Stable carbon isotope variations in surface bloom scum and subsurface seston among shallow eutrophic lakes

Carbon stable isotope analysis of surface bloom scum and subsurface seston samples was conducted in shallow eutrophic lakes in China during warm seasons from 2003 to 2004. δ13C values of bloom scum were always higher (averaged 5‰) than those of seston in this study, and the possible reasons were attributed to (i) direct use of atmospheric CO2 at the air-water interface, (ii) decrease in 13C fractionation due to higher carbon fixation, (iii) active CO2 transport, and/or (iv) HCO3− accumulation. Negative correlation between δ13Cscum − δ13Cseston and pH in the test lakes indicated that phytoplankton at the subsurface water column increased isotopic enrichment under the carbon limitation along with the increase of pH, which might in turn decreased the differences in δ13C between the subsurface seston and the surface scums. Significant positive correlations of seston δ13C with total concentrations of nitrogen and phosphorus in water column suggested that the increase in δ13C of seston with trophic state was depending on nutrient (N or P, or both) supply. Our study showed that δ13C of phytoplankton was indicative of carbon utilization, primary productivity, and nutrient supply among the eutrophic lakes.