Change in Distribution of Plankton as Affected by Environmental Factors in the Estuaries of Feiyunjiang River and Aojiang River in Summer and Autumn

Spatial distribution of phytoplankton and zooplankton in the waters around the estuaries of the Feiyunjiang River and the Aojiang River (27°45-27°08′N，120°05-120°09′E) were investigated in August and November 2010 for analysis of its relationship with some major environmental factors, such as nutrient salts, suspended substances (SS), etc. Results show that concentrations of inorganic nitrogen, labile phosphate and suspended substances gradually decreased, while salinity and pH gradually increased from the estuary waters to offshore waters. In August, the parts of the estuaries were found to be low in DO concentration, and a strip of high sea surface temperature(SST ) was in the center of the investigated region. In November, a strip of low SST was observed in the estuary of the Feiyunjiang River. The average concentration of chlorophyll-a in August and November was 5.2 and 0.9 µg·L^-1, respectively. The abundance of phytoplankton was quite high in August, averaging 2344.6 × 10⁴ m^-3, but quite low in Novermber, being 33.1 × 10⁴ m^-3 on average. The distributions of phytoplankton abundance and meso-zooplankton density were similar to that of SS concentration in trend, being high in the estuary of the Feiyunjiang River and declining towards the offshore zone. Phytoplankton abundance was not much related to environmental factors, except for in August（P ＜ 0.05）when it was negatively related to salinity. West Pacific tidal waves, inland runoff via rivers, coastal currents and sea topography are the factors that jointly determine the ecological processes of the estuaries of the Feiyunjiang River and Aojiang River. However, during the hight water season, inland runoff via rivers stands out as the dominant factor influencing plankton distribution.