The effect of the exceptionally mild European winter of 2006‐2007 on temperature and oxygen profiles in lakes in Switzerland: A foretaste of the future?

Abstract

The European winter of 2006–2007 was unusually mild, with record high mean winter air temperatures comparable with those predicted to become the norm by the end of the current century as a result of climate warming. In Lake Zurich and Greifensee, two neighboring Swiss perialpine lakes with several decades of data, mean lake temperatures for this winter were the highest ever recorded, as was thermal stability. Associated with the high thermal stability, mean winter oxygen concentrations in Lake Zurich were unusually high in the epilimnion and metalimnion, but normal in the hypolimnion. In Greifensee, however, which is much shallower, mean winter oxygen concentrations did not deviate substantially from the norm anywhere in the water column. From 17–19 January 2007, an unusually severe cyclonic storm, “Kyrill”, traversed Europe. Monthly oxygen profiles suggest that the stabilizing effect of the mild winter on the two lakes was greatest before the occurrence of the storm, and that wind mixing resulted in a deepening of the mixed layer in both lakes. The mixing was able to encompass the entire water column of Greifensee, but not of Lake Zurich. These results, supported by more limited data from two other neighboring lakes, suggest that climate warming will likely inhibit complete mixing of some deep, temperate, normally monomictic lakes in winter even when extremely intense cyclonic storms occur. In shallower lakes, however, complete mixing is unlikely to be inhibited.

Investigating hypoxia in aquatic environments: diverse approaches to addressing a complex phenomenon

“…Lake Zurich is a medium-sized perialpine lake with a surface area of 65 km 2 , a volume of 3.3 km 3 and a maximum depth of 136 m (Livingstone, 2003). Although typically monomictic, Lake Zurich can also behave as a dimictic or oligomictic lake depending on the winter weather (Rempfer et al, 2010). The recent shift towards oligomixis, a situation in which the lake does not mix completely in some years (Livingstone, 1993), has resulted in some occurrences of extremely low deep-water oxygen concentrations (Livingstone, 1997).…”

“…1) are deep, wind-sheltered water bodies which are typically monomictic; i.e., they mix completely once a year. Recent climate change has caused shifts towards less frequent mixing (oligomixis) (Livingstone, 1993), which can result in deep-water hypoxia (Livingstone, 1997;Rempfer et al, 2010) and the release of phosphorus from the sediment (North et al, 2014). Current conditions in the lakes, as well as the impact of climate change and the history of hypoxia, were addressed by a combination of measurements and time-series analyses.…”

“…These climate effects might counteract the benefits of the measures introduced to improve lake trophic status. In Switzerland, the availability of long, high-quality lake profile data, especially from Lake Zurich, allows attempts to be made to quantify the effect of recent climate change on deep-water oxygen concentrations (e.g., Livingstone, 1997;Jankowski et al, 2006;Rempfer et al, 2009Rempfer et al, , 2010North et al, 2014). Lake Zurich is a medium-sized perialpine lake with a surface area of 65 km 2 , a volume of 3.3 km 3 and a maximum depth of 136 m (Livingstone, 2003).…”

“…Global warming also results in rising lake surface water temperatures (Adrian et al, 2009) and an increase in the intensity and duration of thermal stratification (Stefan et al, 1998;Peeters et al, 2002;Livingstone, 2003). During winter and spring -when temperate lakes typically undergo deep mixing -shorter periods of homothermy, or their complete absence, will reduce or inhibit deep-water renewal, resulting in lower deep-water oxygen concentrations (Stefan et al, 1996;Livingstone, 1997;Peeters et al, 2002;Jankowski et al, 2006;Rempfer et al, 2009Rempfer et al, , 2010. Climate warming is therefore likely to lead to an increase in the occurrence of hypoxia in lakes (Stefan and Fang, 1994;Jankowski et al, 2006).…”

Comparison of linear and cubic spline methods of interpolating lake water column profiles

“…To facilitate the comparison of such data, some form of interpolation is usually employed to yield estimates of the data at standard depths (e.g., Livingstone 2003;Coats et al 2006;Rempfer et al 2010) before going on to produce time-series based on these data. Because of their simplicity and ease of use, twopoint linear interpolation (between the two measured values on either side of a gap) and cubic spline interpolation (which incorporates information from several measurements on each side of a gap) are both commonly used for this purpose.…”