Mechanisms And Controlling Factors Of Stable Carbon Isotope Fractionation By Phytoplankton In Freshwater

Phytoplankton drives aquatic carbon cycle and its stable carbon isotope composition(?¹³C_phyto)is of great significance for understanding the global carbon biogeochemical cycle.The cascade reservoirs formed after damming in karst river include different phytoplankton community structures in different evolution stages and are the ideal place for studying the carbon isotope fractionation by phytoplankton in freshwater.In this paper,mechanisms and controlling factors of carbon isotope fractionation by phytoplankton inWujiang cascade reservoirs was studied by combining the mechanism of carbon cycle in karst impounding river and environmental factors.The main conclusions are as follows:1)The water temperature?T?in the study area showed obvious seasonal changes with air temperature.The variation trend of dissolved oxygen?DO?was opposite to that of T and the seasonal variation of pH was not obvious.T,pH,DO in the reservoir were all higher than those in the released water.The concentration of CO₂,dissolved inorganic carbon?DIC?and particulate organic carbon?POC?,?¹³C_POC and?¹³C_DICIC all showed obvious temporal and spatial variations.?¹³C_POCOC in the warm seasons were more positive than that in the cold seasons.?¹³C_POCOC in the reservoir was more negative than that in the rivers.2)Phytoplankton community structure in the study area had obvious temporal and spatial variations.In 2011-2012,cyanobacteria,bacillariophyta and pyrophyta were dominant in summer,while cyanobacteria and bacillariophyta were dominant in other seasons.Wujiangdu and Hongfeng Reservoir took cyanobacteria as dominant groups,while rivers and other reservoirs were dominated by bacillariophyta.In 2015-2016,pyrophyta was dominant in spring and bacillariophyta was dominant in other seasons.Wujiangdu Reservoir took pyrophyta as dominant groups,while rivers and other reservoirs were dominated by bacillariophyta.3)Carbon isotope fractionation??_p?by phytoplankton had obvious temporal and spatial variations.?_p was from 8.94‰to 21.76‰with an average of 14.18‰in the study area.?_p was larger in the warm seasons than that in the cold seasons,and?_p in Hongfeng Reservoir was larger than that in other reservoirs.There were significant positive correlations between?_p and T,pH,chlorophyll concentration?Chl?,Chl/CO₂and biomass of cyanobacteria and chlorophyta,and a significant negative correlation between?_p and CO₂ concentration.?_p had no obvious relationship with F_v/F_m representing the photosynthetic efficiency.These indicate that,with the increase of T,the biomass of phytoplankton increases and the amount of CO₂ consumed increases,eventually leading to the increase of?_p.