Invasive primary producers modulate carbon fluxes and associated carbon budgets in temperate shallow lakes.

Lowland shallow lakes are the receiving environments of nutrients and organic carbon from the catchment area. In temperate areas, the synergic action of nutrients and mild temperatures induce carbon emissions from these systems. However, this trend might be modulated by the trophic state of lakes and by their productivity. In this study, we consider blooms of invasive submerged aquatic vegetation (SAV) and cyanobacteria as a valuable proxy for eutrophication and explore their role in carbon pools and associated budgets in temperate shallow lakes. We calculated the mass carbon budget of two large shallow lakes, characterized by different trophic states and colonized by varying degrees of invasive SAV and cyanobacteria, basing on annual carbon pools (input, output, gas exchange, burial) and aquatic metabolism. The oligo-mesotrophic lake behaved as an annual CO2 and CH4 source toward the atmosphere (81.2 ± 14.8 g C m-2 yr-1), mainly due to dominant benthic heterotrophic metabolism, whereas the mesotrophic lake behaved as an annual sink (-6.7 ± 9.7 g C m-2 yr-1), mainly because of a much higher net carbon uptake by invasive SAV and cyanobacteria. In the mesotrophic lake, the fast-growing metabolism of the invasive primary producers also resulted in a strong buffer capacity with respect to the carbon export from the lake. Our study highlights the major role played by the littoral lacustrine zones in the control of regional/global carbon cycle, especially in densely vegetated systems. We suggest that the interplay between eutrophication and biological invasions can switch lakes from carbon source to sink.