Winter conditions are rapidly changing in temperate ecosystems, particularly for those that experience periods of snow and ice cover. Relatively little is known of winter ecology in these systems, due to a historical research focus on summer 'growing seasons'. We executed the first global quantitative synthesis on under-ice lake ecology, including 36 abiotic and biotic variables from 42 research groups and 101 lakes, examining seasonal differences and connections as well as how seasonal differences vary with geophysical factors. Plankton were more abundant under ice than expected; mean winter values were 43.2% of summer values for chlorophyll a, 15.8% of summer phytoplankton biovolume and 25.3% of summer zooplankton density. Dissolved nitrogen concentrations were typically higher during winter, and these differences were exaggerated in smaller lakes. Lake size also influenced winter-summer patterns for dissolved organic carbon (DOC), with higher winter DOC in smaller lakes. At coarse levels of taxonomic aggregation, phytoplankton and zooplankton community composition showed few systematic differences between seasons, although literature suggests that seasonal differences are frequently lake-specific, species-specific, or occur at the level of functional group. Within the subset of lakes that had longer time series, winter influenced the subsequent summer for some nutrient variables and zooplankton biomass.

The concentration of dissolved oxygen in aquatic systems helps regulate biodiveristy 1, 2 , nutrient biogeochemistry 3 , greenhouse gas emissions 4 , and drinking water quality 5 . The long-term declines in dissolved oxygen concentrations in coastal and ocean waters have been linked to climate warming and human activity 6, 7 , but little is known about changes in dissolved oxygen concentrations in lakes. While dissolved oxygen solubility decreases with increasing water temperatures, long-term lake trajectories are not necessarily predictable. Oxygen losses in warming lakes may be amplified by enhanced decomposition and stronger thermal stratification 8, 9 or they may increase as a result of enhanced primary production 10 . Here we analyse 45,148 dissolved oxygen and temperature profiles from 393 temperate lakes spanning 1941-2017. We find that a decline in dissolved oxygen is widespread in surface and deep-water habitats. The decline in surface waters is primarily associated with reduced solubility under warmer water temperatures, although surface dissolved oxygen increased in a subset of highly-productive warming lakes, likely due to increasing phytoplankton production. In contrast, the decline in deep waters is associated with stronger thermal stratification and water clarity losses, but not with changes in gas solubility. Our results suggest that climate change and declining water clarity have altered the physical and chemical environment of lakes. Freshwater dissolved oxygen losses are 2.5-10 times greater than observed in the world's oceans 6, 7 and could threaten essential lake ecosystem services 2,3,5,11 .

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.