Net heterotrophy and CO2 evasion from a productive calcareous reservoir: Adding complexity to the metabolism-CO2 evasion issue

The role of lacustrine systems in the global carbon cycle is a major topic in freshwater research. Most lakes release inorganic carbon to the atmosphere owing to the imbalance of net metabolism toward respiration. Most research on this topic relies on data collected in natural low-alkalinity lakes, and it therefore remains unclear what role human-made calcareous lakes play in the current paradigm. In this study, we reported inorganic carbon fluxes and net ecosystem production in a calcareous human-made reservoir in Spain using mass balances of inorganic carbon and dissolved oxygen. The inorganic carbon loadings reported are among the largest found in the literature, and the inorganic carbon pool in the reservoir depends on the net hydrological balance. The reservoir was identified as a net emitter of carbon dioxide (CO2) and showed persistent net heterotrophy, although we could not find any clear dependence of CO2 emissions on the metabolic balance. The variability of inorganic carbon and the net ecosystem production were poorly correlated, suggesting that additional internal processes govern the transformation of the dissolved inorganic carbon in the water column. Considering the significant decrease in surface alkalinity occurring during the summer, carbonate precipitation may be the dominant process confounding the relationship between metabolism and inorganic carbon variability. These results indicate that the CO2 dynamics may be weakly coupled to net metabolism in calcareous systems and that net heterotrophy may occur simultaneously with eutrophic conditions and low dissolved organic carbon concentration in flushed systems such as reservoirs.