Temporal Trends in Dissolved Inorganic Carbon in a Swedish Boreal Catchment

Sammanfattning: Inland waters are important systems for transforming, storing and transporting carbon along the aquatic continuum, but also by emitting carbon dioxide (CO2) and methane (CH4) to the atmosphere. In light of the last decades observed increase in dissolved organic carbon (DOC) in many inland waters across the northern hemisphere, a logical question arise whether other aquatic carbon species display similar trends. This study examined the measured concentrations of dissolved inorganic carbon (DIC) in a boreal catchment over a 14-year period. The objectives were to determine changes in DIC concentration over time and try to explain the causes for the observed changes. Data from 15 mostly forested sub-catchments were analyzed, both over the full time period, and grouped by season. Over the full 14-year period, only two of the sites exhibited significant trends in DIC concentration, both being negative. However, by seasonally grouping the data distinct patterns for the different seasons emerged. The autumn and winter data displayed no significant trends, whereas the spring flood data showed significant negative trends for almost all sites (14 out of 15). The summer data showed significant negative trends for seven sites, and positive for one site. The DIC concentration data were expectedly positively correlated with pH across most sites (13 out of 15). The correlation with DOC was negative for most sites (11 out of 15), possibly indicating different origins of the different carbon species. The DIC concentration was also negatively correlated with discharge for most sites (13 out of 15), suggesting a diluting effect with increased discharge. In conclusion, significant negative trends were observed during the spring flood and summer periods. Although the cause of these trends will require further investigation, the correlation analysis showed that the DIC concentration was closely related to the catchment hydrology. This suggests changes in terrestrial source areas where DIC is mobilized during spring and summer, and that these changes might continue during altered hydrometeorological conditions. The differences in DIC trends between sub-catchments further show the variability of the boreal landscape and highlight the need for local-scale process understanding when scaling to larger landscape units. We further conclude that trends in DIC concentration do not follow observed DOC changes over time, suggesting that DIC and DOC exports are mechanistically decoupled.