Reduced Ice Cover Research Articles

Exceptional sea ice loss leading to anomalously deep winter convection north of Svalbard in 2018

AbstractAn important question is will deep convection sites, where deep waters are ventilated and air-gas exchange into the deep ocean occurs, emerge in the Arctic Ocean with the warming climate. As sea ice retreats northward and as Arctic sea ice becomes younger and thinner, air-sea interactions are strengthening in the high-latitude oceans. This includes new and extreme deep convection events. We investigate the associated physical processes and examine impacts and implications. Focusing on a region near the Arctic gateway of Fram Strait, our study confirms a significant sea ice cover reduction north of Svalbard in 2018 compared to the past decade, shown in observations and several numerical studies. We conduct our study using the regional configuration Arctic and North Hemisphere Atlantic of the ocean/sea ice model NEMO, running at 1/12° resolution (ANHA12). Our numerical study shows that the open water condition during the winter of 2018 allows intense winter convection over the Yermak Plateau, as more oceanic heat is lost to the atmosphere without the insulating sea ice cover, causing the mixed layer depth to reach over 600 m. Anomalous wind prior to the deep convection event forces offshore sea ice movement and contributes to the reduced sea ice cover. The sea ice loss is also attributed to the excess heat brought by the Atlantic Water, which reaches its maximum in the preceding winter in Fram Strait. The deep convection event coincides with enhanced mesoscale eddy activity on the boundary of the Yermak Plateau, especially to the east. The resulting substantial heat loss to the atmosphere also leads to a heat content reduction integrated over the Yermak Plateau region. This event can be linked to the minimum southward sea ice volume flux through Fram Strait in 2018, which is a potential negative freshwater anomaly in the subpolar Atlantic.

The consequences of winter climate change for plant performance.

With continually increasing summer temperatures and intense heatwaves, it can be easy to neglect the ecological effects of winter climate change. However, shifts in the climate during winter can have profound consequences for eco-evolutionary dynamics in extratropical latitudes and high elevation locales. Climate change has increased winter temperatures, disrupted snowpack, and reduced ice cover (Rixen et al., 2022). Extreme losses of snowpack are projected for many regions by the end of the century (Talsma et al., 2022). Patterns of climate change are complex and region-dependent, but overall winters are becoming less reliable, with elevated temperatures and altered snow dynamics. This article is protected by copyright. All rights reserved.

Evolving relationship of Nares Strait ice arches on sea ice along the Strait and the North Water, the Arctic’s most productive polynya

Nares Strait, the waterway that separates northwest Greenland from Ellesmere Island, is a major pathway along which sea ice leaves the Arctic, including the planet’s oldest and thickest sea ice that is experiencing an accelerated loss. Ice arches that develop during the winter at the Strait’s northern or southern terminus can remain stable for months at a time during which the transport of sea ice ceases. The Arctic’s most productive polynya, the North Water (NOW) or Pikialasorsuaq (West Greenlandic for ‘great upwelling’) forms at the Strait’s southern end. There is evidence that a warming climate and the concomitant thinning of Arctic sea ice is weakening the arches and it has been proposed that this may impact the stability of NOW and the complex ecosystem that it sustains. Here we employ a categorization of recent winters with respect to the presence or absence of ice arches to explore their impact on sea ice along the Strait and over the NOW. We find that winters during which a southern ice arch is absent are associated with a reduced and thinner ice cover along the Strait with ice conditions over the NOW similar to that during winters with a southern arch. In winters, without a southern arch, there is also an acceleration of the winds along the Strait that contributes to the presence of reduced ice cover. Ocean color remote sensing data suggests that current levels of primary productivity over the NOW are independent of the presence or absence of an ice arch. The results suggest more research is needed to assess the stability of the NOW, with respect to reduced ice cover and primary productivity, in a future where ice arches cease to form along Nares Strait.

Ice Freeze‐Up and Break‐Up in Arctic Rivers Observed With Satellite L‐Band Passive Microwave Data From 2010 to 2020

AbstractThe timing of ice freeze‐up and break‐up in the Arctic may be responding to climate change. Passive microwave remote sensing is a powerful technique for monitoring this timing. We processed low‐frequency microwave time series from the European Space Agency Soil Moisture and Ocean Salinity (SMOS) mission for a set of 31 satellite gauging reaches (SGRs) above 65°N between 2010 and 2020 to determine timing of freeze‐up and break‐up and annual river ice durations. We found indication of progressive ice cover reduction over more than half of the monitored river reaches, with possibly the fastest rate occurring over northeast Russia. Some rivers in high‐latitude North America experienced a slight increase in ice cover. Across the data set, we observed an average 2.2 days shift toward later ice freeze‐up in autumn and an average 0.6 days shift toward earlier ice break‐up in spring, resulting in an average decrease of 3.4 days in ice duration between 2010 and 2020. River reaches with the longest duration of ice cover appeared to have experienced the fastest rate of decrease. A possible reduction of the time lag between air temperature rise or fall and corresponding river ice break‐up and freeze‐up was also observed. Yet results on variability are carefuly interpreted given the short length of the time series (2010–2020) and the low statistical confidence rates calculated for the decadal tendency. Still outcomes are consistent with increases in global and Arctic surface air temperature. Following these time series over the next decade using passive microwave satellite sensors can monitor ice cover duration in the Arctic and will further determine temporal and regional trends.

Opening the black box of winter: Full‐year dynamics of crustacean zooplankton along a nearshore depth gradient in a large lake

AbstractPast studies of zooplankton seasonality in large temperate lakes have often neglected the winter period. Winter conditions are rapidly changing (e.g., reduced ice cover extent and duration, altered thermal and mixing regimes) in northern lakes, making it important to fill the existing winter knowledge gap. In this study, we sampled five stations in Lake Superior across a nearshore depth gradient through the full year to assess the phenology of crustacean zooplankton communities and the effect of environmental drivers on them. Across stations, zooplankton densities were the lowest in winter (0.9 ± 0.6 Ind. L−1) and highest in summer (14.2 ± 15.1 Ind. L−1). Zooplankton abundances and community composition were less seasonally variable at deeper stations compared to shallower and more terrestrially affected regions. Cladocerans were the dominant taxonomic group during the summer across all stations, while cyclopoid and calanoid copepods were more important during the fall, winter, and spring. Among feeding groups, herbivores were most abundant in summer while omnivores and carnivores dominated in winter. We found that water temperature and food availability were the main drivers of total zooplankton densities through the year and during the cold seasons, but the effect of these factors varied among the main taxonomic groups. Our study demonstrates seasonal and spatial variation in crustacean zooplankton and environmental parameters, with the highest fluctuation at shallower stations. This study offers new information on seasonal crustacean zooplankton dynamics and contributes to understanding the effects of climate change on large lake ecosystems.

Under‐ice plankton community response to snow removal experiment in bog lake

AbstractAlthough previously overlooked, winter is now seen as a period of significant biological activity in the annual cycle of north‐temperate lakes. Research suggests a future of reduced ice cover duration and altered snow conditions could significantly change the functioning of aquatic ecosystems. This study seeks to explore the possible repercussions of changing ice and snow dynamics on aquatic biological communities, particularly at lower trophic levels. To explore plankton community responses to changing under‐ice light conditions, we performed a whole‐lake manipulation by removing all of the snow from the surface of a north temperate bog lake in northern Wisconsin. Over three winters, samples were collected under ice in the study lake, South Sparkling Bog. The first winter, 2018–2019, served as a reference year during which snow was not removed from the lake and was followed by two subsequent winters of snow removal during 2019–2020 and 2020–2021. Data collected included phytoplankton and zooplankton abundances and taxa, chlorophyll a, dissolved organic carbon, light, Secchi depth, and ice and snow thickness. In our snow removal years, increased light availability in the water column shifted the phytoplankton community from low‐biomass, mixed community of potential mixotrophs, unicellular cyanobacteria and Chlorophytes to dominance by photoautotrophs, and rotifer zooplankton densities increased. Ice condition, specifically the thickness of white ice vs. black ice, was a major driver in the magnitude of change between years. This research improves our understanding of how plankton communities might respond to climate‐change driven shifts in winter dynamics for north temperate systems.

Climate-driven shifts in pelagic fish distributions in a rapidly changing Pacific Arctic

Baseline surveys of offshore pelagic fishes in the eastern Chukchi Sea in 2012 and 2013 found that age-0 Arctic cod (Boreogadus saida) dominated the pelagic fish community in summer, with relatively few adults present in the region. Since this time, drastic changes in the ocean-atmosphere-ice feedback loop have led to continued warming, further reducing ice cover, and increased northward transport has led to an increase in Pacific-origin waters on the Chukchi shelf in summer. To examine potential bottom-up effects of these environmental changes on pelagic fishes in this rapidly changing environment, we extended a time series of large-scale acoustic-trawl surveys with additional surveys in 2017 and 2019. Age-0 Arctic cod were the most abundant pelagic fish in all four survey years, comprising 68–93% of fish abundance. However, age-0 walleye pollock (Gadus chalcogrammus), which were scarce (<0.1% of fishes) and confined to the southern Chukchi in 2012 and 2013, were present in high abundance (>21% of fish abundance) throughout the Chukchi shelf in 2017 and 2019. Age-0 Arctic cod were substantially more abundant in 2017 than in other years, possibly due to increased survivorship of larvae under warm conditions. Unlike in 2017, Arctic cod and pollock were spatially separated in 2019 due to enhanced transport, with Arctic cod primarily present in the northeastern portion of the survey area, which was characterized by cool surface and bottom temperatures. The substantial increase in abundance of age-0 pollock in recent years suggests that environmental conditions now allow this species to extend its northern range into the southern and central Chukchi Sea, at least on a seasonal basis. The changes in abundance and species composition of pelagic fishes in the 2012–2019 time series are tightly coupled to recent changes in sea ice, temperature, and the increasing transport of Bering Sea waters through Bering Strait into the Chukchi Sea. Given that the environment is expected to experience further warming and increased transport, these northward shifts in species distribution are likely to persist in the future.

New distribution records of kelp in the Kitikmeot Region, Northwest Passage, Canada, fill a pan-Arctic gap

Kelps play important roles in ecosystems as they provide structural habitat and protection, and supply food. Given these beneficial roles and observed increases in seaweed biomass and distribution ranges across the Arctic, mapping kelp occurrence around Arctic coasts is both timely and necessary for future conservation. Here, we fill spatial gaps in the knowledge of kelp distribution in the southern Northwest Passage, Canadian Arctic Archipelago; specifically, we report the occurrence of Laminaria solidungula, Saccharina latissima and Alaria esculenta from Victoria and Dease straits and Bathurst Inlet in the Kitikmeot Region at depths mostly from 10 to 30 m (max. 40 m; upper extent vessel-limited). Kelp specimens were found at bottom water temperatures from sub-zero to 1 °C (surface-T to ~ 6 °C) and bottom water salinities of ~ 28 (surface-S < 20) in August–September. Kelp sites were characterized by both strong tidal currents (max. estimates from a tidal model 20–70 cm s−1 in center of passages) and hard substrates, interspersed with finer sediments. Co-occurring identifiable epibenthos was dominated by suspension-feeders preferring currents (sea cucumbers, soft corals, Hiatella clams), potential kelp consumers (sea urchins Strongylocentrotus sp., Margarites snails, limpets) and predatory invertebrates (sea stars, lyre crabs). At the same and some deeper nearby sites, loose kelp fragments were also found at the seabed, suggesting that kelps contribute to the regional detrital food web by supplying carbon to less productive sites. Kelps in the region may expand their ranges and/or growing season with reduced ice cover and warming, although constraints through local turbidity sources, extreme temperatures, low salinity and low nutrient concentrations are also recognized.

A song of ice and vessels: Seasonal trends in the soundscape of the western arm of Lake Superior

The soundscapes of marine systems have been studied for decades to determine spatial and temporal patterns of biological, geological, and anthropogenic activity. However, comparatively little is known about freshwater soundscapes. Lake Superior is the world’s largest freshwater lake by surface area and home to a variety of aquatic animals. The Twin Ports of Duluth and Superior, located in the western arm of Lake Superior, are annually closed because of ice cover, providing a unique opportunity to explore sound levels in the absence of vessel traffic. Passive acoustic monitoring was conducted over 15 months between November 2018 and March 2020 to investigate seasonal patterns. Sound pressure levels were significantly lower (8 dB) at low frequencies (<100 Hz) during the winter and spring months (when ice cover increased). Quieter ambient sound pressure levels may provide an acoustic refuge to soniferous animals, especially those that vocalise during winter spawning. During the ice-free months, commercial shipping introduces a near continuous source of sound, increasing sound levels by up to 25 dB (at frequencies < 1,000 Hz). Climate change is expected to further reduce ice cover and thickness allowing the shipping season into the Twin Ports to be extended, thus warranting concern about the effect of noise from vessel activity on aquatic life. This study provides a baseline of the soundscape in the western arm of Lake Superior, against which scientists and environmental managers can assess future changes to this important habitat.

Changes in Freshwater Content in the Arctic Basin, Sea Ice Cover Reduction and Warming in the Arctic

The paper deals with global changes in regime and climate of Arctic basin. Changes in the content and inflows of freshwater into the upper 100 m layer of the Arctic basin from decade to decade (1950s – 2010s) were determined. Data from national expedition surveys of the Arctic Basin in the 1950s to the 1970s combined with data from expe- ditions in the 1980s to the 2010s were used. The results obtained showed that freshwater content (FWC) decreased in the Eurasian and increased in the Amerasian part of the Arctic basin in the 1990s – 2000s. The decrease in the FWC occurred in the result of increased Atlantic water inflow since the 1990s and the salinization of the upper 100 m layer of Arctic Ocean, despite increased precipitation, river runoff and sea ice melt. The influence from the tropics of the North Atlantic on climatic changes in the structure of Arctic basin waters, on the reduction of the sea ice cover and warming in the Arctic with a delay of 2-3 years was established. Methods of multidimensional correlation analysis, calculation of spectra and coherences, and construction of correlation graphs were used.

Russia, NATO and the Arctic: Rivalry, Security, Possible Scenarios of Geopolitical Competition

The article is devoted to the analysis of key aspects of the geopolitical struggle for the Arctic region. The relevance of the research stems from the fact that due to climate change and the reduction of ice cover in the Northern Hemisphere, the Arctic is of increasing interest to world powers, which are competing for control over new deposits of natural resources and strategic dominance in the region. It is emphasized that the Arctic has become a region where leading states are trying to implement various geopolitical strategies. On the one hand, the four members of the Arctic Five under the auspices of NATO are trying to implement a scenario in which the polar zone is divided into stable zones of influence. On the other hand, there are the interests and position of Russia, which is concerned about the increased military activity of the United States and NATO in the region. The purpose of the article is to study the problems of the Arctic militarization, which will make it possible to identify challenges and threats in this region for Russias national strategy. The research methodology is based on theoretical analysis and dialectical method. In addition, the study used formal logical methods and various approaches to information processing (analysis, synthesis, induction, deduction). The forecasting method helped to determine the range of possible military and political trends in the region. The article identifies the potential interests of Russia, the United States and NATO in the context of increased attention to the Arctic region in recent years. The study reflects a comparative analysis of the policies and interests of the member states of the Arctic Five and NATO, namely Denmark, Canada, Norway and the USA, their practical steps in the High North. The author describes possible prospects for interaction and dialogue on countering the main threats to international security within the framework of a comprehensive strategy. Considering the role and rivalry between Russia and NATO in the Arctic, the article highlights the authors forecasts of further military and political presence of NATO in the Arctic and the necessary actions for Russia to defend its northern territories.

Trends in the intensity of dense water cascading from the Arctic shelves due to ice cover reduction in the Arctic seas

The article discusses the possible relationship between changes in the ice cover area of the shelf seas of the Arctic Ocean and the intensity of dense water cascading, based on calculation data obtained with the NEMO model for the period 1986–2010, with the findings issued at 5-day intervals and a spatial resolution of 1/10°. The cascading cases were calculated using an innovative method developed by the author. The work is based on the assumption that as the ice cover in the seas retreats, the formation of cooled dense water masses is intensified, which submerge and flow down the slope from the shelf to great depths. Thus, in the Arctic shelf seas, the mechanism of water densification due to cooling is added to the mechanism of water densification during ice formation, or, replaces it for certain regions. It was found that in the Barents Sea, the Laptev Sea and the Beaufort Sea, a decrease in the ice cover area causes an increase in the number of cases of cascading. However, in most of the Arctic seas, as the area of ice cover decreases, the number of cases of cascading also decreases. As a consequence, for the whole Arctic shelf area, the number of cases of cascading also decreases with decreasing ice cover. It is shown that in the Beaufort Sea the maximum number of cascading cases was observed in the winter period of 2007–2008, which was preceded by the summer minimum of the ice cover area in the Arctic Ocean. In the Barents Sea after 2000, a situation has been observed where the ice area has been decreasing to zero values, whereas the number of cascading cases has for some time (1 month approximately) remained close to high winter values. This possibly means that the cooling and densification of the waters in ice-free areas occurs due to thermal convection. Based on the calculation of the number of cases of cascading, it can be argued that the intensification of cascading due to a reduction in the ice cover is a feature of individual seas of the Arctic Ocean, those in which there is no excessive freshening of the upper water layer due to ice melting.

Comparative analyses of changes in the ice cover of the Tatar Strait in relation to 1961-1990 and 1991-2020 climate normals

Current warming in the Northern Hemisphere has encompassed not only the surface atmosphere, but also manifested itself in a change in the state of the sea ice cover, as an integral part of the climate system. One of the indicators of the warming was a reduction in ice cover observed in the water area of the Far Eastern seas since the late 1980s. The prospective areas of hydrocarbon deposits discovered in the Tatar Strait, as well as the existing needs for the transportation of hydrocarbons along the shipping routes in the strait, led to the emerging interest in researching the ice regime of the Sea of Japan in the modern climatic era. The proposed paper analyses the changes in ice cover of the Tatar Strait in relation to 1961–1990 and 1991–2020 climate normals. The regularities of the long-term variability of ice cover within non-overlapping 30-year periods were revealed. The analysis of the dynamics of ice cover anomalies, calculated for each normal separately, was carried out. An assessment of the distribution of winter types in the periods under study was carried out. The characteristic features of variations in the intra-seasonal course of ice cover and the dates of the onset of ice phases in different types of winters were identified.

Relative vs. absolute wind stress in a circumpolar model of the Southern Ocean

The transfer of momentum between the atmosphere and ocean is dependent upon the velocity difference between the seawater and overlying air. This is commonly known as relative wind, or ocean current interaction, and its direct effect is to damp mesoscale ocean eddies through the imposition of an opposing surface torque. If an ocean model neglects the ocean velocity in its bulk formulae, this can lead to an increase in power input to the ocean and a large increase in Eddy Kinetic Energy (EKE). Other secondary effects that are dependent upon the current system under consideration may also occur. Here we show that the neglect of relative wind leads to an ∼50% increase in surface EKE in a circumpolar model of the Southern Ocean. This acts to increase the southwards eddy heat transport, fluxing more heat into the seasonal ice zone, and subsequently reducing ice cover in all seasons. The net reduction in planetary albedo may be a way for a largescale impact on climate.

Impact of a warm anomaly in the Pacific Arctic region derived from time-series export fluxes.

Unusually warm conditions recently observed in the Pacific Arctic region included a dramatic loss of sea ice cover and an enhanced inflow of warmer Pacific-derived waters. Moored sediment traps deployed at three biological hotspots of the Distributed Biological Observatory (DBO) during this anomalously warm period collected sinking particles nearly continuously from June 2017 to July 2019 in the northern Bering Sea (DBO2) and in the southern Chukchi Sea (DBO3), and from August 2018 to July 2019 in the northern Chukchi Sea (DBO4). Fluxes of living algal cells, chlorophyll a (chl a), total particulate matter (TPM), particulate organic carbon (POC), and zooplankton fecal pellets, along with zooplankton and meroplankton collected in the traps, were used to evaluate spatial and temporal variations in the development and composition of the phytoplankton and zooplankton communities in relation to sea ice cover and water temperature. The unprecedented sea ice loss of 2018 in the northern Bering Sea led to the export of a large bloom dominated by the exclusively pelagic diatoms Chaetoceros spp. at DBO2. Despite this intense bloom, early sea ice breakup resulted in shorter periods of enhanced chl a and diatom fluxes at all DBO sites, suggesting a weaker biological pump under reduced ice cover in the Pacific Arctic region, while the coincident increase or decrease in TPM and POC fluxes likely reflected variations in resuspension events. Meanwhile, the highest transport of warm Pacific waters during 2017–2018 led to a dominance of the small copepods Pseudocalanus at all sites. Whereas the export of ice-associated diatoms during 2019 suggested a return to more typical conditions in the northern Bering Sea, the impact on copepods persisted under the continuously enhanced transport of warm Pacific waters. Regardless, the biological pump remained strong on the shallow Pacific Arctic shelves.

Winter Climate and Lake Morphology Control Ice Phenology and Under‐Ice Temperature and Oxygen Regimes in Mountain Lakes

AbstractWarming 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.

Formation of dense water dome over the Central Bank under conditions of reduced ice cover in the Barents Sea

In this study on the basis of various data sets we traced formation of a ‘dome’- shaped density structure over the Central Bank - an important morphological element of the Barents Sea bottom topography. The major conclusion, which follows from the performed analysis, is that under changed conditions at the sea surface, transformation of thermohaline structure during the cold season principally differs from that under the mean climate conditions in the 20th century. Transition from the stratified vertical structure (in summer) to the homogeneous one (in winter) is governed by thermal convection. Additional input of warm and salty water with inflowing Atlantic origin water is crucial to allow vertical mixing to reach the seabed before the temperature drops to the freezing point. Cascading of dense water from the bank commences as soon as the convection goes down to the level of seabed. The influence of cascading on the Barents Sea hydrographic structure extends several hundreds of kilometers off the bank. In the absence of advective influx of salt and warm water vertical convection can also reach the seabed. However, under this condition formation of sea ice and haline convection is required. In this case water temperature in the homogeneous water column over the bank is close to the freezing point. Obtained results suggest that in the warmer climate the role of sea ice in winter transformation of thermohaline conditions over the bank is opposite to what it was in the 20th century: imported sea ice blocks convection, thus making the water in the dense ‘dome’ warmer than it typically was in colder climate.

Eutrophication and climatic changes lead to unprecedented cyanobacterial blooms in a Canadian sub-Arctic landscape

Cyanobacterial blooms have been increasing in frequency and intensity but are often considered an issue restricted to temperate and tropical lakes. Here we report on one of the first occurrences of recurring cyanobacterial (Planktothrix spp.) blooms in a sub-Arctic lake from Yellowknife (Northwest Territories, Canada) and provide a long-term environmental context for the recent blooms using local meteorological data and multi-proxy paleolimnological analyses. Multiple co-occurring regional (gold mining emissions and climatic change) and local (land clearance and urbanization) stressors have impacted Jackfish Lake during the 20th and early-21st centuries, which have led to biological responses across multiple trophic levels. The unprecedented post-2013 cyanobacterial blooms were likely a cumulative response to nutrient enrichment and complex climate-mediated changes to lake thermal properties. A regional analysis of eight lakes around Yellowknife revealed that reduced ice cover duration and longer growing seasons have led to an increase in whole-lake primary production, whilst urban lakes were also fertilized by nutrients from local land-use changes in their catchments. Our findings suggest that anthropogenically nutrient-enriched sub-Arctic lakes, akin to their lower-latitude counterparts, may be vulnerable to cyanobacterial blooms in a warming world.

Wintertime growth in Atlantic salmon under changing climate: the importance of ice cover for individual growth dynamics

Understanding the dynamic nature of individual growth in stream-dwelling salmonids may help forecast consequences of climate change on northern fish populations. Here, we performed an experimental capture–mark–recapture study in Atlantic salmon (Salmo salar) to quantify factors influencing wintertime growth variation among juveniles under different scenarios for ice-cover reduction. We applied multiple imputation to simulate missing size observations for unrecaptured fish and to account for individual-level variation in growth rates. The salmon parr exhibited substantial body length shrinkage in early winter, suppressed growth through midwinter, and increased growth rates in late winter and particularly in spring. Unexpectedly, the presence of ice cover had no direct effects on wintertime growth. Instead, our results implied increasing energetic costs with reducing ice cover; individuals exposed to absent or shortened ice-covered period gained mass at a lowered rate in spring, whereas the present, long ice-covered period was followed by rapid growth. This study emphasizes natural resilience of Atlantic salmon to wintertime environmental variation, which may help the species to cope with the reductions in ice-cover duration due to climate change.

The role of internal feedbacks in shifting deep lake mixing regimes under a warming climate

Abstract Climate warming is causing changes in the physics of deep lakes, such as longer summer stratification, increased water column stability, reduced ice cover, and a shallower depth of winter overturns. An ultimate consequence of warming would be a transition to a different mixing regime. Here we investigate the role of physical, chemical, and biological feedback mechanisms that unfold during a shift in mixing regime, and whether these feedbacks could prompt and stabilise the new regime. Although climate, interannual temperature variation, and lake morphometry are the main determinants of a mixing regime, when climate change causes shifts in mixing regime, internal feedback mechanisms may gain in importance and modify lake ecosystem functioning. We review the role of these feedbacks in three mixing regime shifts: from polymictic to seasonally stratified, from dimictic to monomictic, and from holomictic to oligomictic or meromictic. Polymictic lakes of intermediate depth (c. 3–10 m mean depth) could experience seasonal stratification if a stratification event triggers phytoplankton blooms or dissolved organic matter release, reducing transparency and therefore further heating the surface layer. However, this feedback is only likely to have influence in small and clear lakes, it would be easily disturbed by weather conditions, and the resulting stratified state does not remain stable in the long term, as stratification is lost in winter. The ice‐albedo feedback might cause an accelerated shift from ice‐covered (dimictic) to ice‐free (monomictic) winters in sufficiently deep (mean depth 50 m or more) lakes, where temperature memory is carried over from one winter to the next. Nevertheless, there is an ongoing debate into whether this process can persist during natural weather variations and overcome self‐stabilising mechanisms such as thermal insulation by snow. The majority of studies suggest that a gradual transition from dimictic to monomictic is more likely than an abrupt transition. A shift from a holomictic to a meromictic regime can occur if anoxia is triggered by incomplete mixing and an increase in deep‐water density—through the accumulation of solutes—exceeds a density decrease by hypolimnetic warming. A shift to meromixis would strongly alter the biology of a lake and might be difficult to reverse. If solutes accumulate only minimally in the hypolimnion, an oligomictic regime is formed, in which years with complete and incomplete mixing alternate. Understanding the importance of feedback mechanisms and the role of biogeochemistry when lakes shift in mixing regime could lead to a better understanding of how climate change affects lake ecosystems.