西亚试剂：Decrease in CO2 efflux from northern hardwater lakes with i

Boreal lakes are biogeochemical hotspots that alter carbon fluxes by sequestering particulate organic carbon in sediments1, 2 and by oxidizing terrestrial dissolved organic matter to carbon dioxide (CO2) or methane through microbial processes3, 4. At present, such dilute lakes release ~1.4 petagrams of carbon annually to the atmosphere3, 4, and this carbon efflux may increase in the future in response to elevated temperatures5 and increased hydrological delivery of mineralizable dissolved organic matter to lakes6, 7. 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 area4, 8. Here we show that atmospheric warming may reduce CO2 emissions from hardwater lakes. We analyse decadal records of meteorological variability, CO2 fluxes and water chemistry to investigate the processes affecting variations in pH and carbon exchange9, 10 in hydrologically diverse lakes of central North America. We find that the lakes have shifted progressively from being substantial CO2 sources in the mid-1990s to sequestering CO2 by 2010, with a steady increase in annual mean pH. We attribute the observed changes in pH and CO2 uptake to an atmospheric-warming-induced decline in ice cover in spring that decreases CO2 accumulation under ice, increases spring and summer pH, and enhances the chemical uptake of CO2 in hardwater lakes. Our study suggests that rising temperatures do not invariably increase CO2 emissions from aquatic ecosystems.

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