Call it a biologist’s dilemma: no sooner does winter ice form than I wonder what’s going on beneath it. As it turns out, plenty.
To start, for many land organisms, winter means hibernation or brumation (a specialized metabolic slow-down in cold-blooded critters), but in water things aren’t as black and white: differing degrees of movement and activity make winter life in water a little greyer.
As an example, a recent Ontario study found that interactions between lake trout and smallmouth bass change dramatically with seasonal shifts in conditions. Specifically, ice-covered periods—a stressor for all fishes—appear to play a unique role: though denizens of deep, cold, offshore waters during summer, lake trout are drawn by the changing temperature and oxygen gradients of winter into shallower, nearshore areas occupied by bass. Lest these top carnivores suddenly find themselves battling for resources, however, nature has worked it out. While the trout remain active throughout winter, the bass enter a state of semi-dormancy, slowing both swimming and eating to levels that obviate any competition between these typically spatially separated species; when the ice melts and surface waters warm, the trout head back to depth while the bass get back to business.
Appreciating such behaviour requires a nod to the chemistry and physics of seasonal lake hydrology. While the water in shallow lakes mixes frequently and features more uniform temperatures top-to-bottom in all seasons, lakes deeper than six metres tend to stratify during both summer and winter in gradients that prevent mixing of top and bottom layers. That’s because unlike most liquids water is densest at 4 C—meaning it’s lighter at both warmer and cooler temperatures. Thus, in summer, warmer, less-dense water floats atop a cooler, denser layer—it might be 20 C at the surface but single digits only a few metres down (as you’ve doubtless noticed in Whistler’s lakes); in winter, less-dense, close-to-freezing water also floats atop a denser layer—which is why ice both forms on the surface and floats. In between these states, lakes undergo the phenomenon of “turnover.” In autumn, as air temperatures dive, stratification in the water column breaks down; once surface waters reach 4 C they sink toward the lower 4 C layer causing a lake-wide mixing of nutrients and oxygen; in spring, ice-off allows the colder upper layer to rapidly warm to 4 C, precipitating a similar flop.
The biggest winter challenge for fish is the availability of oxygen, which becomes limited under a surface stilled by ice, where light is in short supply and plants consume oxygen instead of producing it. Oxygen can fall so dramatically in small, shallow lakes that fish suffer hypoxia or even suffocate. Dead fish, of course, decompose, which consumes more oxygen and causes more fatalities, a snowball effect that can lead to winterkill—the death of most or all lake organisms. But different lake fish have different tricks to avoid hypoxia: darters emigrate from the lake into flowing water before ice forms; sticklebacks remain in the lake, but move to inlets of freshwater streams where dissolved oxygen is higher; bottom-dwellers move to the lake’s edges where the water is oxygenated down to the substrate; mud minnows can breathe gaseous air, surviving on oxygen trapped in air bubbles beneath the ice.
Lakes lose oxygen from the bottom up, so fish that frequent deeper habitats—such as the aforementioned lake trout—move to shallows, where oxygen in the ice slowly dissolves into the water. Northern pike, on the other hand, create their own areas of high oxygen: resting at an angle, noses almost touching the ice, they slowly fan pectoral fins and gill-covers to create a weak current of warm water that melts a dome of oxygen-rich water into the ice.
But hanging out close to ice has its own challenges, as this is now the coldest part of the lake, at or below 1 C with a risk of hypothermia. Different fish species counter this by: a) conserving energy by reducing metabolic, respiratory and feeding activity to rely more on fat reserves, or; b) actually increasing feeding, respiration and general activity during ice-over as the weed cover that once protected their small prey dies back; this strategy of feeding actively throughout winter is why ice-fishermen exist.
River fish employ a mix of the two, drastically reducing body temperature and metabolic rates compared to summer, but continuing to hunt for food (low prey densities, however, mean some reliance on stored fat). Minimizing energy use, most species hole-up in slow, deep pools, while smaller fish and newly hatched fry find cover in coarse gravel and cobble. An unstable ice environment (ephemeral dams, frazil, anchor ice, etc.) caused by frequent freeze-thaw events (hello climate change!) can force fish from holding pools, rapidly stripping energy reserves. Some rivers, however, have built-in respite—because, beavers. Beaver ponds are vital overwintering habitat for many trout; dams halt the movement of frazil and anchor ice and form deep pools that maintain consistent water levels.
Areas with groundwater are another important overwintering habitat for fish and their eggs, as well as other water critters like turtles and amphibians. In lakes, both phytoplankton and zooplankton settle downward to overwinter in the relatively warmer sediments. Bottom-dwelling invertebrates typically burrow into the substrate of the littoral zone where, in spring, light will reach the bottom in sufficient quantity to promote plant growth. But these behaviours are context-dependent, hinging on the type of waterbody, amount of oxygen, and weather—which is being altered by climate change. Indeed, under certain conditions that have become common—like, thin, clear ice—winter biological activity can be high, and even include algal blooms.
What does this all mean? Basically, that seasonal transformations are themselves transforming, and that understanding their impacts is going to get greyer than ever.
Leslie Anthony is a biologist, writer and author of several popular books on environmental science.
