Future projections of temperature and mixing regime of European temperate lakes

Abstract

The physical response of lakes to climate warming is regionally variable and highly dependent on individual lake characteristics, making generalizations about their development difficult. To qualify the role of individual lake characteristics in their response to regionally homogeneous warming, we simulated temperature, ice cover, and mixing in four intensively studied German lakes of varying morphology and mixing regime with a one-dimensional lake model. We forced the model with an ensemble of 12 climate projections (RCP4.5) up to 2100. The lakes were projected to warm at 0.10-0.11 • C decade −1 , which is 75 %-90 % of the projected air temperature trend. In simulations, surface temperatures increased strongly in winter and spring, but little or not at all in summer and autumn. Mean bottom temperatures were projected to increase in all lakes, with steeper trends in winter and in shallower lakes. Modelled ice thaw and summer stratification advanced by 1.5-2.2 and 1.4-1.8 days decade −1 respectively, whereas autumn turnover and winter freeze timing was less sensitive. The projected summer mixed-layer depth was unaffected by warming but sensitive to changes in water transparency. By midcentury, the frequency of ice and stratification-free winters was projected to increase by about 20 %, making ice cover rare and shifting the two deeper dimictic lakes to a predominantly monomictic regime. The polymictic lake was unlikely to become dimictic by the end of the century. A sensi-tivity analysis predicted that decreasing transparency would dampen the effect of warming on mean temperature but amplify its effect on stratification. However, this interaction was only predicted to occur in clear lakes, and not in the study lakes at their historical transparency. Not only lake morphology, but also mixing regime determines how heat is stored and ultimately how lakes respond to climate warming. Seasonal differences in climate warming rates are thus important and require more attention. Recent studies highlighted that lakes respond quite individually to climate change. Although surface water temperature (T s ) is perhaps the most predictable indicator of warming , trends in T s remain highly variable at a global scale (O'Reilly et al., 2015). Deep water temperatures respond less predictably to warming and have been Published by Copernicus Publications on behalf of the European Geosciences Union.

Global lake responses to climate change

“…environment of lakes, especially their seasonal mix ing regimes 120 (Box 1). In response to climate induced variations in these lake surface conditions, the mix ing regimes of lakes are projected to change through time 11,121,122 , with numerous consequential implications for lake ecosystems.…”

“…The mixing regimes of marginal lakes have also been described as being very sensitive to changes in water clarity 122,129 . Specifically, a browning of lake surface waters (resulting in a decrease in water transparency), due mainly to terrestrial inputs of dissolved organic matter [130][131][132] , affects the depth at which shortwave radia tion is absor bed within a lake.…”

Simulations of future changes in thermal structure of Lake Erken: proof of concept for ISIMIP2b lake sector local simulation strategy

“…For example, global average warming rates of 0.34 • C per decade have been observed between 1985by O'Reilly et al (2015. Hypolimnetic temperature responds less clearly to warming and has been observed to be warming, cooling or not changing significantly with increasing air temperature (Shimoda et al, 2011;Butcher et al, 2015;Winslow et al, 2017). And, A. I.…”

“…Ayala et al: Simulations of future changes in thermal structure of Lake Erken these changing water temperatures have also led to an increased stability and duration of stratification (Butcher et al, 2015;Kraemer et al, 2015). A final consequence of warming lake temperature is projected to be the shift in the mixing regime (Kirillin, 2010;Shimoda et al, 2011;Shatwell et al, 2019;. For example, loss of ice cover in deep lakes is likely to turn amictic lakes into cold monomictic lakes and cold monomictic lakes into dimictic lakes (Nõges et al, 2009).…”

Attention: Incorrect implementation/interpretation of one of the models

“…The latter is not resolved in FLake numerically, but parameterized via few "shape factor"-constants related to the spatial integrals over the stratified layer. Several recent publications, including co-authorship of one of the authors of the present study, (Shatwell et al, 2016;Kirillin et al, 2017;Shatwell et al, 2019;Su et al, 2019) discussed the appropriate choice of the shape factor constants in FLake and have demonstrated that the set of constants used in the NWP-version of the model should be amended if the vertical thermal structure is in question apart from the lake surface temperatures. In particular, the unrealistically weak deep stratification and the corresponding high depth of the surface mixed layer, as reported in the present study, are the results of applying the NWP-constants together with the maximum lake depth as the model lake depth.…”