Internal and external sources of CO2 release in subarctic lakes

Internal and external sources of CO₂ release in subarctic lakes

Most lakes are supersaturated with CO₂, and are thus net sources of CO₂ to the atmosphere. Lakes can only sustain annual net CO₂ emissions through either 1) mineralization of terrestrial carbon flushed into the system or 2) transfer of inorganic carbon from the terrestrial landscape, but the relative contribution of these sources are unknown. We studied 14 lakes in the Southern Swedish Scandes during the open-water season. We used loggers to infer CO₂-emissions and net internal production of CO₂ during the open water season, and DIC mass balance to determine winter CO₂-buildup and emission at spring ice-off. All lakes emitted CO₂ to the atmosphere on an annual basis, even though some took up CO₂ during the open water season. Emissions during spring ice off dominated (mean 7,5 g C m^-2 yr^-1) the yearly CO₂-balance of the lakes. The CO₂-emissions from the lakes increased with increasing DOC concentrations, similar to findings from many other regions. However, only a small part of the CO₂-emissions originated from the internal production of CO₂ (mean 31%), implying dominance of external CO₂ sources in these lakes. The external source contribution also increased over the DOC gradient, from 30% in clear lakes to 91% in high DOC-lakes. Our results highlight the importance of seasonal and landscape factors in maintaining net CO2 release from lake waters.