From dimictic to monomictic: Empirical evidence of thermal regime transitions in three deep alpine lakes in Austria induced by climate change

Abstract

Duration and timing of thermal stratification and the period of vertical mixing are crucial for internal chemical and biological processes of deep temperate lakes. Climatic changes induced a prolongation of thermal stratification in many lakes over the last decades. This study provides a comparative assessment of observed climate‐induced changes in thermal and limnological properties of three originally dimictic temperate lakes. We analysed 41 years of vertical sampling data of Lake Irrsee (IR), Mondsee (MO) and Hallstätter See (HA) located in the Salzkammergut lake district of Austria. We examined temperatures in surface and bottom waters, quantified changes in thermal regimes and investigated the development of hypoxia, anoxia and phosphorus concentrations of the three lakes from 1975 to 2015. Mean annual surface‐water temperatures increased substantially, whereas bottom‐water temperatures showed insignificant trends in IR and MO and a slight rising trend in HA. Resulting higher vertical temperature gradients caused stronger thermal stabilities of the lakes. Based on calculations of daily thermal stability, all three lakes showed a significant prolongation of the stratification period with +31 days (±7 days) in IR, +37 days (±7 days) in MO and +28 (±8 days) in HA. We observed greater changes in the timing of onset of stratification compared to the changes in the timing of stratification termination (IR onset −19 days versus termination +11 days, MO onset −21 days versus termination +16 days, HA onset −16 days versus termination +12 days). We found that expanding seasonal hypoxia and anoxia were correlated to prolonged seasonal stratifications and to increasing phosphorus concentrations in bottom‐water layers. However, higher phosphorus concentrations in bottom waters did not raise the lakes′ volume‐weighted total phosphorus concentrations. Under ongoing climate change, alterations in the thermal characteristics will inevitably lead to a transition of mixing regimes from a dimictic to a monomictic mixing pattern in originally ice‐covered temperate lakes. Our investigation revealed that this transition will be earlier in larger and deeper lakes.

Future projections of temperature and mixing regime of European temperate lakes

“…The comparatively rapid increase in winter air temperature was projected to strongly affect ice cover and winter stratification, and push the two deeper dimictic lakes (Stechlinsee and Arendsee) towards a monomictic regime. Such shifts have been projected as a consequence of climate warming for temperate stratified lakes (Ficker et al, 2017;Kirillin, 2010;Livingstone, 2008). Both of these lakes switched regularly between monomixis and dimixis and the climate-induced transition between regimes is projected to be gradual.…”

“…Although surface water temperature (T s ) is perhaps the most predictable indicator of warming (Adrian et al, 2009), trends in T s remain globally highly variable (O'Reilly et al, 2015). Deep water temperatures respond less predictably to warming and have been observed to increase, decrease or not change with increasing air temperature (Dokulil et al, 2006;Ficker et al, 2017;Kirillin et al, 2013;Kirillin et al, 2017;Richardson et al, 2017;Winslow et al, 2017). Stratification strength and duration generally increase due to warming (Butcher et al, 2015;Kirillin, 2010), but patterns of change may have little regional coherence and cannot be reliably inferred from surface water trends (Read et al, 2014).…”

Do current European lake monitoring programmes reliably estimate phytoplankton community changes?

“…The last complete freeze-over occurred in 1963 (Einsele, 1963). Lake Mondsee is an example of an originally ice-covered temperate lake undergoing a transition from a dimictic to a monomictic mixing pattern in the course of the ongoing lake warming (Ficker et al, 2017). Lake Mondsee is one of the best studied lakes in Austria with long-term phytoplankton datasets from the last 40 years (Findenegg, 1969;Dokulil & Skolaut, 1986;Dokulil & Jagsch, 1992;Greisberger et al, 2008;Dokulil & Teubner, 2012).…”

Evidence for higher-than-average air temperatures after the 8.2 ka event provided by a Central European δ18O record

“…Results (Supplementary Table 2) are given relative to the Vienna Standard (2) and (3), considering the temperaturedependent fractionation factor (10 3 Â ln(a calcite-water )) between calcite and water (Friedman and O'Neil, 1977) at an average hypolimnetic water temperature (t hypo ) of 4.5 C (Ficker et al, 2017). Considering the 1.25% counting error of the Mondsee varve chronology (chapter 3.1.2), the absolute dating uncertainty for the interval around 8.2 ka BP can be estimated to ± 100 years and the error for the duration of the 8.2 ka event itself is ±2 years.…”

“…18,000e19,000 cal years BP (Reitner, 2007;van Husen, 1977van Husen, , 1997. The lake is at present mainly monomictic with a long stratification period between late April and December and mixing during winter/early spring; dimictic conditions with a short winter stagnation of a few weeks occur only sporadically during the rare years with ice cover (Dokulil and Skolaut, 1986;Ficker et al, 2017;K€ ampf et al, 2015). Mondsee is fed by three major tributaries (Fuschler Ache, Zeller Ache, Wangauer Ache), which account for~70% of the total inflow, as well as several minor creeks.…”