Spatial and seasonal distributions of photosynthetic picoeukaryotes along an estuary to basin transect in the northern South China Sea

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.