Rebekah Garza | April 22, 2026

Weathering the cold New England winters can be difficult, even with modern comforts like space heaters and fuzzy socks. That’s especially true this past winter of 2026, when Massachusetts experienced a cold snap that brought with it the chilliest temperatures seen in the last decade [1]. Now, imagine what it might be like trying to survive all winter at the bottom of the icy frozen campus pond (Fig. 1).

Figure 1. Ducks and a single goose sit atop the half frozen campus pond in winter. (Photo by Rebekah Garza)

Talk about a cold plunge! But over the past few weeks you may have noticed life returning to, and emerging from, the pond as it thaws. So, how exactly do animals like turtles and fish survive all winter under extreme conditions? The key is in their breath.

Many aquatic organisms slow down their breathing, and with it their metabolisms, to conserve energy under the ice. This process is called “brumation” in reptiles and amphibians, which is similar to hibernation for mammals. It is common in aquatic ectotherms, aka “cold-blooded” animals who cannot regulate their own body temperatures but whose bodies instead match the temperature of their surrounding environment. If you see a snapping turtle (Chelydra serpentina) under the pond ice, you may worry that it’s frozen. But it is very much alive, and can even move about under the ice. It turns out that the cold layer of ice that forms over lakes and ponds in the winter actually benefits ecotherms, acting as a blanket that insulates them from freezing winds. Using this to their advantage, animals can burrow at the bottom of lakes and ponds, slow their breathing, and pass the winter under the ice at temperatures just above freezing. 

Lake Seasonality

Throughout the year, lakes in temperate regions undergo broad changes in temperatures, which results in seasonal patterns of mixing and layering. During summer months the top of the lake is hot and the bottom is cold. That difference in temperatures causes a difference in densities, inhibiting mixing between the top and bottom of the lake like oil and water. This process is known as “thermal stratification”, or layering of distinct water temperatures [2]. It’s worth noting that full stratification is most common in deeper lakes, and probably doesn’t occur in the campus pond, since  it’s fairly shallow. When stratified, the benthic biolayer, or layer of muck at the bottom of the lake, becomes depleted in oxygen because it is chock full of microorganisms using it for respiration. In the autumn, as surface temperatures begin to cool, the layers in the lake begin to fade into each other and mix throughout the water column [2]. This is called turnover (Fig. 2). During turnover, deeper circulation restores oxygen to the benthic zone and brings important nutrients to the surface. Water is most dense at 4°C/39.2°F, so when ice forms at the top of a lake in early winter, the water at the bottom remains just above freezing, allowing organisms to survive the winter in the murky depths.

Figure 2: Seasonal density-driven mixing patterns for lakes. In the spring and fall, mixing occurs, but in summer and winter, layers are distinct. (Source: Hydrated via Wikimedia Commons.)

Built for the Cold

In addition to using the physics of water density to their advantage, aquatic organisms have also evolved physiological adaptations to help them survive winter in cold, oxygen-limited environments. The first adaptation is called torpor. Torpor is a sleep-like state, first discovered in fish, which involves a slowing of the metabolism and features discontinuous breathing to conserve oxygen but maintain enough mobility to feed and flee predation [3]. While fish breathe underwater year-round, even air-breathing animals have adapted to survive winters under ice. For example, turtles breathe air and have lungs instead of gills (Fig. 3). But in the winter, they can slow down their metabolisms so much that they don’t need to use them. Instead, they can diffuse oxygen into their bodies by moving water across blood vessels in their skin, mouth and throat, and butts [4]. That’s right – during the winter, turtles activate butt breathing to survive. The scientific term for this is called “cloacal respiration.” Turtles have been shown to be able to remain in this state for more than 100 days before the build up of lactic acids in their bodies becomes dangerous [5]. 

Figure 3: A snapping turtle coming up for breath in a pond. (Source: Matthew Hoobin via Wikimedia Commons)

Spring Thaw

When winter ends and the landscape begins to thaw, warm rains accelerate the erosion of the layer of ice atop the pond. When the ice finally melts, spring turnover occurs, mixing nutrients throughout the water column. The timing and duration of spring turnover is important for water chemistry and microbes throughout the lake [6]. Certain microorganisms called phytoplankton rely on circulation to bring key nutrients to the surface [7]. Like plants, phytoplankton make their own food via photosynthesis, so they’re vital to form the base of a lake’s food web. When more mixing occurs, phytoplankton blooms are larger, and there is greater total lake biomass overall  [8]. During emergence from hibernation, this is very important as animals need access to abundant food sources. Since their bodies are still warming up, animals can move very slowly in the spring, leaving them vulnerable to predation. Slowly but surely, as the animals warm up under the spring sunshine, they return to life as usual. So this spring, when you see the geese returning in their migration to the pond, keep an eye out for the fish and turtles too as they emerge from the cold depths to bask in the sun.

References

[1] Noyes, Danielle. “Why Boston feels so much colder than usual this winter.” WBUR, February 12, 2026.

[2] Wetzel, Robert. “Chapter 6: Fate of Heat.” In Limnology (Third Edition). Academic Press (2001).

[3] Crawshaw, L.I. “Low-temperature dormancy in fish.” American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 246:4 (1984).

[4] FitzGibbon S, Franklin C. “The importance of the cloacal bursae as the primary site of aquatic respiration in the freshwater turtle, Elseya albagula.” Australian Zoologist 35, 276–282 (2010).

[5] Litzgus J. “The secret to turtle hibernation: Butt‑breathing.” The Conversation, November 21, 2017.

[6] Kling G.W., Giblin A.E., Fry B., Peterson B.J., (1991), “The role of seasonal turnover in lake alkalinity dynamics.” Limnology and Oceanography 36:1 (1991).

[7] Pettersson, K. “The spring development of phytoplankton in Lake Erken: species composition, biomass, primary production and nutrient conditions — a review.” Hydrobiologia 191, 9–14 (1990).

[8] Gantzer, C., and Stefan, H.G. “A model of microbial activity in lake sediments in response to periodic water-column mixing.” Water Research 37:12 2833-2846 (2003).