We investigated the abundance of different picophytoplankton groups and the phytoplankton pigment ratio in relation to environmental factors such as nutrients and suspended solids along a salinity gradient in the Changjiang River Estuary.The average numbers of Synechococcus spp.（Syn） and picoeukaryotes （Euk） were （2.7 ±5.1）×l03 and （1.1±1.4）×l03 cells mL-1,respectively.Prochlorococcus spp.（Pro） was only found in the high-salinity brackish water with the concentration of 3.0× 10^3 cells mL-1.Syn and Euk numbers both tended to increase offshore and Syn showed a larger variation in cell abundance than Euk.The contribution of picophytoplankton to total phytoplankton biomass increased with increasing salinity and decreasing nutrient concentrations from the estuary to the open ocean.The response of different picophytoplankton groups to environmental variables was different.Water temperature was more important in its control over Euk than over Syn,while nutrients were more important in their influence over Syn than over Euk.Phytoplankton pigment ratios were different in the three different ecological zones along the salinity gradient （i.e.,freshwater zone with 0-5 range,fresh and saline water mixing zone with 5-20 range,and high-salinity brackish water zone with 20-32 range）,where three different phytoplankton communities were discovered,suggesting that phytoplankton pigment ratios can be considered as a complementary indicator of phytoplankton community structure in the Changjiang River Estuary.
WANG BaoliLIU CongqiangWANG FushunLI SiliangSivaji Patra
This study quantifies surface denudation of carbonate rocks by the first application of in-situ cosmogenic36Cl in China. Concentrations of natural Cl and in-situ cosmogenic36Cl in bare carbonates from Guizhou karst areas were measured with isotope dilution by accelerator mass spectrometer. The Cl concentration varied from 16 to 206 ppm. The 36Cl concentrations were in range of (0.8-2.4)106 atom g-1 , resulting in total denudation rates of 20-50 mm ka-1 that averaged over a 104-105 a timescale. The 36Cl-denudation rates showed roughly a negative correlation with the local mean temperature. This preliminary observation may suggest the variations of proportions of chemical weathering and physical erosion in denudation process, depending upon local climatic conditions.
To determine the impact of damming on the geochemical behavior of dissolved inorganic carbon(DIC)in the Wujiang River basin,field measurements and samples were collected semimonthly for a year.The surface waters of the reservoirs contained concentrations of chlorophyll a up to 5.6 times higher than the upstream river.Compared with the entering waters,the contributions of HCO3-and dissolved CO2to DIC decreased,and the contribution of CO32-to DIC and the carbon isotopic composition of DIC(d13CDIC)increased in the surface waters of the reservoirs,while in the waters released,the DIC species showed reverse geochemical behaviors.The d13CDICranged from-10.2%to 2.5%,indicating that significant contributions were from carbonate weathering,photosynthesis,and respiration.After the damming of a river,the bioactivity of phytoplankton was enhanced,and this affected the geochemistry of DIC compared to an unimpacted river and d13CDICcan be used to discern these changes.High-frequency monitoring of river-reservoir systems is necessary to evaluate the efflux of CO2and provide a better understanding of the carbon sinks and sources in the impounded river.
