Warming winters in lakes: Later ice onset promotes consumer overwintering and shapes springtime planktonic food webs

Global climate warming is causing the loss of freshwater ice around the Northern Hemisphere. Although the timing and duration of ice covers are known to regulate ecological processes in seasonally ice-covered ecosystems, the consequences of shortening winters for freshwater biota are poorly understood owing to the scarcity of under-ice research. Here, we present one of the first in-lake experiments to postpone ice-cover onset (by ≤21 d), thereby extending light availability (by ≤40 d) in early winter, and explicitly demonstrate cascading effects on pelagic food web processes and phenologies. Delaying ice-on elicited a sequence of events from winter to spring: 1) relatively greater densities of algal resources and primary consumers in early winter; 2) an enhanced prevalence of winter-active (overwintering) consumers throughout the ice-covered period, associated with augmented storage of high-quality fats likely due to a longer access to algal resources in early winter; and 3) an altered trophic structure after ice-off, with greater initial springtime densities of overwintering consumers driving stronger, earlier top-down regulation, effectively reducing the spring algal bloom. Increasingly later ice onset may thus promote consumer overwintering, which can confer a competitive advantage on taxa capable of surviving winters upon ice-off; a process that may diminish spring food availability for other consumers, potentially disrupting trophic linkages and energy flow pathways over the subsequent open-water season. In considering a future with warmer winters, these results provide empirical evidence that may help anticipate phenological responses to freshwater ice loss and, more broadly, constitute a case of climate-induced cross-seasonal cascade on realized food web processes.

Investigation of the Effect of Block Size, Shape, and Freeze Bond Strength on Flexural Failure of Freshwater Ice Rubble Using the Discrete Element Method

As part of the Mechanics of Ice Rubble project, recent experiments have been carried out to study the strength and failure behavior of ice rubble beams and the freeze bonds that form between individual ice blocks. In this study, we present results obtained from a newly developed model for the three-dimensional (3D) discrete element modeling (DEM) open-source code LIGGGHTS. The ice model contains normal and shear springs that operate between neighboring particles which are bonded or that overlap due to compressional stresses. Energy dissipation is accounted for by using a viscous damping model. Using this DEM model, medium-scale freshwater ice rubble punch tests have been simulated. Rubble specimens were generated by “raining” individual DEM ice pieces into a rectangular form and compacting the rubble mass to achieve the target porosity. Before the compacting pressure was removed, bonds between contacting blocks were introduced with parameter values based on representative freeze bond experiments. The rubble beam was then deformed by pushing a platen vertically downward through the center of the beam until failure occurred. Two types of block size and shapes have been simulated: cuboid blocks generated based on the size distribution of the actual rubble, and rubble blocks generated by image processing of actual blocks of broken ice used in the comparison experiments. The mechanism of flexural rubble failure for both cuboid block [s4.2] simulations and empirical block [s4.3] simulations is in line with experimental results; however, the empirical block simulations provide a significantly better estimation of the failure force.

Physical processes behind interactions of microplastic particles with ice

<p>Microplastic particles (MPs) are found in marine ice in larger quantities than in seawater, indicating that the ice is an important link in the chain of spreading of this contaminant. Some studies indicate larger MPs abundance near the ice surface, while others did not find any consistent pattern in the vertical distribution of MPs within sea ice cores. We discuss physical mechanisms of incorporation of MPs in the ice and present the results of laboratory tests, underpinning our conclusions.</p><p>First, plastic hydrophobicity is shown to cause the effect of pushing the floating MPs further up of the newly-forming ice. This leads to a concentration of MPs at the ice surface in the laboratory, while in the field the particles at the surface may by covered by snow and become a part of the upper ice layer. Under open-air test conditions, the bubbles of foamed polystyrene (density 0.04 g/cm<sup>3</sup>), initially floating at the water surface, were gone by weak wind when the firm ice was formed.</p><p>Second, the difference between freshwater and marine ice is considered. Since fresh water has its temperature of the density maximum (Tmd=3.98 C) well above the freezing point (Tfr=0 C), the freshwater ice is formed when the water column is stably stratified for a relatively long period of cooling from the Tmd down to the Tfr. Under such steady conditions, even just slightly positively/negatively buoyant MPs have enough time to rise to the surface / to settle to the bottom. In contrast, the ice in the ocean freezes when thermal convection is at work, further enhanced by the brine release. Thus, strong convection beneath the forming marine ice keeps slightly positively/negatively buoyant MPs in suspension and maintains the contact between the MPs and the forming ice. Laboratory tests show both the difference between the solid-and-transparent freshwater ice and the layered, filled with brine marine ice, and the difference in the level of their contamination.</p><p>Lastly, it is demonstrated that MPs tend to be incorporated in the ice together with air bubbles and in-between the ice plates (in brine channels). This is most probably due t plastics’ hydrophobicity.</p><p>Investigations are supported by the Russian Science Foundation, grant No 19-17-00041.</p>

Observation-derived ice growth curves show patterns and trends in maximum ice thickness and safe travel duration of Alaskan lakes and rivers

The formation, growth, and decay of freshwater ice on lakes and rivers are fundamental processes of northern regions with wide-ranging implications for socio-ecological systems. Ice thickness at the end of winter is perhaps the best integration of cold-season weather and climate, while the duration of thick and growing ice cover is a useful indicator for the winter travel and recreation season. Both maximum ice thickness (MIT) and ice travel duration (ITD) can be estimated from temperature-driven ice growth curves fit to ice thickness observations. We simulated and analyzed ice growth curves based on ice thickness data collected from a range of observation programs throughout Alaska spanning the past 20–60 years to understand patterns and trends in lake and river ice. Results suggest reductions in MIT (thinning) in several northern, interior, and coastal regions of Alaska and overall greater interannual variability in rivers compared to lakes. Interior regions generally showed less variability in MIT and even slightly increasing trends in at least one river site. Average ITD ranged from 214 d in the northernmost lakes to 114 d across southernmost lakes, with significant decreases in duration for half of sites. River ITD showed low regional variability but high interannual variability, underscoring the challenges with predicting seasonally consistent river travel. Standardization and analysis of these ice observation data provide a comprehensive summary for understanding changes in winter climate and its impact on freshwater ice services.

Observation-derived ice growth curves show patterns and trends in maximum ice thickness and safe travel duration of Alaskan lakes and rivers

The formation, growth, and decay of freshwater ice on lakes and rivers are fundamental processes of northern regions with wide ranging implications for socio-ecological systems. Ice thickness at the end of winter is perhaps the best integration of cold-season weather and climate, while the duration of thick and growing ice cover is a useful indicator for the winter travel and recreation season. Both maximum ice thickness (MIT) and ice travel duration (ITD) can be estimated from temperature-driven ice growth curves fit to ice thickness observations. We simulated and analyzed ice growth curves based on ice thickness data collected from a range of observation programs throughout Alaska spanning the past 20–60 years to understand patterns and trends in lake and river ice. Results suggest reductions in MIT (thinning) in several northern, interior, and coastal regions of Alaska and overall greater interannual variability in rivers compared to lakes. Interior regions generally showed less variability in MIT and even slightly increasing trends in at least one river site. Average ITD ranged from 214 days in the northern-most lakes to 114 days across southern-most lakes with significant decreases in duration for half of sites. River ITD showed low regional variability, but high interannual variability, underscoring the challenges with predicting seasonally-consistent river travel. Standardization and analysis of these ice observation data provide a comprehensive summary for understanding changes in winter climate and its impact on freshwater ice services.