不同水文情景下洪泽湖二氧化碳排放通量特征及影响因素

内陆水域二氧化碳(CO₂)排放是全球碳平衡的重要组成部分,全球CO₂排放通量估算通常有很大不确定性,一方面源于CO₂排放数据观测的时空离散性,另一方面也是缺少水文情景与CO₂排放通量关联性的研究.本文观测了2018年洪泽湖不同水文情景表层水体CO₂排放通量特征,并探讨其影响因素.结果表明,洪泽湖CO₂排放通量为丰水期((106.9±73.4) mmol/(m²·d))>枯水期((18.7±13.6) mmol/(m²·d))>平水期((5.2±15.5) mmol/(m²·d)),且碳通量由丰(310.2~32.0 mmol/(m²·d))、枯(50.8~2.2 mmol/(m²·d))、平(-17.3~39.8 mmol/(m²·d))3种水文情景的交替表现出湖泊碳源到弱碳汇的转变,空间上CO₂排放通量总体呈现北部成子湖区低、南部过水湖区高的分布趋势.洪泽湖CO₂排放对水文情景响应敏感,特别是上游淮河流域来水量的改变,是主导该湖CO₂排放时空分异的重要因子.丰水期湖泊接纳了淮河更多有机和无机碳的输入,外源碳基质的降解和矿化显著促进了水体CO₂的生产与排放,同时氮、磷等营养物质的大量输入,加剧了水体营养化程度,进一步提高CO₂排放量,间接反映出人类活动对洪泽湖CO₂变化的深刻影响.平、枯水期随着上游淮河来水量的减少,驱动水体CO₂排放的因素逐渐由外源输入转变为水体有机质的呼吸降解.此外,上游河口区DOM中陆源类腐殖质的累积与矿化能够促进CO₂的排放,而内源有机质组分似乎并没有直接参与CO₂的排放过程.研究结果揭示了水文情景交替对湖库CO₂排放的重要影响,同时有必要进行高频观测以进一步明晰湖泊的碳通量变化及其控制因素.

Characteristics and influence factors of carbon dioxide efflux from Lake Hongze under different hydrological scenarios

CO₂ emission from inland waters is an important component in assessing the global carbon budgets, while the large uncertainty during the budget estimation not only results from incomplete spatial coverage of sampling sites, but also from the lack of studies on the linkages between hydrological heterogeneity of inland waters and CO₂ emission. Here we examined the CO₂ efflux from Lake Hongze under different hydrological scenarios in 2018, to investigate the characteristics and influencing factors of CO₂. Our results indicated that the CO₂ fluxes in the wet season ((106.9±73.4) mmol/(m²·d)) was significantly higher than in the dry season ((18.7±13.6) mmol/(m²·d)), and further higher in the wet-to-dry transition season ((5.2±15.5) mmol/(m²·d)). The alternation of CO₂ efflux from the wet season to the dry, and further to the wet-to-dry transition season (310.2-32.0 mmol/(m²·d), 50.8-2.2 mmol/(m²·d), and -17.3-39.8 mmol/(m²·d), separately) suggested a shift from the net carbon source to a weak sink of CO₂ emission from Lake Hongze. The CO₂ efflux was generally low in the northern open water and high in the southern waterway region. The emission of CO₂ from Lake Hongze was highly sensitive to the hydrological scenarios, especially the inflow runoff from the upstream Huaihe River to the lake. In the rainy wet season, the lake received large quantities of organic and inorganic carbon inputs from the Huaihe River. The labile inorganic carbon could be easily transformed into CO₂, and the degradation and mineralization of terrestrial organic carbon could significantly promote the production and emission of CO₂. The input of nitrogen, phosphorus and other nutrients enhanced the eutrophication and further increased the emission of CO₂ from Lake Hongze, especially the upstream inflowing lake regions. Relatively low inflow runoff during the dry and the wet-to-dry transition seasons resulted in a prolonged water residence time and allochthonous input to autochthonous degradation of organic matter potentially favored the production of CO₂ emission in the water. We further found the accumulation and degradation of terrestrial humic-like could promote CO₂ emission, while autochthonous components of dissolved organic matter (DOM) were linked weakly to the process and emission of CO₂. Our results demonstrate the importance of hydrological scenarios fueling the emission of CO₂ from freshwater lakes, and it is necessary to conduct high-frequency observations to further clarify the efflux of CO₂ from lakes and the corresponding influencing factors.