Inland waters, include lakes, are important sources of atmospheric greenhouse gas emissions and organic carbon pool. The changes of lake carbon budget and the conversion of source/sink function caused by warming and nutrient load increase are the key scientific issues of regional carbon budget estimation. The middle and lower reaches of the Yangtze River are the main distribution areas of freshwater lakes in China. In the past few decades, these lakes have been widely affected by climate change and human activities, and the understanding of their carbon cycling processes in response to climate warming and nutrient loading is still limited. In this study, Lake Caizi and Lake Chaohu, which present different nutrient levels in the middle and lower reaches of the Yangtze River, was taken as the research objects. A 3-year field observation of CO₂ and CH₄ fluxes across the lake - atmosphere interface and related physicochemical parameters were conducted in both lakes. With laboratory experiments of nutrient addition in the microcosm, the study systematically investigated the impact of nutrient loading on the carbon mineralization processes of lake ecosystems. Then, the carbon budget of lakes with different nutrient levels in the middle and lower reaches of the Yangtze River was quantitatively estimated. The results showed that: (1) CO₂ and CH₄ fluxes from lakes with different nutrient levels exhibit certain spatiotemporal dynamics. The spatial variation characteristics of CO₂ flux from Lake Caizi and Lake Chaohu were not obvious, but the obvious seasonal patterns with CO₂ emissions in autumn and CO₂ absorption in other seasons were recorded. The CH₄ flux exhibited significant spatiotemporal variability, with the emission peak occurring in summer. In addition, the closer to the shoreline, the higher the CH₄ flux were found. (2) The spatiotemporal variation of CO₂ flux in lakes with different nutrient levels was mainly controlled by the dissolved oxygen (DO) and dissolved CO₂ concentration. The spatiotemporal variation of diffusive CH₄ flux was mainly regulated by dissolved CH₄ concentration, sediment temperature (Ts), and dissolved oxygen (DO), while the total CH₄ flux was only affected by dissolved CH₄ concentration. (3) There was a priming effect on CO₂ production in lakes when additive N and P nutrient loading occurred, and this effect was stronger in lakes with relatively high nutrient levels. The effects of N and P loading on CH4 production in lakes were varied. N addition inhibited CH₄ production in lake water, while P addition significantly promoted the production rate of CH₄ in lakes. In addition, the additive N and P loading lead to a decrease in DO in the water, which inhibited CO2 production and promoted CH₄ production in lakes. (4) Lake Caizi and Lake Chaohu were important CH₄ sources, with annual rates of approximately 1.54 g m^-2 yr^-1 and 2.49 g m^-2 yr^-1, respectively. Bubble emissions of CH₄ were the main pathway for entering the atmosphere. The functions of lakes in the middle and lower reaches of the Yangtze River as CO₂ sources or sinks varied between different years. During the observation period, Lake Chaohu, which had relatively high nutrient levels, was a CO₂ sink, while Lake Caizi showed a weak CO₂ source or sink. The results revealed the response mechanism of key carbon cycle processes to global change, providing necessary basic data and theoretical basis for the compilation of regional greenhouse gas emission inventories and carbon balance estimation in China.

Methane; Carbon dioxide; Inland waters; Lake; Trophic levels