Limnetica 42

Reservoirs are significant sources of greenhouse gases (GHG), such as carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O), to the atmosphere. These systems receive and metabolize a larger amount of organic and inorganic carbon and nitrogen from their watersheds than lakes, resulting in the production of CO₂, CH₄ and N₂O. Despite their global relevance, there are still important uncertainties regarding the magnitude, variability and drivers of their emissions that undermine global estimates. Therefore, a comprehensive understanding of the origin of these emissions is required. Here, I investigate the fluxes of CO₂, CH₄ and N₂O and their concentrations in the water column of twelve Mediterranean reservoirs during the stratification and mixing periods to discern the main pathways involved in their production and the spatial and seasonal variability among these gases and their emissions and radiative forcing. Finally, I provide a theorical framework to understand GHG emissions as a response of reservoirs to eutrophication and external forcing. I integrate Margalef’s ideas about how eutrophication perturbs the biogeochemistry of inland waters with the main findings of my previous work to analyze how the C, N and P inputs from reservoir watersheds modify the biogeochemical cycling of C, N, P and O, and determine the production and emission of CO₂, CH₄, and N₂O. This perturbation effect is especially notable for CH₄, and N₂O emissions, which increase significantly in eutrophic waters, even exceeding the climate forcing of CO₂. Therefore, emission of GHG should be seen as part of the reservoir response to the external forcing that displaces a fraction of the materials to the atmospheric boundary.