Evaporation and groundwater fluxes are thought to regulate hydrologic variability in lakes of the northern Great Plains, but little is known of how the relative importance of these processes may vary in time or space. To address this issue, we measured the isotopic composition of water (d 18 O, d 2 H) from 70 closed-basin lakes in southern Saskatchewan, Canada. All lakes occupied endorheic basins along a long gradient of salinity (0.2-115 g total dissolved solids L 21 ). Lakes exhibited synchronous seasonal changes in salinity (synchrony, S 5 0.78) and d 18 O (S 5 0.84) during the dry summer of 2003 (,195 mm rain), whereas coherence was reduced to 0.56 and 0.22, respectively, during the wet summer of 2004 (,295 mm rain). However, despite evaporative enrichment of isotopic ratios during dry summers, hydrologic balances were regulated mainly by changes in water inflow (I) rather than evaporation (E) in both wet and dry years, with particularly strong influence of inflow (lowest E : I ratio) in dry southwestern regions. Analysis of isotopic composition also identified winter precipitation or groundwater as the most influential source of water to most lakes, despite only ,30% of annual precipitation being delivered during winter. Therefore, although seasonal variability in lake chemistry was influenced by evaporation during summer, long-term mean chemical characteristics of prairie lakes were regulated mainly by changes in winter precipitation or groundwater influx.Lakes are abundant in the northern Great Plains (Last 1992) despite intense evaporation (Pham et al. 2008) and net precipitation deficits of 40 cm yr 21 to 60 cm yr 21 (Laird et al. 1996). In general, inter-annual variability of meteorological conditions that affect the persistence of lakes (temperature, seasonal precipitation) is regulated by the interplay between air masses arising over the Arctic, Pacific Ocean, and Gulf of Mexico (Bryson and Hare 1974), as well as global atmosphere-ocean systems such as the El Niñ o-Southern Oscillation (ENSO; Trenberth and Hurrell 1994), North Atlantic Oscillation (NAO; Hurrell 1995), and the Pacific Decadal Oscillation (PDO; Mantua et al. 1997). In addition, at a regional scale, the spatial and temporal variability of prairie lake hydrology is also affected by groundwater fluxes (van der Fritz et al. 2000) and by an unusually high supply of runoff from large catchments, most of which is derived from winter precipitation (Steppuhn 1981;Akinremi et al. 1999). However, despite these generalities, little is known of the specific conditions under which evaporation or water influx may control changes in lake chemistry and persistence, nor of the relative importance of summer and winter precipitation in the hydrologic budget of local and regional lakes. Consequently, an improved understanding of the basic hydrology of prairie lakes is needed to both forecast the effects of future climate change on lakes of the northern Great Plains, and to better interpret paleolimnological records of past climate variability in th...

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Six hard-water lakes were sampled May-August for 14 yr in a 52,000 km 2 catchment to identify the mechanisms that regulate the spatial and temporal variability of net atmospheric exchange of CO 2 of lakes on the Northern Great Plains. Annual mean daily fluxes ranged from 2100 to .200 mmol C m 22 d 21 , while pCO 2 values varied between 0.3 and 5500 Pa. We observed periods of net CO 2 uptake (1995, 2000) and release (1998, 2006) resulting in synchronous variations in net CO 2 flux among lakes. Furthermore, pCO 2 , pH, and chemical enhancement of CO 2 influx all varied coherently among sites. Interannual variation in net CO 2 flux and pCO 2 was correlated strongly with pH, correlated weakly with other physical and chemical conditions, and was uncorrelated to algal biomass, productivity, or ecosystem respiration. In contrast, spatial variability of water-column pCO 2 was correlated negatively to concentrations of soluble reactive phosphorus, total dissolved nitrogen, pH, and gross primary productivity, suggesting an important role of lake metabolism at large spatial scales. Finally, comparison with an additional 20 saline lakes demonstrated that changes in mean annual pH, pCO 2 , and CO 2 flux during 2002-2007 were coherent in diverse lakes within a region of .100,000 km 2 and suggest that climatic control of pH and pCO 2 had an unexpectedly great effect on net CO 2 flux through productive hard-water lake ecosystems.

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 (CO2) 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 mineralizable 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 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 exchange 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.

Eutrophication can initiate sudden ecosystem state change either by slowly pushing lakes toward a catastrophic tipping point beyond which self-reinforcing mechanisms establish an alternate stable state, or through rapid but persistent changes in external forcing mechanisms. In principle, these processes can be distinguished by determining whether historical changes in focal parameters (phytoplankton) exhibit transient (rising then declining) or continuously-elevated variability characteristic of alternate stable states or a "paradox of enrichment," respectively. We tested this hypothesis in the south basin of Lake Winnipeg, Canada, a site with intense blooms of N 2 -fixing cyanobacteria since 1990, but for which little is known of earlier limnological conditions, causes of eutrophication, or whether modern conditions represent a alternate stable state. Paleolimnological analysis revealed that the basin was naturally mesotrophic (15-20 lg P L 21