Preliminary Studies On Plankton Community Changes In The Changjiang Estuary And Adjacent Sea At Different Time Scales

Estuarine plankton communities can respond to global climate change and human activities at different timescales,and there are obvious relationships between the changes in plankton communities and water environmental factors.Hence plankton may serve as a bioindicator to monitor estuarine environmental conditions. Since the 1950s,many scientists have conducted an extensive number of studies on plankton seasonal changes in spatial distribution in the Changjiang Estuary and adjacent sea,but rare of them has focused on diel changes of picoplankton and viruses. In addition,nutrient and suspended matter discharges from the Changjiang River will tremendously decrease due to the water storage of the Three Gorges Dam,thus may lead to some changes in spatial distribution and seasonal variation patterns of plankton.Further more,the impacts of global climate change and human activities on estuarine eco-environment exhibit on a long time scale,and then interannual study on plankton community is necessary.Based on the data from cruise monitoring in July and November 2005 and in June 2006,anchor monitoring in June 2006,and historical observations from 1996 to 2005,we studied the changes in plankton community in Changjiang Estuary and adjacent sea at different timescales.The results are as follows:(1)Diel changesTime-series data sets on the fluctuations of the Chlorophyllα,the picoplankton and viruses with tide were obtained and treated by a tidal model based on a rectangle equation.The results reveal that tidal action is a major factor in affecting diel variations of chlorophyllα,picoplankton and viruses in the Changjiang Estuary.The chlorophyll a concentration varied with a semidiurnal period of～11 hr in surface waters,and the cell abundances of picoplankton and viruses～13 hr,coinciding with the tidal period.However,different periods were found in middle and bottom waters, due to the influences of tidal induced physical forcings such as resuspension and stratification.The diel variations of chlorophyllα,picoplankton and viruses are no significant correlation with the environmental factors(temperature,salinity,turbidity, inorganic nutrients).(2)Changes in summer and autumnTwo hundred and three phytoplankton species in summer and 298 in autumn were identified.Skeletonema costatum was the dominant species identified.The average cell abundance was lower in summer(5.48×10⁴ cells L^-1)than in autumn (2.70×10⁵ cells L^-1),however,the average chlorophyllαconcentration was higher in summer(2.34 mg m^-3)than in autumn(1.32 mg m^-3).The average diversity index(H') was higher in summer(1.51)than in autumn(0.86),as was the average evenness(J) (0.59 and 0.34,respectively).Spatial distribution of phytoplankton featured distinct regionality,and the seasonal variation was controlled by factors such as water source, nutrient,suspended matter,and so on.Although the spatial distribution and seasonal variation patterns were found no difference between before and after the water storage of the Three Gorges Dam,identified species number was higher than that before the dam,which may be related to the consequent changes such as reduced sediment discharge and intensified seawater intrusion after the dam.Cell abundances of picoplankton and viruses were simultaneously determined by flow cytometry.The average abundances of Synechococcus,picoeukaryotes, heterotrophic bacteria and viruses were at the order of 10⁷,10⁶,10⁹ and 10⁹ cells L^-1, respectively.Regression analysis results reveal that viral abundance was positively correlated with salinity,and negatively correlated with turbidity and inorganic nutrient concentrations,indicating that viruses can be the most sensitive to environmental changes compared with the picoplankton groups.Cell abundances of picoplankton and viruses were found changes after the water storage of the Three Gorges Dam.Particularly,viral abundance decreased to one order of magnitude. Therefore,viruses may serve as a high sensitive indicator to monitor estuarine environmental changes.(3)Interannual changesPlankton community structure has changed in the Changjiang Estuary in the last ten years.The annual average concentrations of suspended matter,COD,TIN and silicate have clearly decreased since 2000,which is attributed mainly to the Three Gorges Project.Although eutrophication has declined to a certain extent since 2000, N/P ratio was always much higher than 40 and Si/N ratio reduced from 1.35 in 1996 to 0.94 in 2005.As an ecological consequence to such nutrient balance changes,the dominance of diatoms decreased,dinoflagellates increasingly dominated the phytoplankton population,and the frequency of harmful algal blooms caused by dinoflagellates increased.Furthermore,due to the reduced diatom availability,the dominance of copepods decreased,and medusas increasingly dominated the zooplankton population.Nutrient ratios,therefore,are more important in controlling the stability of plankton community structure than are the amounts of contamination loadings from land.