Long‐term changes in hypoxia and soluble reactive phosphorus in the hypolimnion of a large temperate lake: consequences of a climate regime shift

Abstract

The (Lower) Lake of Zurich provides an ideal system for studying the long-term impact of environmental change on deep-water hypoxia because of its sensitivity to climatic forcing, its history of eutrophication and subsequent oligotrophication, and the quality and length of its data set. Based on 39 years (1972-2010) of measured profiles of temperature, oxygen concentration and phosphorus (P) concentration, the potentially confounding effects of oligotrophication and climatic forcing on the occurrence and extent of deep-water hypoxia in the lake were investigated. The time-series of Nürnberg's hypoxic factor (HF) for the lake can be divided into three distinct segments: (i) a segment of consistently low HF from 1972 to the late-1980s climate regime shift (CRS); (ii) a transitional segment between the late-1980s CRS and approximately 2000 within which the HF was highly variable; and (iii) a segment of consistently high HF thereafter. The increase in hypoxia during the study period was not a consequence of a change in trophic status, as the lake underwent oligotrophication as a result of reduced external P loading during this time. Instead, wavelet analysis suggests that changes in the lake's mixing regime, initiated by the late-1980s CRS, ultimately led to a delayed but abrupt decrease in the deep-water oxygen concentration, resulting in a general expansion of the hypoxic zone in autumn. Even after detrending to remove long-term effects, the concentration of soluble reactive P in the bottom water of the lake was highly correlated with various measures of hypoxia, providing quantitative evidence supporting the probable effect of hypoxia on internal P loading. Such climate-induced, ecosystem-scale changes, which may result in undesirable effects such as a decline in water quality and a reduction in coldwater fish habitats, provide further evidence for the vulnerability of large temperate lakes to predicted increases in global air temperature.

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

“…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).…”

“…During this period, monitoring was carried out consistently by the City of Zurich Water Supply, with North et al (2013North et al ( , 2014. Watercolumn profiles of temperature and of the concentrations of oxygen, total phosphorus and soluble reactive phosphorus were measured at the location of the deepest point of the lake (136 m).…”

“…Based on the temporal evolution of deep-water oxygen concentrations and the thickness of the hypoxic zone in September (Fig. 15a and b) the study period can be divided into three segments 2 (North et al, 2014). In Segment I (1972Segment I ( -1987, with the exception of the first two years, the mean deep-water oxygen concentration (i.e., below 120 m water depth) is generally at or above 62.5 µmol L −1 and the hypoxic zone is rarely thicker than 11 m. By contrast, in Segment III (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010), the hypoxic zone is consistently thicker than 11 m. Segment II (1988-1999 represents a transitional phase during which deep-water oxygen concentrations and the thickness of the hypoxic zone are extremely variable.…”

“…Most of the increase in both water and air temperatures occurred abruptly from 1987-1988 as a result of the late 1980s climate regime shift (CRS) (North et al, 2013), which is known to have affected temperatures in oceans (e.g., Conversi et al, 2010) and lakes (e.g., Anneville et al, 2004Anneville et al, , 2005, and which appears to have resulted ultimately from a shift in the Arctic Oscillation (Rodionov and Overland, 2005). Thus the reason for the long-term increase in the extent of hypoxia appears to be at least partially associated with changes in the large-scale climatic regime (North et al, 2014).…”