Bacterial Activity in the Water Column and its Impact on the CO₂ Efflux

Abstract

As part of a comprehensive study intended to elucidate mechanisms that drive carbon dioxide (CO₂) emissions from hydroelectric reservoirs, we examined bacterial abundance and production in the water column of three hydroelectric reservoirs of different ages and their nearby lakes, in relation to temperature, dissolved organic carbon (DOC), Chlorophyll a, phytoplankton production and CO₂ fluxes from these ecosystems to the atmosphere. The summer values of bacterial production and bacterial specific production in each reservoir were similar to those in the nearby lakes. There was no clear evidence that the age of the reservoir per se had a strong effect on the measured bacterial activities, even though the highest values of these activities were found in the youngest reservoir. DOC and nutrient availability were among the major factors driving bacterial activities. DOC was indeed positively related to bacterial production, bacterial specific production and the proportion of bacteria with high nucleic acid content (i.e. bacteria with higher activity = % HNA) in these sites, where nutrients were, most of the time, found to be limiting for bacterial growth. Among the bacterial variables tested, the % HNA appeared to be important in determining changes in CO₂ emissions at least in reservoirs, where it explained 38% of the variance of CO₂ fluxes to the atmosphere. Such a relationship was not found in lakes. These results indicate that examining different aspects of the functioning of bacterial communities may help to understand the mechanisms underlying CO₂ emissions from aquatic ecosystems, and suggest that the relative importance of factors driving bacterial activities and CO₂ efflux may be quite different in lakes versus reservoirs.