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The spatial and seasonal distributions of marine photosynthetic picoeukaryotes (PPEs) and the underlying structuring mechanisms are not well understood. Here, we performed Fluorescent in situ hybridization associated with tyramide signal amplification along an estuary to basin transect in the northern South China Sea (SCS) across three seasons (spring, summer and autumn). We disentangled the PPE assemblage variances by combining these data with the results obtained in our previous study conducted in winter to evaluate the relative importance of environmental, spatial and temporal effects. Our results showed that Mamiellophyceae was the most abundant class and accounted for an average of 33.1% (spring), 34.2% (summer) and 30% (autumn) of the depth-integrated picoeukaryotic abundances in different seasons. Prymnesiophyceae species were widely detected across the three seasons, and Pelagophyceae species were remarkably abundant in the chlorophyll maximum depth in summer. Bolidophyceae represents an important PPE class and made considerable contributions (up to 25.4% in autumn) to the depth-integrated picoeukaryotic abundances. Moreover, the PPE assemblages were significantly explained by purely environmental (6.5%) and purely temporal (42.7%) components, and they were also explained by a weak and non-significant spatial component (1.9%). In summary, our results provide insights into PPE distributions that are differently influenced by environmental, spatial and temporal components in the northern SCS. [ABSTRACT FROM AUTHOR]

This study finds a relationship between the distribution of the seven-year mean relative vorticity of surface winds (RVSW) derived from 25-kilometer-resolution measurements of ocean winds from the Quick Scatterometer (QuikSCAT) satellite and the genesis locations of tropical cyclones (TCs) in the South China Sea (SCS). The results show the effects of the background mean RVSW on TC formation in the SCS. During the summer monsoon, TCs are formed in the northern SCS, where monsoon trough and coastal mountains contribute to positive mean RVSW, while during the winter monsoon TCs form in the southern SCS with positive mean RVSW. No TCs formed in the southern part of the SCS during the summer monsoon nor in the northern part during the winter monsoon during the period from 1945 to 2005, both of which coincide with negative mean RVSW induced by the coastal mountains. The mean RVSW prior to TC formation in regions around the TC genesis locations are calculated using the QuikSCAT measurements averaged over 3 d before the geneses of 36 TCs formed in the SCS from July 1999 to June 2006. The value of this mean RVSW in the SCS is 6.19 × 10−5 s−1, which is about 1.7 times the planetary vorticity at the TC genesis positions and about 5.0 times the seven-year monthly mean RVSW. The results of these quantitative calculations using the direct satellite observations support the hypothesis given by Gray in 1975 that above-normal low-level relative vorticity is necessary for the occurrence of tropical cyclogenesis. [ABSTRACT FROM AUTHOR]