title: 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.
authors.affiliation: Department of Water Resources and Drinking Water, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, CH-8600, Dübendorf, Switzerland; Department of Environmental Systems Science, Institute of Biogeochemistry and Pollution Dynamics, ETH Zurich, Universitätstrasse 16, CH-8092, Zürich, Switzerland.
authors.forename: Ryan P
authors.initials: RP
authors.lastname: North
authors.forename: Rebecca L
authors.initials: RL
authors.lastname: North
authors.forename: David M
authors.initials: DM
authors.lastname: Livingstone
authors.forename: Oliver
authors.initials: O
authors.lastname: Köster
authors.forename: Rolf
authors.initials: R
authors.lastname: Kipfer
mesh_terms: D057231:Climate Change; D004784:Environmental Monitoring; D060106:Lakes / Q000737:chemistry*; D010100:Oxygen / Q000032:analysis*; D010758:Phosphorus / Q000032:analysis* / Q000737:chemistry; D012995:Solubility; D013557:Switzerland
chemical_list: D010758:Phosphorus; D010100:Oxygen
keywords: climate change; internal phosphorus loading; mixing dynamics; nutrients; oligotrophication; oxygen; wavelet analysis