Decrease in CO_2 efflux from northern hardwater lakes with increasing atmospheric warming

摘要

我们对于气候变化对来自富含碳酸盐的硬水湖的碳通量的潜在影响知之甚少。这项研究报告了过去二十年北美中部这样的湖泊中从二氧化碳源到汇所发生的一个变化以及年平均pH值的升高。所观察到的pH值及二氧化碳吸收量的变化被归因于由大气变暖诱导的春季冰层的减少，这种减少降低了冰下二氧化碳积累、升高了春季和夏季的pH值、并且增强了硬水湖中二氧化碳的化学吸收。%Boreal lakes are biogeochemical hotspots that alter carbon fluxes by sequestering particulate organic carbon in sediments and by oxidizing terrestrial dissolved organic matter to carbon dioxide (CO_2) or methane through microbial processes. At present, such dilute lakes release ～1.4 petagrams of carbon annually to the atmosphere, and this carbon efflux may increase in the future in response to elevated temperatures and increased hydrological delivery of mineraliz-able dissolved organic matter to lakes. Much less is known about the potential effects of climate changes on carbon fluxes from carbonate-rich hardwater and saline lakes that account for about 20 per cent of inland water surface area Here we show that atmospheric warming may reduce CO_2 emissions from hardwater lakes. We analyse decadal records of meteorological variability, CO_2 fluxes and water chemistry to investigate the processes affecting variations in pH and carbon exchange in hydrologically diverse lakes of central North America. We find that the lakes have shifted progressively from being substantial CO_2 sources in the mid-1990s to sequestering CO_2 by 2010, with a steady increase in annual mean pH. We attribute the observed changes in pH and CO_2 uptake to an atmospheric-warming-induced decline in ice cover in spring that decreases CO_2 accumulation under ice, increases spring and summer pH, and enhances the chemical uptake of CO_2 in hardwater lakes. Our study suggests that rising temperatures do not invariably increase CO_2 emissions from aquatic ecosystems.