Effects of climate change on deepwater oxygen and winter mixing in a deep lake (Lake Geneva): Comparing observational findings and modeling

Abstract

Low concentrations of dissolved oxygen remain a global concern regarding the ecological health of lakes and reservoirs. In addition to high nutrient loads, climate‐induced changes in lake stratification and mixing represent additional anthropogenic menace resulting in decreased deepwater oxygen levels. The analysis of 43 years of monitoring data from Lake Geneva shows no decreasing trend neither in the areal hypolimnetic mineralization rate nor in the extent of hypoxia. Instead, hypoxic conditions are predominantly controlled by deep mixing in winter and much less by the trophic variations over the past decades. To reproduce winter mixing, the one‐dimensional hydrodynamic model SIMSTRAT was specially adapted to deep lakes and run for several climate scenarios. The simulations predicted a decrease in the maximum winter mixing depth from an average of ∼172 m for 1981–2012 to ∼136 m and ∼127 m in response to predicted atmospheric temperatures between 2045–2076 and 2070–2101 according to Intergovernmental Panel on Climate Change scenarios. Concurrently, events with complete homogenization of temperature and oxygen in winter will decrease by ∼50%. Consequently, the hypolimnetic oxygen concentrations will significantly decrease. These results demonstrate that changes in deep mixing can have stronger impact than eutrophication on the deepwater oxygen levels of oligomictic lakes.

Tributaries affect the thermal response of lakes to climate change

“…Area covered by regional climate models (dark red dashed-dotted line) with (a) predicted air temperature increase T in the near-future (blue, 2030-2049) and far-future (orange, 2080-2099) for medium (thick lines) and upper/lower estimates (thin lines) under the A1B emission scenario (CH2011, 2011). fifth winter but is predicted to become less frequent with ongoing climate change (Perroud and Goyette, 2010;Schwefel et al, 2016). Whereas the global average lake surface temperature has increased by ∼ 0.34 • C decade −1 between 1985and 2009(O'Reilly et al, 2015, the Rhône River supplying ∼ 75 % of LG's inflow has experienced a temperature increase of ∼ 0.21 • C decade −1 from 1978 to 2002 (Hari et al, 2006).…”

“…The model calculates heat fluxes and vertical mixing driven by wind and the internal wave field using a k − ε turbulence closure scheme. It has been adapted to and validated for multiple lakes including Lake Zürich , LG (Perroud and Goyette, 2010;Schwefel et al, 2016), Lake Neuchâtel , Lake Constance (Fink et al, 2014b;Wahl and Peeters, 2014) and LB (Råman Vinnå et al, 2017). The model contains seven tunable parameters, including p 1 (irradiance absorption), p 2 (sensible heat flux) and K (vertical light absorption) for heat flux adjustments from the atmosphere to the lake.…”

“…The internal seiche energy balance can be adjusted through α (production), C Deff (loss by bottom friction) and q (vertical distribution of turbulent kinetic energy). To include the effect of seasonally varying stratification strength, we followed Schwefel et al (2016) and varied α: α S for summer (April to September) and α W for winter (October to March), where α S > α W . Here we used the best-fit parameter setup (Table 3) already established and validated for LG and LB by Schwefel et al (2016) and Råman Vinnå et al (2017).…”

“…Studies of climate impact typically do not address these sorts of subtleties in lake dynamics due to a lack of data or difficulties in predicting future temperature and discharge conditions (Fenocchi et al, 2017). Several studies of large lakes suggest that major tributaries play only a minor role in climate-induced warming and deep-water oxygen renewal (Fink et al, 2014a;Schwefel et al, 2016). Mediumand smaller-scale lakes are, however, more abundant than large lakes (Verpoorter et al, 2014) and exhibit a greater degree of sensitivity to point sources of anthropogenic thermal input which can affect temperature and stratification (Kirillin et al, 2013;Råman Vinnå et al, 2017).…”

“…River temperature and suspended sediment content determine water density and, by extension, the depth of river plume intrusions into downstream lakes or reservoirs. The depths and volumes of river intrusion plumes determine deep-water oxygen renewal, especially for deeper lakes where climate-related warming can reduce seasonal deep convective mixing and thereby deplete deep-water oxygen (Schwefel et al, 2016). Major (deep penetrating) river intrusion events typically occur due to flooding, which flush large sediment loads into the river (Fink et al, 2016).…”