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Ice and snow cover on frozen lakes is a natural barrier to solar radiation, reducing the transfer of energy that controls under-ice thermal dynamics and biological productivity. Direct measurements of under-ice irradiance remain scarce due to logistical constraints. We assembled data from 46 freshwater lakes across the Northern Hemisphere, including 722 daily irradiance observations and ice and snow thickness records. Ice quality data (black ice, white ice, and snow cover) were available for 15 lakes (626 measurements). Using this data set and a Bayesian implementation of the Beer-Lambert law, we estimated statistical distributions of albedo and attenuation coefficients. Median albedo values were 0.55 for black ice, 0.60 for white ice, 0.56 for total ice, and 0.94 for snow, with corresponding attenuation coefficients of 0.79, 4.35, 1.75, and 8.96 , respectively. These refined optical properties address critical data gaps, improving under-ice irradiance predictions and enhancing understanding of lake processes under climate-driven ice conditions.

Ice and snow cover on lakes plays a fundamental role for under-ice ecology by reducing water column mixing and light availability. Previous studies have shown that such reductions can significantly influence the growth and reproduction of phytoplankton, primarily focusing on changes in ice-on and ice-off dates in a warming climate. This study goes beyond studying the effects of ice phenology on phytoplankton by addressing two fundamental questions: (1) how does a snow cover on ice influence below-ice phytoplankton chlorophyll-a and community composition and (2) how do variations in ice phenology influence spring phytoplankton chlorophyll-a and community composition after ice-off? To address these two questions, we assessed long-term monitoring data collected at least monthly on phytoplankton chlorophyll-a and community composition. We combined the phytoplankton data with annual ice phenology and nearby meteorological data on daily snow depth between 1997 and 2019 in a mesotrophic lake (Erken) in Sweden. Snow cover resulted in an exponential decrease of phytoplankton chlorophyll-a, with detectable effects during all 3 months studied (January-March). Deeper snow cover changed the community dominance from autotrophs to mixotrophs in two of the months studied (January and March), which we explain by reduced light availability caused by snow cover. In spring, phytoplankton chlorophyll-a increased with longer ice periods and delayed ice-off dates. A wide range of taxa in the spring community increased with delayed ice-off dates, while delayed ice-on dates mainly promoted diatoms. Convective mixing is important to keep non-motile taxa in the photic zone and could explain the increased phytoplankton growth with longer ice duration. Our results highlight seasonal ice and snow cover as key regulators for the timing of growth and reproduction of primary producers below ice, with effects of the ice cover period lasting after ice-off. Snow on ice causes light constraints, which commonly result in reduced under-ice primary production and a higher proportion of mixotrophs in the phytoplankton community. Losing high nutritional phytoplankton groups such as mixotrophs following changes in ice phenology and snow cover can have consequences for the trophic transfer and the biogeochemical cycling in lakes.

Permanent ponds are key landscape units that supply various ecosystem services. Notably, the export of aquatic subsidies to land via emerging insects may significantly influence terrestrial food webs. Polyunsaturated fatty acids (PUFA), which enhance consumer fitness, are among the essential exported components. The patterns and drivers of dietary exports from ponds via insects remain poorly known, particularly at continental scales. We analyzed the exports of biomass, lipid, and fatty acid contents from emerging insects, sampled in 36 ponds across 11 European countries, from 36 degrees N to 59 degrees N and from 26 degrees W to 19 degrees E, over four seasons. We found that biomass and fatty acid exports decreased with increasing latitude and were higher in spring and summer. Seasonal effects also increased with higher latitudes. Temperature was the most important predictor of insect biomass, explaining 27.6% of the total variation and showing an unimodal response. Thus, increasing temperature may promote exports in colder regions and seasons but may negatively influence biomass exports in already warm regions. The exports of total lipids, PUFA, and eicosapentaenoic acid were correlated to exported biomass, while those of docosahexaenoic acid were linked to the emergence of Chaoboridae. Our findings indicated that PUFA contents were affected by taxonomic insect community composition and pond trophic state (indicated by chlorophyll a). Two of the correlates identified here (temperature and trophic state) are influenced by anthropogenic activity via climate and land use change, respectively. Thus, human activity impacts the food webs in and around ponds by influencing the quantity and quality of nutritional exports.

Lakes are rapidly losing ice under global warming, but little is known about ice structure changes. Ice structure is a key regulator of ice stability and thus safety, affecting activities on ice. Here, we analysed spatial and temporal variations in ice structure across 21 Swedish lakes, spanning from 55 to 69 degrees N, and over five decades. We found regional differences in ice structure, with fastest changes occurring in southern Sweden. The stable clear ice layer was particularly sensitive to warming, showing a rapid decline. The number of days when temperatures exceeded the freezing point during the ice cover period was identified as a strong driver for how ice was structured. Since there is a high risk for increased occurrences of unsafe ice conditions under predicted air temperature changes, we recommend re-establishing ice structure monitoring programmes, informing society on the increased risks of being on ice and including ice structure to safety guidelines.

Ice phenology has shifted with anthropogenic warming such that many lakes are experiencing a shorter ice season. However, changes to ice quality - the ratio of black and white ice layers - remain little explored, despite relevance to lake physics, ecological function, human recreation and transportation. In this Review, we outline how ice quality is changing and discuss knock-on ecosystem service impacts. Although direct evidence is sparse, there are suggestions that ice quality is diminishing across the Northern Hemisphere, encompassing declining ice thickness, decreasing black ice and increasing white ice. These changes are projected to continue in the future, scaling with global temperature increases, and driving considerable impacts to related ecosystem services. Rising proportions of white ice will markedly reduce bearing strength, implying more dangerous conditions for transportation (limiting operational use of many winter roads) and recreation (increasing the risk of fatal spring-time drownings). Shifts from black to white ice conditions will further reduce the amount of light reaching the water column, minimizing primary production, and altering community composition to favour motile and mixotrophic species; these changes will affect higher trophic levels, including diminished food quantity for zooplankton and fish, with potential developmental consequences. Reliable and translatable in situ sampling methods to assess and predict spatiotemporal variations in ice quality are urgently needed. Lake ice has witnessed considerable changes in its phenology, but less is known about ice quality - the ratio of black ice to white ice. This Review assesses the changes in lake ice quality and its ecosystem services, noting diminished ice quality in observations and projections.

Cyanobacterial blooms have substantial direct and indirect negative impacts on freshwater ecosystems including releasing toxins, blocking light needed by other organisms, and depleting oxygen. There is growing concern over the potential for climate change to promote cyanobacterial blooms, as the positive effects of increasing lake surface temperature on cyanobacterial growth are well documented in the literature; however, there is increasing evidence that cyanobacterial blooms are also being initiated and persisting in relatively cold-water temperatures (< 15 °C), including ice-covered conditions. In this work, we provide evidence of freshwater cold-water cyanobacterial blooms, review abiotic drivers and physiological adaptations leading to these blooms, offer a typology of these lesser-studied cold-water cyanobacterial blooms, and discuss their occurrence under changing climate conditions.

Permanent ponds are valuable freshwater systems and biodiversity hotspots. They provide diverse ecosystem services (ESs), including water quality improvement and supply, food provisioning, and biodiversity support, despite significant pressure from multiple anthropogenic stressors and the impacts of ongoing global change. However, ponds are largely overlooked in management plans and legislation, and ecological research has focused on large freshwater ecosystems, such as rivers or lakes. Protection of ponds is often insufficient or indirectly provided via associated habitats such as wetlands. This situation is likely exacerbated by the lack of a full-scale understanding of the importance of ponds. In this review, we provide a detailed overview of permanent ponds across Europe, including their usages and the biodiversity they support. By discussing the concepts of pondscape and metacommunity theory, we highlight the importance of connectivity among and between ponds and identified fluxes of emerging insects as another ES of ponds. Those insects are rich in essential nutrients such as polyunsaturated fatty acids (PUFAs), delivered through them to the terrestrial environment; however, the extent and impact of this ES remains largely unexplored. Several potential stressors, especially related to ongoing global change, that influence pond diversity and integrity are discussed. We provide our insights on future pond management. Adaptive measures, taking into account the pond system per se within the pondscape, are the most promising to mitigate the loss of natural ponds and restore and conserve natural small waterbodies as refuges and diversity hotspots in increasingly urbanized landscapes.

The quality of lake ice is of uppermost importance for ice safety and under-ice ecology, but its temporal and spatial variability is largely unknown. Here we conducted a coordinated lake ice quality sampling campaign across the Northern Hemisphere during one of the warmest winters since 1880 and show that lake ice during 2020/2021 commonly consisted of unstable white ice, at times contributing up to 100% to the total ice thickness. We observed that white ice increased over the winter season, becoming thickest and constituting the largest proportion of the ice layer towards the end of the ice cover season when fatal winter drownings occur most often and light limits the growth and reproduction of primary producers. We attribute the dominance of white ice before ice-off to air temperatures varying around the freezing point, a condition which occurs more frequently during warmer winters. Thus, under continued global warming, the prevalence of white ice is likely to substantially increase during the critical period before ice-off, for which we adjusted commonly used equations for human ice safety and light transmittance through ice. Under continued global warming, lakes will increasingly be covered by white ice, in particular towards the end of the ice cover season when fatal winter drownings occur most often and light limits the growth and reproduction of primary producers.