Water’s expansion when it freezes keeps ice on the surface of lakes, rivers, and oceans rather than letting it sink to the bottom, and that single physical quirk is arguably the most important reason Earth’s aquatic ecosystems survive cold seasons at all. Because ice is roughly nine percent less dense than the liquid water beneath it, frozen surfaces act as insulating lids, preventing water bodies from freezing solid from the bottom up. The consequences ripple outward from freshwater ponds to polar seas, from soil formation on mountainsides to plausible scenarios for life on other worlds.
Why Ice Floats in the First Place
Most substances become denser when they solidify. Water does the opposite, and the reason comes down to how its molecules link together through hydrogen bonds. In liquid water, molecules are constantly forming and breaking these bonds, tumbling past one another in a somewhat disordered way. As the temperature drops, the molecules slow down and the hydrogen-bond network becomes more organized. By the time water reaches its freezing point, the molecules lock into a rigid, open hexagonal lattice that takes up more space than the same number of molecules occupied as a liquid. Research modeling this behavior has shown that the average number of hydrogen bonds correlates tightly with water’s so-called density anomaly, the fact that water reaches peak density near 4 °C and then becomes less dense as it continues cooling toward 0 °C.1PubMed. Correlation of structural order, anomalous density, and hydrogen bonding network of liquid water
That density peak near 4 °C is just as important as the expansion at freezing. In autumn, a lake’s surface water cools, becomes denser than the water below, and sinks. This overturning mixes oxygen and nutrients throughout the water column. But once the entire lake reaches about 4 °C, any further surface cooling actually makes that water lighter rather than heavier. The coldest water now stays on top, eventually freezing into a floating ice layer. Without that density reversal, cold water would keep sinking, promoting freezing throughout the entire depth of the lake.
The Insulating Lid That Protects Freshwater Life
Once a sheet of ice forms on a lake surface, it dramatically slows further heat loss. Ice conducts heat far less efficiently than liquid water, and a snow layer on top of the ice adds even more insulation. The result is that even during prolonged subzero air temperatures, the water beneath the ice stays liquid, hovering around 0–4 °C depending on depth. Fish, invertebrates, and microorganisms continue to function in this cold-but-liquid environment all winter.
If water behaved like most other substances, the densest form would be solid, and ice would sink as fast as it formed. Lakes would freeze from the bottom up. Shallow bodies of water would become solid blocks of ice each winter, killing off most organisms unable to migrate or go dormant in frozen sediment. Even deep lakes would gradually accumulate ice on the bottom year after year, because sunlight in summer might not penetrate deeply enough to melt everything that froze the previous winter. The floating-ice arrangement prevents that catastrophic scenario.
Establishing the winter ice cover is not always straightforward. Because the thermal expansivity of water is very small near 4 °C, temperature differences alone do not strongly resist mixing just below that threshold. A lake’s water column can keep cooling past 4 °C for some time before stable winter stratification finally sets in, because the density differences created by small temperature gradients near maximum density are too weak to resist wind-driven mixing.2Limnology and Oceanography. What controls the onset of winter stratification in a deep, dimictic lake? This means the transition into winter is a gradual stabilization rather than a sudden freeze-over, and the exact timing depends on lake depth, wind exposure, and local climate.
Life Under the Ice
An ice-covered lake is not a lifeless refrigerator. Even in winter, sunlight penetrates through the ice and can warm the water just below it. The type of ice matters: clear “black ice” transmits far more light than opaque “white ice” (which contains air bubbles and refrozen snow). When sunlight heats a thin layer of water near the ice surface above 4 °C, that warmer, lighter water sits over colder, denser water, creating an unstable situation. Eventually the warm layer overturns, generating convective currents beneath the ice. This process, called radiatively driven convection, becomes a major source of water movement in late winter and spring. The amount of opaque white ice relative to clear black ice controls how quickly this convection kicks in, which in turn affects how nutrients and dissolved gases get mixed through the water column.3Geophysical Research Letters. How Ice Composition Controls Radiatively Driven Convection Under Lake Ice
That mixing matters because winter and early spring are critical periods for nutrient cycling. Oxygen that was mixed into the water during autumn turnover gets consumed by bacteria decomposing organic matter on the lake bottom. Without some degree of under-ice circulation, deep water could become oxygen-depleted, stressing or killing bottom-dwelling organisms. The fact that the ice lid stays on the surface, allowing light in while trapping heat, helps maintain the gentle circulation that prevents complete stagnation.
Sea Ice and Its Brine Channel Ecosystems
In the ocean, the picture gets more complicated because seawater contains salts. Salt lowers the freezing point, so seawater typically freezes around −1.8 °C rather than 0 °C. And unlike freshwater, seawater does not have a density maximum above its freezing point; it just keeps getting denser as it cools, until it finally freezes at the surface because the ocean loses heat to the frigid air faster than currents can mix it downward. The ice that forms still floats, though, because the crystallization process excludes most of the dissolved salt. The rejected brine drains downward, and the remaining ice is mostly fresh and less dense than the seawater below.
This salt-rejection process creates networks of tiny channels and pockets within the ice, filled with extremely salty liquid. These brine channels are a habitat in their own right. A review of sea-ice biology documented a remarkable community of organisms living in these channels: microalgae, bacteria, viruses, fungi, and even small animals, all tolerating temperatures as low as −25 °C and salinities up to roughly 300 (for reference, normal seawater is about 35).4Polar Biology. Sea ice as habitat for microalgae, bacteria, virus, fungi, meio- and macrofauna: A review of an extreme environment Ice algae growing at the bottom of sea ice are a foundational food source in polar marine food webs. Krill feed on these algae, fish and seabirds feed on krill, and so the entire Antarctic and Arctic marine food chain partly depends on organisms that live in and on floating ice.
If sea ice sank, polar oceans would lose this habitat entirely. The seafloor beneath polar waters receives very little light, so algal communities that now thrive at the ice-water interface would have nowhere to grow. The cascading effects on polar food webs would be enormous.
Rivers and Anchor Ice
Lakes and oceans are the usual examples, but rivers have their own relationship with ice that shapes the organisms living in them. In turbulent, shallow rivers, a phenomenon called anchor ice forms directly on the streambed rather than on the surface. Tiny ice crystals called frazil ice form in supercooled water, drift to the bottom, and accumulate on rocks and gravel. In some events, anchor ice grows at rates between about 0.5 and 3.5 centimeters per hour depending on whether it forms through in-place crystal growth or through accretion of suspended frazil particles.5Journal of Hydrology. Investigation of anchor ice evolution in rivers and the impact of hydrometeorological conditions
Anchor ice can temporarily bury stream-bottom habitats, displacing insects and other invertebrates. But when it detaches during daytime warming, it lifts sediment and organic material, redistributing nutrients and reshaping the channel. For stream organisms, this freeze-thaw cycle is a regular disturbance they have adapted to, much like seasonal flooding. It is one more example of how ice’s buoyancy and expansion interact with living systems in ways that go beyond simply keeping a lid on a lake.
Freeze-Thaw Weathering and Soil Formation
Expansion upon freezing does not only protect aquatic life. It also helps create the soil that terrestrial ecosystems depend on. When water seeps into cracks in rock and then freezes, the roughly nine percent expansion exerts enormous pressure, widening those cracks over time. Repeated freeze-thaw cycles are one of the dominant forces that break solid bedrock into transportable sediment.6Earth-Science Reviews. A review on freeze-thaw action and weathering of rocks Over geological timescales, this mechanical weathering produces the mineral particles that combine with organic matter to form soil.
The process also operates within existing soil. Ice formation between soil aggregates breaks them apart, changing the soil’s physical structure and its chemical and biological properties.6Earth-Science Reviews. A review on freeze-thaw action and weathering of rocks In cold climates, these seasonal freeze-thaw cycles keep soil loose and aerated, promote the release of minerals from rock fragments, and create the varied textures that plant roots and soil organisms need. Without water’s expansion on freezing, mountain landscapes would erode much more slowly, and the deep, productive soils of temperate regions would take far longer to develop.
Soil freezing is also influenced by dissolved salts. In real-world soils, salinity turns out to be a dominant factor controlling how much liquid water remains unfrozen at temperatures below 0 °C, because salt exclusion during ice crystal growth concentrates solutes in the remaining pore water and depresses the freezing point further.7Water Resources Research. A Model for the Soil Freezing Characteristic Curve That Represents the Dominant Role of Salt Exclusion This means that even in frozen ground, thin films of liquid water persist around soil particles, allowing slow microbial activity and chemical processes to continue through winter.
How Organisms Cope with Freezing
While floating ice protects aquatic ecosystems from freezing solid, many organisms that live on land or in shallow water have to deal with ice forming in or around their own tissues. The strategies they use are broadly split into two camps: avoiding ice formation altogether or tolerating it under controlled conditions.
Freeze-avoiding insects, for instance, clear their body fluids of anything that could serve as a seed for ice crystal formation, and they accumulate sugar alcohols (polyols) that depress their freezing point dramatically, sometimes allowing them to remain supercooled to around −20 °C without ice forming. Freeze-tolerant insects take the opposite approach, deliberately producing ice-nucleating agents in their extracellular fluid so that ice forms in a controlled way outside their cells at only a few degrees below zero, rather than inside cells where it would be lethal.8PubMed. Physiology of cold tolerance in insects
A key player in freeze tolerance across many species is a class of molecules called ice-binding proteins (sometimes called antifreeze proteins, though the name can be misleading in freeze-tolerant organisms). In freeze-tolerant creatures, these proteins do not prevent ice formation. Instead, they inhibit recrystallization, the process by which large, damaging ice crystals grow at the expense of small, manageable ones. In some cases, ice-binding proteins also prevent extracellular ice from propagating into the cell interior, which would be fatal.9Journal of Experimental Biology. Animal ice-binding (antifreeze) proteins and glycolipids: an overview with emphasis on physiological function
Plants face a similar challenge. When temperatures drop, ice typically forms first in the spaces between cells, where the freezing point is slightly higher than inside the cell. As extracellular ice grows, it draws water out of cells by osmosis, causing severe dehydration. On top of that, the expanding ice exerts direct mechanical stress on cell membranes.10PubMed Central. Extracellular freezing-induced mechanical stress and surface area regulation on the plasma membrane in cold-acclimated plant cells Cold-acclimated plants have evolved membrane compositions and surface-area regulation strategies that let their cells survive this double assault of dehydration and compression. The fact that ice forms outside cells rather than inside them is itself a consequence of water’s freezing behavior. If intracellular freezing happened first, most plant tissues in temperate and boreal climates would die every winter.
Ice and the Origins of Life
One of the more surprising connections between ice and life reaches back to before biology existed at all. The very chemistry that may have launched life on Earth could have been helped along by freezing. When a dilute solution freezes, dissolved molecules get concentrated into tiny pockets of remaining liquid between ice crystals, called eutectic phases. This natural concentration effect can push reactants together at levels that would be impossible in a warm, dilute ocean.
Research has demonstrated that eutectic ice phases promote the synthesis of nucleotide precursors, the condensation of activated nucleotides into random RNA strands, and even the copying of RNA templates by a ribozyme (an RNA molecule that acts as an enzyme). These findings support a broad role for ice in facilitating the steps from prebiotic chemistry all the way to the emergence of RNA self-replication and early Darwinian evolution.11Nature Communications. Ice as a protocellular medium for RNA replication The implication is striking: ice may not only sustain existing life but could have helped get life started in the first place, by providing tiny, concentrated reaction chambers that no warm-water environment could match.
Life Beneath Glaciers and on Other Worlds
If ice can shelter and concentrate the chemistry of life, then thick ice covers might not be barriers to biology but rather enabling environments. On Earth, Subglacial Lake Whillans in Antarctica lies beneath roughly 800 meters of ice. When researchers drilled through to sample it in 2013, they found a shallow water column that was oxygenated and contained enough carbon, nitrogen, and phosphorus to support microbial growth.12PubMed Central. Physiological Ecology of Microorganisms in Subglacial Lake Whillans Microbial communities were indeed living there, sustained by chemical energy derived from minerals in the underlying bedrock and from organic carbon trapped when the ice sheet advanced over what was once open land. The ice above acts as a blanket, trapping geothermal heat and maintaining liquid water despite surface temperatures far below freezing.
This discovery fuels speculation about ice-covered oceans elsewhere in the solar system. Jupiter’s moon Europa has a global ocean beneath an ice shell estimated to be kilometers thick. One of the key questions for astrobiologists is whether nutrients from the irradiated ice surface can reach the ocean below. Several mechanisms have been proposed, including brine drainage through the ice, subduction of surface ice plates, and melt columns created by impacts that punch through to the ocean.13PubMed Central. A Review on Hypothesized Metabolic Pathways on Europa and Enceladus: Space-Flight Detection Considerations If oxidants created by radiation on Europa’s surface can migrate downward through such processes, they could provide the chemical energy that microbial life would need. The same principle that keeps Earth’s lakes from freezing solid, ice’s buoyancy, keeps Europa’s ocean liquid beneath its shell, insulated from the extreme cold of space by kilometers of floating ice.
What Happens When Ice Cover Shrinks
The protective role of ice makes the ongoing loss of ice cover under climate change an especially urgent concern for ecosystems that evolved under its shelter. High-latitude lakes are experiencing shorter ice-covered seasons, higher summer temperatures, and shifts in hydrology. Long-term monitoring of these lakes has documented cascading effects across entire food webs, from changes in phytoplankton composition to restructuring of zooplankton and fish communities.14PubMed Central. Impacts of climate and reduced ice cover on trophic structure and community dynamics in a high latitude lake
In some rapidly warming Arctic lakes, the consequences have been abrupt rather than gradual. Researchers have observed sharp year-to-year shifts in how these lakes mix, stratify, and distribute oxygen, driven by variability in air temperature and weather events. Even small changes in ice cover duration can provoke state shifts in the lake’s physical and chemical regime.15Geophysical Research Letters. Regime Shifts in Lake Oxygen and Temperature in the Rapidly Warming High Arctic A lake that once maintained stable oxygen levels all year under a predictable ice cover can flip to one that develops deep oxygen depletion in summer when ice-free periods lengthen and thermal stratification intensifies. For cold-adapted species that rely on well-oxygenated bottom waters, these shifts can be devastating.
The same dynamic plays out in polar oceans. As sea ice retreats, the brine-channel communities that form the base of polar food webs lose habitat. Species that depend on the ice edge for feeding and breeding, from krill to seals to seabirds, face compressed seasons and shrinking territory. Ice’s role in supporting life is easiest to appreciate when you watch what happens as it disappears. The systems that evolved around floating ice are not simply inconvenienced by its loss; they are structurally dependent on it, and unraveling the ice means unraveling the communities that grew up beneath and around it.