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Winters are changing rapidly across the globe but the implications for aquatic productivity and food webs are not well understood. In addition, the degree to which winter dynamics in aquatic systems respond to large‐scale climate versus ecosystem‐level factors is unclear but important for understanding and managing potential changes. We used a unique winter data set from the Upper Mississippi River System to explore spatial and temporal patterns in phytoplankton biomass (chlorophylla, CHL) and associated environmental covariates across 25 years and ∼1,500 river km. To assess the role of regional climate versus site‐specific drivers of winter CHL, we evaluated whether there were coherent long‐term CHL dynamics from north to south and across lotic‐lentic areas. We then estimated the degree to which these patterns were associated with climate variability (i.e., the Multivariate El Nino‐Southern Oscillation Index), winter severity (freezing degree days), river discharge, or site‐specific environmental variables (ice depth, snow depth, and nutrient concentrations). We found that winter CHL was typically highest in ice‐free reaches and backwater lakes, occasionally exceeding summer values. We did not find highly synchronous CHL dynamics across the basin, but instead show that temporal trends were independent among river reaches and lotic‐lentic areas of the river. Moreover, after accounting for these spatial dynamics, we found that CHL was most responsive to winter air temperature, being consistently higher in years with warmer winters across the basin. These results indicate that although productivity dynamics are highly dynamic within large river ecosystems, changes in the duration and severity of winter may uniformly increase wintertime productivity.

 

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.

 

Abstract Nearshore (littoral) habitats of clear lakes with high water quality are increasingly experiencing unexplained proliferations of filamentous algae that grow on submerged surfaces. These filamentous algal blooms (FABs) are sometimes associated with nutrient pollution in groundwater, but complex changes in climate, nutrient transport, lake hydrodynamics, and food web structure may also facilitate this emerging threat to clear lakes. A coordinated effort among members of the public, managers, and scientists is needed to document the occurrence of FABs, to standardize methods for measuring their severity, to adapt existing data collection networks to include nearshore habitats, and to mitigate and reverse this profound structural change in lake ecosystems. Current models of lake eutrophication do not explain this littoral greening. However, a cohesive response to it is essential for protecting some of the world's most valued lakes and the flora, fauna, and ecosystem services they sustain. 

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.

 

The extent to which terrestrial organic matter supports aquatic consumers remains uncertain because factors regulating resource flows are poorly understood. We sampled 12 lakes throughout the Sierra Nevada (California, USA) spanning large gradients in elevation and size to evaluate how watershed attributes and lake morphometry influence resource flows to lake carbon pools and zooplankton. We found that the size and composition of carbon pools in lakes were often more strongly determined by watershed or lake features rather than by elevational position. Using three different tracers of resource origin (δ¹³C, Δ¹⁴C, C:N ratio), we found terrestrial contributions to most lake resource pools (dissolved organic carbon, particulate organic matter (POM), sediments) and pelagic consumers (zooplankton) were more strongly related to local‐scale watershed features such as vegetation cover or watershed area: lake area rather than to elevation. Landscape patterns in multiple tracers indicated consistent contribution of within‐lake C sources to bulk resource pools across elevations (POM, sediments, zooplankton). δ¹³C‐enrichment of lake C pools and overlap with δ¹³C of terrestrial resources can arise due to reduced fractionation of¹³C by phytoplankton under CO₂limitation, therefore we recommend careful consideration of potential environmental drivers when interpreting among‐lake patterns in δ¹³C. Our findings emphasize the importance of local‐scale variation in mediating terrestrial contributions to lake food webs.

 

Ice cover plays a critical role in physical, biogeochemical, and ecological processes in lakes. Despite its importance, winter limnology remains relatively understudied. Here, we provide a primer on the predominant drivers of freshwater lake ice cover and the current methodologies used to study lake ice, including in situ and remote sensing observations, physical based models, and experiments. We highlight opportunities for future research by integrating these four disciplines to address key knowledge gaps in our understanding of lake ice dynamics in changing winters. Advances in technology, data integration, and interdisciplinary collaboration will allow the field to move toward developing global forecasts of lake ice cover for small to large lakes across broad spatial and temporal scales, quantifying ice quality and ice thickness, moving from binary to continuous ice records, and determining how winter ice conditions and quality impact ecosystem processes in lakes over winter. Ultimately, integrating disciplines will improve our ability to understand the impacts of changing winters on lake ice.