[Relationship between pCO2 and algal biomass in Xiangxi Bay in spring]

Water temperature (WT), pH, HCO3-, dissolved oxygen and the concentration of Chl-a in Xiangxi Bay were measured in spring. The partial pressure of carbon dioxide was calculated by the chemical balance and Henry's Law while CO2 flux by the thin-layer diffusion model. Results showed that pCO2 of surface water in Xiangxi Bay varied from 8.34 microatm to 168.70 microatm with a mean of 49.01 microatm. pCO2 increased along the stream with Chl-a decreased from upstream to the estuary. The mean of pCO2 was 117.92 microatm on the diurnal scale, ranging from 74.43 microatm to 168.70 microatm, and the mean concentration of Chl-a was 3.04 mg x m(-3) with the range from 2.22 mg x m(-3) to 4.55 mg x m(-3). There were statistically significant negative correlations between values for pCO2 and Chl-a (r = -0.844, P < 0.01). Phytoplankton photosynthesis in Xiangxi Bay played a crucial role in the spatial and temporal distribution of pCO2. The undersaturation observed suggests that surface waters in Xiangxi Bay represented a sink for atmospheric CO2 in spring, and the exchange rate of CO2 was estimated to be -35.17 mmol x (m2 x d)(-1).