Historically, research on inland waters has focused on the warmer months of the year. Limnologists have mostly avoided studying lakes in winter, especially lakes that experience seasonal ice cover, as if dynamics beneath the ice were unimportant.
But multiple lines of evidence now present a compelling case that winter is indeed a fascinating and important time for lakes. Under dark conditions, when snow and ice obscure light penetration, degradation of organic material already in lakes still occurs, and when clear ice allows some light through, this light can fuel primary production to levels even higher than those in summer.
Recent high-profile data syntheses of lake water temperatures, ice cover, and ecology under lake ice [Hampton et al., 2017] are galvanizing the scientific community to focus on winter studies, and new data streams are being amassed by in situ sensors deployed during seasonal ice cover. Furthermore, recognition of the magnitude and rapidity of ice loss trends combined with recent work highlighting substantial socioeconomic impacts for people whose livelihoods are associated with winter lake ice cover suggests that winter presents an important research frontier within limnology, from the biology and biogeochemistry of lakes to the dynamic physics of cold water and the sociological and cultural ramifications of change.
Winter fieldwork on lakes is still difficult and dangerous, particularly on ice-covered lakes. Thus, although basic understanding about winter limnology has increased in the past decade, the pace of scientific progress has not kept pace with rates of ecological change.
To accelerate progress in winter limnology, we convened a Chapman Conference at Flathead Lake Biological Station in northwestern Montana in October 2019 to address hypotheses associated with five winter limnology topics: climate and ice, biogeochemistry, seasonal biological connections, temperature dependency of biotic processes, and food web interactions. Chapman Conferences are small meetings designed to facilitate transformative, innovative research discussions. Ours lived up to the billing, with 44 researchers presenting their latest research via brief “lightning” talks and posters and most of the time together devoted to discussing the state of winter knowledge and where we go from here.
Climate and Ice
Long-term records of seasonal ice cover on many northern temperate lakes show declines in seasonal ice duration over the past several decades. However, such records are far from complete and not available for many lakes. Lake ice records that are available are clustered within selected regions of the historic freeze zone, especially the midwestern United States, southeastern Canada, and northern Europe, leaving gaping holes in our knowledge of lake ice changes at higher latitudes and elevations, where winter warming may be occurring most rapidly.
In addition, knowledge about lake ice loss has been derived mainly from lakes larger than 10 square kilometers, but these lakes are far outnumbered globally by smaller lakes. And analyses of lakes that sometimes experience ice-free winters—the proverbial canaries in the coal mine with respect to warming climate—have been rare. One such analysis by Sharma et al.  integrated available ground-based ice observations with climate projections. That team found that ice-free winters could be arriving at hundreds of thousands of lakes across the Northern Hemisphere in coming decades.
Biogeochemistry and Biological Connections
Changes in lake ice quality and overlying snow depth can dramatically affect primary production and trophic pathways for carbon and nutrients. One recent study showed that in permanently ice covered Antarctic lakes, nutrient processing and active metabolism occurred not only in summer but also in the total darkness of winter [Vick-Majors and Priscu, 2019].
The thinning of Antarctic lake ice—which has happened so rapidly that recent work has predicted the disappearance of permanent ice cover within 10 to 30 years [Obryk et al., 2016, 2019]—will dramatically change under-ice biological communities. Ice-adapted algae, for example, will have less time available in this specialized niche and may be outcompeted by other algae under ice-free conditions. And the eventual disappearance of lake ice would spell the loss of an entire habitat in these polar desert aquatic systems for microbial communities that thrive in temporary ice melt pools on the surface of lake ice [Santibáñez et al., 2019].
Because of under-ice biological activity, changing lake ice conditions may be particularly influential on global carbon dynamics. Many studies, for example, have confirmed that biogenic gases like carbon dioxide and methane accumulate in lakes beneath seasonal ice cover as algal biomass produced during the previous summer degrades [Ducharme-Riel et al., 2015; Denfeld et al., 2018]. The accumulation (or lack) of snow cover on ice can also affect carbon dynamics. Snow cover limits how much light reaches the under-ice environment, which frequently controls under-ice photosynthesis by phytoplankton. In turn, this photosynthesis controls the uptake of nutrients mobilized by degradation of organic matter at the sediment-water interface. Photosynthesis under ice cover also influences the production of dissolved oxygen, which has cascading influences on other biogeochemical transformations, such as nitrification (the oxidation of ammonia to nitrite and nitrate) and mineralization of dissolved organic matter.
Depending on the rates of these processes, nutrients can accumulate during winter and can ultimately be available to support phytoplankton growth in summer. In just one example, many phytoplankton taxa preferentially use ammonium as a nitrogen source, and those organisms will be at a disadvantage as nitrate becomes a larger fraction of the nitrogen pool.
The lasting effects of winter biogeochemical processes on ecology during the ice-free season and effects of winter ecology on summer biogeochemical processes together present a rich area for new research. Research on the drivers and dynamics of ice conditions, such as clarity and thickness, also remains a frontier.
Adapted for Cold
In Siberia’s Lake Baikal, both the bottom and the top of the food web are physically dependent on ice: The largest algal biomass is associated with endemic diatoms that bloom under clear winter ice, and the lake’s top predator, the Baikal seal, gives birth and molts on ice.
In Antarctica, meanwhile, autotrophic communities are well adapted to winter conditions and can thrive in the extreme dark conditions of polar winter. For such cold-water specialists, their year-round coexistence with other species may depend on temporal heterogeneity in lake conditions provided by winter because many other species that might otherwise dominate all the time are disadvantaged by winter conditions. For example, fish kills in lakes are a widely known winter phenomenon that occurs when high respiration rates combine with low primary production to deplete oxygen concentrations. Fish tolerant of low oxygen, cold temperatures, or low light thus may find refuge from competition in winter.
Despite these examples, when it comes to cold temperatures in lakes, we know relatively little about under-ice population dynamics, animal behavior, and trophic interactions, particularly for fish. Winter conditions commonly limit growth and reproduction for both micro- and macroorganisms, but many taxa are well adapted to life with ice. The diverse ways in which organisms survive winter conditions are of increasing interest. For example, zooplankton undergo major shifts in biochemical composition as winter approaches, developing high lipid concentrations to cope with low temperatures; however, the reverberations of such processes throughout the food web are poorly understood.
Accelerating Winter Research
The conference last October revealed even more about how much we have left to learn. With the increased scientific dialogue and new collaborations resulting from the meeting, limnology is poised for a major leap forward in understanding winter processes and the nature of changing seasonal cycles in inland waters.
Limnologists have now left behind old ideas that winter is a time of low biological activity and is of low priority for study. And it is likely that limnologists will eventually abandon using the common but ill-defined term “growing season” to vaguely refer to relatively warm summer months. Limnologists will find it more useful in the future to define summer not as the growing season but instead as a time when algal blooms are more likely to occur, especially now that durations of ice-free conditions are getting longer with the worldwide trend toward less ice cover.
Soon, advanced methods of remote sensing of seasonal ice cover may uncover regional differences and uncertainties in lake ice loss as more frequent and higher spatial resolution imagery becomes available and as solutions to complications in interpreting such imagery from cloud cover emerge. In addition, increasing winter hardiness of in situ sensors may allow under-ice measurements that support more detailed inquiry into winter biogeochemistry, the physical dynamics of water, and even the behavior of overwintering organisms such as fish.
Attendees emerged from the conference with a consensus on the knowledge status and feasibility of future projects that can help sustain research momentum. Continued acceleration of winter research by a geographically diverse group of investigators will generate detailed information about lake dynamics and will support development of models describing lake-climate interactions. These data and tools can be used to help predict the fate of lake ecosystems that so many organisms—and communities—rely upon as we continue to head toward an increasingly ice-free world.