Spatio-temporal variability of phytoplankton assemblages in the Pearl River estuary, with special reference to the influence of turbidity and temperature

The Pearl River Estuary is among the largest estuaries in the subtropical areas of the world. Along the salinity and turbidity gradient between the freshwater reach of the Pearl River and the marine water of the South China Sea, the spatial and temporal composition and abundance of phytoplankton was examined in relation to physic-chemical variables during the dry and wet seasons of 2009. Water samples for phytoplankton and environmental parameters were collected from 18 stations during two seasons along a transect from upper estuary to estuarine and marine sectors. A total of 162 species belonging to 7 phyla were identified, with diatoms dominated in both seasons while dinoflagellates proliferated in autumn. Two main clusters and three sub-clades under each main cluster corresponding to seasons and water sectors were defined with multivariate analysis (cluster and nMDS). Based on the species composition and abundance of phytoplankton, both seasonal and spatial variability were observed at a significant level (ANOSIM: season effect, R=0.896, P<0.01; station effect, R=0.463, P<0.01). The correlation analysis between biotic and abiotic variables indicated that instead of the “proverbial” anthropogenic nutrients loading and salinity gradient, the best 2-variable combination (temperature and turbidity) showed a significant effect on the pattern of phytoplankton assemblages (ρ_w=0.49, BIOENV analysis) between wet and dry seasons in the Pearl River Estuary. This result suggests that physical disturbance either natural or manmade is a more important factor in regulating the phytoplankton community structure within the hydrologically distinct zone of estuaries.