Long-term and seasonal dynamic patterns and drivers of dissolved carbon in a shallow eutrophic lake.

Dissolved carbon is a crucial component of freshwater ecosystems and plays an important role in the Earth's carbon cycle. This paper delivers a groundbreaking exploration of dissolved carbon (DOC and DIC) variations spanning 12 years in a eutrophic lake where nutrient levels are gradually declining to reveal their spatial and temporal distribution patterns and the key drivers behind this variation. Our findings indicate that both DIC and DOC concentrations in Lake Chaohu exhibit a westward spatial gradient, with an overall upward trend in DIC levels from 2012 to 2023, contrasting with a downward trend in DOC. A similar variation trend in the inflowing rivers suggests that exogenous inputs predominantly dictate long-term carbon concentration changes in the lake. Additionally, a reduction of eutrophication levels in the lake may have also contributed to a decrease in endogenous DOC generated by algal metabolism. Seasonally, DIC and DOC concentrations display inverse trends throughout the year, with DOC levels closely tracking the seasonal patterns of the cyanobacterium Microcystis. Correlation and regression analyses reveal a significant positive relationship between Microcystis and DOC and a negative association with DIC, while Dolichospermum shows no correlation with either. This suggests that the impact of phytoplankton on DOC and DIC variations is species-specific. In Lake Chaohu, the annual succession of Microcystis may be a major driving factor for the intra-annual variations in DOC and DIC. Our findings deepen the comprehension of how phytoplankton modulates the intra-annual dynamics of DOC and DIC.