Hypoxia (low dissolved oxygen) is a mounting concern for aquatic ecosystems as its prevalence increases with rising anthropogenic nutrient inputs. Hypoxia is most commonly defined as 2.0 mg l–1 of dissolved oxygen, although this level varies widely across studies and agency regulations. Such definitions may be too conservative, as ecologically‐relevant non‐lethal effects (e.g. consumption and growth) of hypoxia on important aquatic species, such as fish, often occur at oxygen levels much higher than 2.0 mg l–1. In addition, many mechanisms that regulate hypoxia tolerance in fish have been proposed, including temperature, habitat, location in the water column, and body size, but there is ongoing debate over which mechanisms are most important. Using a structured meta‐analysis of published studies, we showed consistent, significant negative effects on fish growth and consumption below 4.5 mg l–1. While the total amount of variation explained was generally low, below 4.5 mg l–1 of dissolved oxygen, phylogenetic relationships accounted for most of the explained variation in fish growth. Ecological factors including body size, location in the water column (pelagic, demersal, or benthopelagic), habitat (freshwater, marine, or diadromous), and temperature explained very little of the effect of hypoxia on fish growth and explained only a moderate level of variation in consumption. Our results suggest a dramatically higher threshold for sub‐lethal effects of hypoxia on fish than oxygen levels generally set for regulation purposes, and provide little support for accepted ecological mechanisms thought to influence hypoxia tolerance.

Lakes are traditionally classified based on their thermal regime and trophic status. While this classification adequately captures many lakes, it is not sufficient to understand seasonally ice‐covered lakes, the most common lake type on Earth. We describe the inverse thermal stratification in 19 highly varying lakes and derive a model that predicts the temperature profile as a function of wind stress, area, and depth. The results suggest an additional subdivision of seasonally ice‐covered lakes to differentiate underice stratification. When ice forms in smaller and deeper lakes, inverse stratification will form with a thin buoyant layer of cold water (near 0°C) below the ice, which remains above a deeper 4°C layer. In contrast, the entire water column can cool to ∼0°C in larger and shallower lakes. We suggest these alternative conditions for dimictic lakes be termed “cryostratified” and “cryomictic.”

The ice-cover period in lakes is increasingly recognized for its distinct combination of physical and biological phenomena and ecological relevance. Knowledge gaps exist where research areas of hydrodynamics, biogeochemistry and biology intersect. For example, density-driven circulation under ice coincides with an expansion of the anoxic zone, but abiotic and biotic controls on oxygen depletion have not been disentangled, and while heterotrophic microorganisms and migrating phytoplankton often thrive at the oxycline, the extent to which physical processes induce fluxes of heat and substrates that support under-ice food webs is uncertain. Similarly, increased irradiance in spring can promote growth of motile phytoplankton or, if radiatively driven convection occurs, more nutritious diatoms, but links between functional trait selection, trophic transfer to zooplankton and fish, and the prevalence of microbial versus classical food webs in seasonally ice-covered lakes remain unclear. Under-ice processes cascade into and from the ice-free season, and are relevant to annual cycling of energy and carbon through aquatic food webs. Understanding the coupling between state transitions and the reorganization of trophic hierarchies is essential for predicting complex ecosystem responses to climate change. In this interdisciplinary review we describe existing knowledge of physical processes in lakes in winter and the parallel developments in under-ice biogeochemistry and ecology. We then illustrate interactions between these processes, identify extant knowledge gaps and present (novel) methods to address outstanding questions.Plain Language Summary Winter is an important but poorly understood period for lake ecosystems at high latitudes. Incoming solar radiation is diminished by ice and (often) snow, flows of oxygen and substrates such as organic matter or nutrients from outside the lake are limited, and wind no longer causes turbulent mixing of the water column. The sediments become a source of heat as well as of solutes which drive denser water toward the bottom. The resulting density stratification creates a template for the development of winter ecosystems. Distinct oxygenated and oxygen-depleted zones will affect microbial community structure and the habitat and behavior of zooplankton and fish. Conditions can rapidly change in spring with increased irradiance and incoming snowmelt. This paper reviews how physical, biogeochemical and biological processes act together to shape aquatic ecosystems in winter and in spring. In addition, we present an overview of the unknowns regarding the interactions between the different processes, which can now be posed due to improved understanding of under-ice hydrodynamics and the nature of lake ice, of biogeochemistry, and of ecology. However, work to date has largely been conducted within distinct disciplines. We therefore outline interdisciplinary approaches that can bridge current knowledge gaps in winter limnology. JANSEN ET AL.