Assessing factors underlying variation of CO2 emissions in boreal lakes vs. reservoirs

Reservoirs and lakes were compared to test the hypothesis that they are similar with respect to factors driving the variation in CO2 emissions to the atmosphere. Understanding this variation is necessary for the assessment of the contribution of these freshwater ecosystems to the global carbon cycle. This study, in contrast to previous ones, included analyses of the relationships between CO2 emissions and microbial communities. Pooled data (lakes and reservoirs) showed that variations in CO2 emissions were strongly related to variations in temperature, dissolved organic matter (DOM) quality, and bacterial production (BP). Results also showed that lakes were characterized by higher water temperature, lower DOM quality, larger size of Daphnia, and enriched d13C zooplankton compared to reservoirs. Moreover, interactions within plankton communities and relationships between CO2 emissions and zooplankton d13C signatures differed in lakes vs. reservoirs, indicating among-system type differences in food web structure and carbon cycling. As a result of these ecosystem-type characteristics, CO2 emission variation was mainly explained by temperature and BP in lakes, and by DOM quality and the ratio of phytoplankton biomass to microheterotroph biomass in reservoirs. These results showed that differences in temperature and DOM quality between lakes and reservoirs translate into differences in microbial interactions and ultimately in the importance of factors driving CO2 emissions to the atmosphere. They indicated that considering microbial communities and environmental variables such as temperature and DOM quality can help improve our understanding of the variation in CO2 emissions from freshwater ecosystems.