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Few coupled lake-watershed studies examine long-term effects of climate on the ecosystem function of lakes in a hydrological context. We use 32 years of hydrological and biogeochemical data from a high-elevation site in the Sierra Nevada of California to characterize variation in snowmelt in relation to climate variability and explore the impact on factors affecting phytoplankton biomass. The magnitude of accumulated winter snow, quantified through basinwide estimates of snow water equivalent (SWE), was the most important climate factor controlling variation in the timing and rate of spring snowmelt. Variations in SWE and snowmelt led to significant differences in lake flushing rate, water temperature, and nitrate concentrations across years. On average in dry years, snowmelt started 25 days earlier and proceeded 7 mm/day slower, and the lake began the ice-free season with nitrate concentrations ~2 μM higher and water temperatures 9°C warmer than in wet years. Flushing rates in wet years were 2.5 times larger than those in dry years. Consequently, particulate organic matter concentrations, a proxy for phytoplankton biomass, were 5–6 μM higher in dry years. There was a temporal trend of increase in particulate organic matter across dry years that corresponded to lake warming independent of variation in SWE. These results suggest that phytoplankton biomass is increasing as a result of both interannual variability in precipitation and long-term warming trends. Our study underscores the need to account for local-scale catchment variability that may affect the accumulation of winter snowpack when predicting climate responses in lakes.

Our changing climate is having effects on freshwater ecosystems in all seasons, especially winter. High latitude lakes, wetlands, and rivers are experiencing shorter periods of ice cover, and lower latitudes systems that used to freeze are experiencing open water conditions throughout the winter. A 2019 AGU Chapman conference convened aquatic scientists to examine these changes and address the implications of changing winters to aquatic life, chemistry, and physics. Several studies demonstrate decreased ice cover duration than in the past. The removal of an ice "lid" from lakes and rivers impacts the exchange of gases with the atmosphere and the predominant types of metabolism occurring in the waters below, with the potential for more photosynthesis and an increase in oxic versus anoxic metabolism when the lid is removed. Multiple studies indicated an increase in the interannual variability of winters, especially in terms of ice‐cover duration and ice quality. Increased variability may simply be an outcome of a more variable winter climate or small differences in environmental conditions such as temperature that can have strong effects on gas exchange, light transmission, and turbulence when ice forms. A question that merits further consideration is whether and how winters of shorter duration and severity will change the dynamics of freshwater systems. Are there memory or legacy effects that carry over to the next season or year? There is much work to be done to understand how changing winters will impact the biogeochemical behavior of lakes and rivers in the coming decades. Plain Language Summary: Winters at mid‐high latitudes are changing rapidly with important implications for freshwater ecosystems. An AGU Chapman conference, convened in 2019, brought together scientists to outline what we know about the changes that are occurring and what questions need further study. This special issue is a collection of the papers that developed from that workshop. These studies demonstrate that lakes and rivers at mid‐high latitudes are experiencing less ice cover duration and greater variability in year‐to‐year ice cover. Changing ice and snow cover impacts processes occurring under ice in winter by altering the light regime, mixing, biota present, and the availability of oxygen. There are many important research questions regarding winter dynamics that still need to be addressed but one of the most important ones has to do with how changing winters might influence the biota in other seasons. Key Points: Several of the papers point to interannual variability in winter and seasonal ice cover along with decreased ice cover durationAvailable winter datasets have grown supporting conceptual developments, but increased efforts focused on ecological responses are neededFuture research should examine connections between seasons, deploy submersed sensors year‐round, and use remote sensing approaches [ABSTRACT FROM AUTHOR]

Warming winters will reduce ice cover and change under‐ice conditions in temperate mountain lakes, where snow contributes most of winter cover on lakes. Snow‐dominated mountain lakes are abundant and highly susceptible to climate warming, yet we lack an understanding of how climate variation and local attributes influence winter processes. We investigated climatic and intrinsic controls on ice phenology, water temperature, and bottom‐water dissolved oxygen (DO) in 15 morphologically diverse lakes in the Sierra Nevada and Klamath Mountains of California, USA, using high‐frequency measurements from multiple (2–5) winters. We found that ice phenology was determined by winter climate variables (snowfall and air temperature) that influence ice‐off timing, whereas ice‐on timing was relatively invariant among years. Lake size and morphology mediated the effect of climate on lake temperature and DO dynamics in early and late winter. Rates of hypolimnetic DO decline were highest in small, shallow lakes, and were unrelated to water temperature. Temperature and oxygen dynamics were more variable in small lakes because heavy snowfall caused ice submergence, mixing, and DO replenishment that affected the entire water column. As the persistence of snow declines in temperate mountain regions, autumn, and spring climatic conditions are expected to gain importance in regulating lake ice phenology. Water temperature and DO will likely increase in most lakes during winter as snowpack declines, but morphological attributes such as lake size will determine the sensitivity of ice phenology and under‐ice processes to climate change. Plain Language Summary: Mountain lakes in temperate regions are at risk of losing ice cover as winters become warmer and snowfall decreases. Small mountain lakes are numerous and provide important habitats for aquatic species, yet we currently lack an understanding of what controls ice duration and under‐ice conditions in these snow‐dominated systems because most are remote and inaccessible. We used hourly measurements within a diverse set of California mountain lakes across multiple winters to explore how climate variables and lake or watershed characteristics influence the duration and timing of ice cover, as well as under‐ice temperature and oxygen concentrations. We found that winter snowfall and air temperature controlled the timing of ice breakup and the length of ice duration. Lake size and shape controlled oxygen and temperature immediately after ice formation, but late winter conditions depended on winter climate variables that dictated ice cover duration. Oxygen depletion rates and the prevalence of low oxygen conditions were greater in small, shallow lakes than in large, deep lakes. As winters warm, most mountain lakes will have shorter ice cover, warmer water temperatures, and increased oxygen concentrations, but small and large lakes will respond differently to the loss of snow and ice cover. Key Points: Winter air temperature and snowfall drive ice‐off timing and ice phenology of temperate mountain lakesLake size and morphology mediate how climate affects water temperature and dissolved oxygen dynamics during early and late winterRates of hypolimnetic oxygen decline are highest in shallow lakes and do not vary with water temperature [ABSTRACT FROM AUTHOR]

Mountain lakes experience extreme interannual climate variation as well as rapidly warming air temperatures, making them ideal systems to understand lake‐climate responses. Snowpack and water temperature are highly correlated in mountain lakes, but we lack a complete understanding of underlying mechanisms. Motivated by predicted declines in snowfall with future temperature increases, we investigated how surface heat fluxes and lake warming responded to variation in snowpack, ice‐off, and summer weather patterns in a high elevation lake in the Sierra Nevada, California. Ice‐off timing determined the phenology of lake exposure to solar radiation, and was the dominant mechanism linking snowpack to lake temperature. The relative importance of heat loss fluxes (longwave radiation, latent and sensible heat exchange) varied among wet and dry years. Declines in snowpack and ice cover in mountain systems will reduce variability in lake thermal responses and increase the responsiveness of lake warming to atmospheric forcing. [ABSTRACT FROM AUTHOR]

Environmental drivers such as climate change are responsible for extreme events that are critically altering freshwater resources across the planet. In the continental US, these events range from increases in the frequency and duration of droughts and wildfires in the West, to increasing precipitation and floods that are turning lakes and reservoirs brown in the East. Such events transform and transport organic carbon in ways that affect the exposure of ecosystems to ultraviolet ( UV) radiation and visible light, with important implications for ecosystem services. Organic matter dissolved in storm runoff or released as black carbon in smoke selectively reduces UV radiation exposure. In contrast, droughts generally increase water transparency, so that UV radiation and visible light penetrate to greater depths. These shifts in water transparency alter the potential for solar disinfection of waterborne parasites, the production of carcinogenic disinfection byproducts in drinking water, and the vertical distribution of zooplankton that are a critical link in aquatic food webs. [ABSTRACT FROM AUTHOR]