Dissolved organic carbon export and internal cycling in small, headwater lakes

Carbon (C) cycling in freshwater lakes is intense but poorly integrated into our current understanding of overall C transport from the land to the oceans. We quantified dissolved organic carbon export (DOC_X) and compared it with modeled gross DOC mineralization (DOCR) to determine whether hydrologic or within-lake processes dominated DOC cycling in a small headwaters watershed in Minnesota, USA. We also used DOC optical properties to gather information about DOC sources. We then compared our results to a data set of approximately 1500 lakes in the Eastern USA (Eastern Lake Survey, ELS, data set) to place our results in context of lakes more broadly. In the open-basin lakes in our watershed (n = 5), DOC_X ranged from 60 to 183 g C m⁻² lake area yr⁻¹, whereas DOCR ranged from 15 to 21 g C m⁻² lake area yr⁻¹, emphasizing that lateral DOC fluxes dominated. DOC_X calculated in our study watershed clustered near the 75th percentile of open-basin lakes in the ELS data set, suggesting that these results were not unusual. In contrast, DOC_X in closed-basin lakes (n = 2) was approximately 5 g C m⁻² lake area yr⁻¹, whereas DOCR was 37 to 42 g C m⁻² lake area yr⁻¹, suggesting that internal C cycling dominated. In the ELS data set, median DOC_X was 32 and 12 g C m⁻² yr⁻¹ in open-basin and closed-basin lakes, respectively. Although not as high as what was observed in our study watershed, DOC_X is an important component of lake C flux more generally, particularly in open-basin lakes.