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.

<p>The ice-covered period in lakes is increasingly recognized for its unique hydrodynamic and biogeochemical phenomena and ecological relevance yet it remains poorly studied compared to the ice-free season. Knowledge gaps exist where research areas – 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. While heterotrophic microorganisms and migrating phytoplankton often thrive at the oxycline, the extent to which physical processes induce fluxes of heat and substrates that further support under-ice food webs is uncertain. Similarly, radiatively-driven convection under ice in spring can promote growth of motile phytoplankton or diatoms depending on flow velocity, water clarity and mixing depth, 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 presentation, we briefly review existing knowledge regarding 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 whose solution requires interdisciplinary approaches, and present (novel) methods to address outstanding questions.</p>

This study measures the presence of bacteria-sediment associations (BSAs) in an alpine, glacier-fed watershed in the Southern Coast Mountains of British Columbia, Canada. The impact of BSAs on the creation of flocculated particles and their settling velocity are quantified using a laser transmissometer. Results from the study indicate that BSAs are present in the watershed and vary over both space and time. The percentage of bacteria associated with sediment particles was found to range from < 1% to 40%. Major sources of planktonic bacteria such as agricultural land and wastewater treatment outflow co-occur with large decreases in the BSA ratio. Laboratory analysis demonstrates that an increase in the concentration of bacteria was associated with a decrease in the volume concentration of small particles, and a decrease in both estimated density and measured settling velocity for particles in larger size classes; consistent with flocculated particles of increasing complexity arising from combinations of primary particles and/or BSAs. Paleoenvironmental reconstructions using laminated lake sediments in alpine, glacier-fed systems benefit from a fuller understanding of the geomorphologic processes by which they formed. While bacteria are noted to enhance formation of flocculated particles in laboratory systems, their impact upon geomorphic processes in natural systems have yet to be fully explored.