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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We thank I. Phillips for estimates of lake area in Saskatchewan; D. Conrad for water chemistry data from Buffalo Pound Lake, Katherine Miller for compilation and analysis of Buffalo Pound data; J. Piwowar for assistance with GIS data; S. Pham and the University of Regina Limnology Laboratory for field work in 1996–2010; and Y. T. Prairie, E. G. Stets, J. A. Downing and D. E. Schindler for reviewing the manuscript. NSERC Canada, the Canada Foundation for Innovation, the Province of Saskatchewan, and the Canada Research Chair programme provided funding for this work.
The authors declare no competing financial interests.
Extended data figures and tables
Hardwater lakes of the Qu’Appelle catchment (triangles) were monitored every two weeks from May to September during 1995–2010 (ref. 19), and closed-basin lakes were monitored monthly (open circles) or annually (filled circles) during 2002–2010 (except 2006)29. Weekly monitoring of pH occurred at Buffalo Pound Lake (black triangle) during 1979–2007. All lakes are situated in prairie grassland ecozones with pronounced precipitation deficits (annual precipitation − potential evaporation) of 40–60 cm yr−1 (dashed lines)18,29. Source data
Extended Data Figure 2 Principal components analysis of the relationship between mean annual surface water pH, annual meteorological conditions and mean summer lake parameters during 1995–2010.
a, Ordination of mean summer pH in Qu’Appelle lakes (n = 6) during 1 May to 31 August in relation to mean annual meteorological conditions revealed that pH was correlated positively with mean annual and spring (not shown) temperatures, correlated negatively with the date of ice melt, and was unrelated to mean annual levels of precipitation or irradiance. Variables include log10-transformed mean annual temperature (temperature), total annual rainfall (rain), total annual precipitation (precipitation), total snowfall (snow), untransformed daily hours of bright sunlight (irradiance) and the calendar day of the year when ice was completely melted from the lake surface (ice melt date). b, Ordination of mean summer pH in relation to coeval chemical, hydrological and physical conditions in Qu’Appelle lakes, as well as indices of relevant global climate systems. Abbreviations include water temperature (T°CH20), total inorganic carbon (TIC), dissolved organic carbon (DOC), total dissolved nitrogen (TDN), log10-transformed chlorophyll a (Chl), conductivity (Cond), log10-transformed soluble reactive phosphorus (SRP), log10-transformed total dissolved phosphorus (TDP), log10-transformed dissolved ammonia/ammonium (NH4), turbidity (Secchi depth), log10-transformed volume of river inflow (inflow), dissolved oxygen (O2), log10-transformed dissolved nitrite + nitrate (NO3) and climate indices representing the Pacific Decadal Oscillation (PDO), the North Atlantic Oscillation (NAO) and the winter (SOIwinter) or annual (SOImean) Southern Oscillation Index. This analysis reveals that mean summer pH is correlated positively with the PDO and negatively with the SOI, consistent with the interpretation that warm winters and reduced ice cover result in higher summer pH in Qu’Appelle lakes. Source data
Extended Data Figure 3 Elastic net analysis to identify and rank predictors of changes in under-ice pH in Buffalo Pound Lake during winter.
Water quality parameters at 1.5 m above the lake bottom were analysed weekly using uniform methods during 1985–2003 from the date of ice-cover formation to the date of ice melt. Analysis was performed using 125 weekly observations with complete water chemistry. Parameters include concentrations of dissolved oxygen (O2), sodium (Na+), carbonate (CO32−), loge-transformed dissolved aluminium (logeAl), fluoride (F−), potassium (K+), loge-transformed orthophosphate (logePO43−), calcium (Ca2+), dissolved magnesium (Mg+), loge-transformed nitite + nitrate (logeNO3−), loge-transformed dissolved manganese (logeMn), bromide (Br−), total phosphorus (TP), dissolved iron (Fe), chloride (Cl−), loge-transformed ammonium (logeNH4+), bicarbonate (HCO3−), sulphate (SO42−) and temperature (temp). Coloured lines indicate how standardized regression coefficients (y axis, left) develop (right to left) as the initial pool of predictors (y axis, right) is refined by removing collinear and non-significant variables. Evaluation of the standardized coefficients of the most parsimonious model (vertical dashed line; equation under graph) demonstrates that changes in microbial metabolism (O2 decline × respiratory quotient of 1.2 = CO2 production)4,11 was the main factor regulating variation in water-column pH under ice, showing a nearly fourfold greater coefficient (0.14) than did either HCO3− or Ca2+ (0.04). Although dissolved logeAl, logeNH4+ and CO32− were also significant predictors of changes in winter pH (standardized coefficients 0.03–0.07), concentrations of these solutes (means ± s.e.m.; n = 17) were too low (<0.01 M) to regulate lake-water pH relative to the effects of changes in O2 (0.62 M), HCO3− (3.69 M) or Ca2+ (1.22 M). This analysis suggests that metabolically produced CO2 mainly regulates variation in winter pH by the production of carbonic acid (reduces pH), but that pH decline is slightly tempered by CO2-induced dissolution of sedimentary CaCO3, the main form of sedimentary carbon in Buffalo Pound9. Source data
Extended Data Figure 4 Effects of ice melt on water chemistry and spring carbon efflux from Buffalo Pound Lake, 1985–2003.
a, Mean pH recorded at 1.5 m above the lake bottom from four weeks before to four weeks after ice melt (week = 0). The rate of pH increase was linear with time (r = 0.96, P < 0.0001) with a slightly higher magnitude of increase (0.28 units) occurring in the two weeks after ice melt. b, Changes in mean concentration (mg C l−1) of total inorganic carbon (TIC) during spring. The maximum extent of TIC decline (6.57 mg C l−1; 14.6% of TIC stock at week 0) occurred during the two weeks after ice melt as a result of a combination of CO2 efflux (0.86 mg C l−1) and dilution by snowmelt (5.71 mg C l−1), as documented in d. c, Changes in concentrations of HCO3− and CO32−, and log10 of calculated CaCO3 saturation index. Patterns reveal that the decline in TIC was due to a decrease in HCO3− concentrations, rather than to the precipitation of CaCO3. d, Changes in mean concentrations (mg l−1) of chloride (Cl−) and fluoride (F−) during spring. Because concentrations of conservative tracers declined by an average of 12.7% in the two weeks after ice melt, yet TIC declined by 14.6%, we estimated that 1.9% of the decline in TIC stock at week 0 (0.86 mg C l−1) was caused by loss of inorganic C, particularly atmospheric evasion. Geochemical modelling of water chemistry observed at week 0 demonstrated that this magnitude of CO2 loss should increase the pH by 0.23 ± 0.08 units by week 2, a value equivalent to the observed increase in pH (see a). e, Observed changes in water temperature were too small (1.7 °C) to influence water chemistry strongly during the two weeks after ice melt. f, Calculated changes in chemically enhanced CO2 efflux modelled with observed water chemistry. Ice cover was assumed to prevent potential atmospheric CO2 exchange (grey shading), whereas most CO2 efflux seemed to occur one to two weeks after ice melt. All water samples were collected weekly at 1.5 m above the bottom of 3.0-m-deep Buffalo Pound Lake. Error bars represent s.e.m. (n = 17). During each year, sampling intervals were standardized to the documented week of ice melt (week = 0) before the calculation of long-term means. Changes in CaCO3 saturation and water-column were modelled with observed water-column parameters and Geochemists Workbench version 9.0.9 (see Methods). Source data
Extended Data Figure 5 Relationship between surface water pH in hardwater lakes of the Qu’Appelle River catchment and hydrologically closed lakes of southern Saskatchewan.
a, Qu’Appelle lakes (n = 6) were monitored every two weeks during summer 1995–2010 (ref. 19); closed-basin lakes (n = 20 in most years) were sampled monthly or seasonally (spring, summer and autumn) during 2002–2009 (except 2006, when there were no samples)29. b, Seasonal change in mean surface water pH of 15 closed-basin lakes monitored monthly during 2002–2009. Error bars represent one s.e.m. These patterns demonstrate that the pH of closed-basin lakes varied synchronously with that of Qu’Appelle lakes on both annual and seasonal scales, despite large differences in hydrological properties10,19. Source data
Extended Data Figure 6 Global map of regions where climatic conditions and soil types resemble those of southern Saskatchewan, Canada.
Hardwater lake distribution is not well quantified; however, this map depicts the region in which subsoil composition favours hardwater lakes and where climatic conditions produce substantial winter ice cover. Soil data originate from the FAO–UNESCO Soil Map of the World; regions highlighted in black have subsoil concentrations of CaCO3 in excess of 10% (for example Cambisol, Xerosol, Yarsol, Kastanozem and Chernozem soils). These data were overlain with temperature data (10 arcminute resolution, averaged monthly during 1950–2000) obtained from the WorldClim Global Climate Data that were restricted to regions where the monthly average temperature was below 0 °C for December–February (June–August for the Southern Hemisphere) but where the temperature was above 0 °C in October (April in the Southern Hemisphere), to exclude high-latitude lakes with permanent ice cover. The highlighted area (15,200,000 km2) has pronounced winter and calcareous soils, spanning the prairie and steppe regions of North America, South America, Europe and Asia. If we assume this region to have a similar surface water distribution to that of southern Saskatchewan, the area occupied by permanent lakes should be between 740,678 km2 (at 1:50,000 scale) and 538,892 km2 (at 1:250,000). If these basins also experienced a decline in CO2 efflux of 100 g C m−2 per summer during the past 15 years (Fig. 1d), global hardwater lakes may have sequestered 74.1 Mt (53.9 Mt at the coarser resolution) more C per summer than they did during the mid-1990s, a change greater than 5% of global efflux from dilute boreal lakes3,4. This value should increase in the future as ice cover declines. Source data
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Finlay, K., Vogt, R., Bogard, M. et al. Decrease in CO2 efflux from northern hardwater lakes with increasing atmospheric warming. Nature 519, 215–218 (2015) doi:10.1038/nature14172
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