Mechanisms controlling the carbon stable isotope composition of phytoplankton in karst reservoirs

摘要

In order to systematically understand the mechanisms controlling the carbon stable isotope composition of phytoplankton (δ~(13)CPHY) in freshwater ecosystems, seasonal changes in δ~(13)CPHY and related environmental factors were determined in karst reservoirs from the Wujiang river basin, China. Substantial and systematic differences within seasons and reservoirs were observed for δ ~(13) C PHY, which ranged from -39.2‰ to -15.1‰. An increase in water temperature triggered fast growth of phytoplankton which assimilated more dissolved inorganic carbon (DIC), resulting in the increase of δ~(13)CPHY, δ~(13)CDIC and pH. When the concentration of dissolved carbon dioxide (CO_2) was less than 10 mmol L~(-1), phytoplankton shifted to using HCO3- as a carbon source. This resulted in the sharp increase of δ~(13)CPHY. The carbon stable isotope composition of phytoplankton tended to decrease with the increase of Bacillariophyta, which dominated in January and April, but tended to increase with the increase of Chlorophyta and Dinophyta, which dominated in July. Multiple regression equations suggested that the influence of biological factors such as taxonomic difference on δ~(13)CPHY could be equal or more important than that of physical and chemical factors. Thus, the effect of taxonomic differences on δ~(13)CPHY must be considered when explaining the δ~(13)C of organic matter in lacustrine ecosystem.