The effectiveness of controlling nitrogen(N)loading(in addition to phosphorus[P])to manage the eutrophication of aquatic ecosystems has been debated despite the role of N in producing algal biomass and toxins.Longterm...The effectiveness of controlling nitrogen(N)loading(in addition to phosphorus[P])to manage the eutrophication of aquatic ecosystems has been debated despite the role of N in producing algal biomass and toxins.Longterm,controlled tests of the efficacy of N loading reductions are largely missing from the scientific record,perhaps due to the historical focus on P control.To address this knowledge gap,we examined the results from a unique,long-term study conducted in 24 flow-through(2.5-month retention time)lake ecosystem-scale mesocosms in Denmark,operating since 2003 at two contrasting nutrient loading levels crossed with three temperature scenarios(ambient,IPCC(Intergovernmental Panel on Climate Change)A2 scenario,and A2+50%).For 1 year,the N loading,apart from groundwater inputs,was stopped in high nutrient loading mesocosms,while P loading was maintained.Wefollowed the changes in key environmental variables and systemmetabolismfor 5 years,including the 2 years prior to N loading reduction and 2 years after N loading resumption.The low nutrient loading treatments,which only received N and P fromgroundwater,were used as a reference.We found a strong effect of N loading on total N(TN),N oxides(NO_(2)+NO_(3)),and N:P ratios.After reducing the excess external N loading,which had lasted for 15 years,TN and N oxides declined to similar levels as those in the low nutrient treatments at all temperature scenarios and increased quickly when N loading was resumed.Algal biomass(as chlorophyll a)and ecosystem production and respiration were also affected.The results showed(1)a rapid response of water N concentrations to external N loading,(2)major ecosystem effects,including reduced algal biomass and system metabolism,and(3)overall low sensitivity in response to the IPCC temperature scenarios.This study was conducted under semi-natural conditions,providing strong experimental support for the key role of N at the ecosystem level in shallow lakes.Our results have profound implications for lake management and suggest that external N loading reductions may strengthen the recovery of shallow lakes from eutrophication.展开更多
基金supported by the AQUACOSM and AQUACOSM-plus projects funded by the European Union’s Horizon 2020 research and innovation programme under grant agreement nos.731065 and 871081 and AnaEE Denmark(anaee.dk)supported by the TÜB_ITAK-B_IDEB 2232 program(grant/award no.118C250)+3 种基金supported by the National Science Foundation of China(no.32371629)the Jiangxi Provincial Natural Science Foundation(20232ACB215006)supported by the Estonian Research Council(PRG1954 and PRG709)AQUACOSM/AQUACOSM-plus.
文摘The effectiveness of controlling nitrogen(N)loading(in addition to phosphorus[P])to manage the eutrophication of aquatic ecosystems has been debated despite the role of N in producing algal biomass and toxins.Longterm,controlled tests of the efficacy of N loading reductions are largely missing from the scientific record,perhaps due to the historical focus on P control.To address this knowledge gap,we examined the results from a unique,long-term study conducted in 24 flow-through(2.5-month retention time)lake ecosystem-scale mesocosms in Denmark,operating since 2003 at two contrasting nutrient loading levels crossed with three temperature scenarios(ambient,IPCC(Intergovernmental Panel on Climate Change)A2 scenario,and A2+50%).For 1 year,the N loading,apart from groundwater inputs,was stopped in high nutrient loading mesocosms,while P loading was maintained.Wefollowed the changes in key environmental variables and systemmetabolismfor 5 years,including the 2 years prior to N loading reduction and 2 years after N loading resumption.The low nutrient loading treatments,which only received N and P fromgroundwater,were used as a reference.We found a strong effect of N loading on total N(TN),N oxides(NO_(2)+NO_(3)),and N:P ratios.After reducing the excess external N loading,which had lasted for 15 years,TN and N oxides declined to similar levels as those in the low nutrient treatments at all temperature scenarios and increased quickly when N loading was resumed.Algal biomass(as chlorophyll a)and ecosystem production and respiration were also affected.The results showed(1)a rapid response of water N concentrations to external N loading,(2)major ecosystem effects,including reduced algal biomass and system metabolism,and(3)overall low sensitivity in response to the IPCC temperature scenarios.This study was conducted under semi-natural conditions,providing strong experimental support for the key role of N at the ecosystem level in shallow lakes.Our results have profound implications for lake management and suggest that external N loading reductions may strengthen the recovery of shallow lakes from eutrophication.
