size and higher diatoms proportion. More offshore species were found in Sunda Strait when surface waters flowed from EIO to JS in April of 2011. Nutrient source can be different in Sunda Strait during inflow or outflow of surface waters. The spatial variation of phytoplankton community was greatly determined by Chaetoceros spp., Navicula sp., Pleurosigma affine, Thalassionema frauenfeldii and nano-dinoflagelage. Salinity, nitrate and silicate were the most important environmental factors regulating the variation of phytoplankton community. This study suggests that spatial distribution of phytoplankton and environmental factors are significantly influenced by the surface current patterns and river discharges, and these influences should be greater in dry season.