Dynamic carbon budget of a large shallow lake assessed by a mass balance approach.

To study the role of large and shallowhemiboreal lakes in carbon processing, we calculated a3-year carbon mass balance for Lake Vo˜rtsja¨rv (Estonia)based on in situ measurements. This balance tookinto account hydrological and biogeochemical processesaffecting dissolved inorganic (DIC), dissolvedorganic (DOC) and particulate organic (POC) carbonspecies. Accumulation varied greatly on a seasonal andyearly basis. The lake exported carbon during most ofthe year except during spring floods and in late autumn.In-lake processes were responsible for exporting POCand storing DOC while DIC switched between storageand export. The carbon cycle was alternatively dominatedin 2009 by biogeochemical processes and in2011 by riverine fluxes, whereas in 2010 the twoprocess types were of the same magnitude. Theseresults suggest that the role of large shallow lakes likeVo˜ rtsja¨rv in the global C cycle is equally driven byhydrological factors, in particular seasonal water levelchanges, and by biogeochemical in-lake reactions.