The seasonal variation of the phytoplankton size class in northern South China Sea

Phytoplankton size class variation is a good indicator of water dynamics changes, alga blooms or ecosystem change, for it usually corresponds to different phytoplankton group composition. Remote sensing technique is a very effective method for studying phytoplankton at large scale by virtue of synoptic view and large area cover. Here we detected seasonal and annual phytoplankton size class variation using remote sensing data based on a bio-optical algorithm in northern South China Sea. Result showed that the phytoplankton size class turned to be smaller during summer time, because of the stability of the water body and lack of nutrients. While in winter, the north-east monsoon made the water dynamics become complex. Upwellings, mesoscale eddy, typhoon and a series of drifts bring underwater nutrients up to the sea surface. As a result, phytoplankton with bigger cell size began to grow and the cell size turned to be bigger. The fraction of micro-phytolankton changed from 7% to 10%, while nanoand pico-phytolankton fraction decreased from 29% to 24% and 64% to 62% respectively. Annual change of phytoplankton size classes was in a relatively slow pace, and no big difference was noticed in the normal year. Usually about 1-5 percent change for each size class. When compared 2008 with 1998, a decade variances could be monitored. All the results in this study showed that phytoplankton size class can be an effective indicator of the ecosystem or environmental change.