Seasonal and spatial variations of cladocerans on the northwest continental shelf of the South China Sea

Abstract

The seasonal and spatial variations of cladocerans were studied based on samples collected with 169 μm mesh plankton nets from July to August 2006 (summer), December 2006 to January 2007 (winter), and in April 2007 (spring) on the northwestern continental shelf of the South China Sea. Three species of cladocerans (Pseudevadne tergestina, Penilia avirostris, and Podon schmackeri) were observed. Cladocerans occur in spring and summer, but not in winter. They showed an average abundance of 12.82 (± 48.44) inds m−3 in spring and 64.66 (± 188.82) inds m−3 in summer. P. avirostris occurred in the inshore waters of the Leizhou Peninsula during spring, but in summer they were found in large number along the inshore waters from Leizhou Peninsula to the southeast of Hainan Island, with the highest abundance (1121.50 inds m−3). P. tergestina was the most dominant species both in spring and summer, being widely distributed throughout the whole surveyed area. P. schmackeri was distributed mainly along the inshore waters of the Leizhou Peninsula during spring, and mainly along the inshore waters of Hainan Island during the summer (with low density). Our results suggest that the unstable water environment resulting from the strong northeastern monsoon in winter may explain the absence of cladocerans in a large area of the northwest continental shelf of the South China Sea. High chlorophyll a concentration owing to physical factors (e.g., cold eddy, coastal upwelling and mixing) may be the main reason for the difference in the spatial distribution of cladocerans in the subtropical and tropical areas of our survey.