2.1. Field sampling and measurements

China, situated in eastern Asia, predominantly occupies the temperate climatic zone, with only marginal portions extending into the subtropical and tropical zones. The country boasts an impressive tally of approximately 2700 lakes, each spanning an area exceeding 1 km² in China, contributing to a cumulative lake expanse covering 81,415 km² [12,34]. Thirteen field research campaigns were conducted on Chinese lakes between 2017 and 2021 to measure the pCO₂ values. Detailed information on the sampling campaigns has been provided in Table S1. To compare the seasonal variations of pCO₂ in the lakes, seven lakes with different trophic states, which are ice-free all year round, were sampled in January, April, July, and November of 2019. These included the Baipenzhu Reservoir, Zhelin Lake, Xinfengjiang Reservoir, Fengshuba Reservoir, Poyang Lake, Gaoyou Lake, and Hongze Lake. To compare the effect of trophic variations on lake pCO₂, a total of 43 lakes were sampled in summers between 2017 and 2021 (Table 1). These lakes were distributed over five different regions of China: eight in Northeast China, four in the Yangtze River region, five in the Huang-Huai-Hai region, four in the Greater Pearl region, and twenty-three in the Qingzang Plateau (Table 1). The sampled lakes in the Qingzang Plateau included eight freshwater lakes and fifteen saline lakes.

Detailed information on the sampling lakes.

Gas samples were collected at 6–10 sampling stations from every lake on average using the headspace equilibrium method as described in Supplementary Materials [7,38,39]. The measurements of CO₂ concentrations were made with a gas chromatography analyzer (Agilent, 7890B). Approximately 2 L of water was collected at every station at the water depth of 0.5–1 m for measuring water quality parameters in the laboratory by a standard procedure [40,41], including total nitrogen, total phosphorus, chemical oxygen demand, and chlorophyll a. The water samples were placed in a portable refrigerator and transported to the laboratory within 24–48 h. During gas and water sampling, water pH, salinity, water temperature, and electrical conductivity were all determined in situ using a portable multi-parameter water quality analyzer (YSI 6600, U.S.). Wind speed at 1.0 m above the water surface was measured by a portable anemometer (Pro'sKit MT-4615, China), while lake clarity was measured by a Secchi disc. The water and gas sampling were conducted between 9:00 a.m. and 2:00 p.m.