Linking summer conditions to CO<inf>2</inf> undersaturation and CO<inf>2</inf> influx in a subtropical coastal lake

© 2015, The Japanese Society of Limnology. In this study, we tested the hypothesis that pCO2 and air–water CO2 fluxes in the surface waters of a subtropical lake vary on two time scales (diel and seasonally) and that CO2 concentrations would decrease during the day and in summer. We estimated the variability of pCO2 and the air–water CO2 flux from pH-alkalinity in four 48-h periods that were representative of each subtropical season. There was high variability in pCO2 and the air–water gas flux over 48 h, but there was no clear pattern between day and night. CO2 concentrations showed a strong positive correlation with heterotrophic bacterial biomass and a negative correlation with dissolved organic carbon concentrations and water temperature. The lake was predominantly diel and seasonally CO2 supersaturated; the highest CO2 efflux was observed in the spring and a CO2 influx was observed in summer. Our hypothesis was confirmed; pCO2 was lowest in summer and during the daytime in spring and summer due to physical and biological conditions that favoured photosynthetic activities. These findings suggest that temporal shifts in the microbial community and meteorological variables, which are indirectly related to temperature, may be important drivers of CO2 concentrations in Peri Lake. In conclusion, pCO2 and the air–water CO2 flux vary temporally (diel and seasonally) in the littoral zone of this subtropical coastal lake, with shifts between CO2 influx and efflux throughout the sampling periods