Staying warm in the winter is hard. Chickadees eat constantly in order to survive long, cold winter nights. Squirrels spend precious time and energy creating complex insulated nests. Deer browse on nutrient-poor twigs to get as many calories out of their surroundings as possible. Yet compared to fish and other aquatic organisms, terrestrial wildlife breathe easy – literally. As fish battle the cold through the long winter, they are steadily running out of oxygen.
Life underwater would be impossible without a unique characteristic of water: its solid form is less dense than its liquid form. In other words, ice floats. As air temperatures plummet in the fall, ice forms on the tops of lakes and ponds and stays there. In deep lakes, the temperature under the ice remains above freezing and allows fish and other aquatic organisms to remain active.
However, the same ceiling of ice that allows aquatic life to persist causes one of the main problems for fish in winter. When fish respire through their gills, they absorb oxygen that has been dissolved into the water through two main mechanisms. Aquatic plants produce oxygen during photosynthesis, and waves and currents mix oxygen from the air with surface water. When the surface of the water is frozen in place, both mechanisms stall.
Most fish cope with the low oxygen levels and cold temperatures by slowing down. The less they swim, eat and breathe, the less oxygen they need. Some fish go so far as to burrow into the lake bottom and reduce their metabolisms so that they go into diapause, a state of suspended development. Other fish migrate toward areas with more oxygen, like inlets or the upper regions of the water column.
Despite these behavioral and evolutionary adaptations, low oxygen levels are one of the main causes of winter mortality in Vermont’s fish. Different fish species are more and less susceptible to winterkill. Despite being considered a coldwater fish, trout populations can be decimated when oxygen levels drop too far. Other groups of fish like mud minnows and fathead minnows rarely succumb to mass die-offs. Species popular to ice fishermen, like northern pike and perch, generally are moderately tolerant of low oxygen.
Winterkill is fairly common during long cold winters. When the ice melts, winterkilled fish provide a boon of nutrients to scavengers, such as bald eagles and river otters. Fish populations usually rebound within a few years. However, for fish populations already stressed by invasive species, overfishing or low population levels, winterkill can be catastrophic. Most importantly, lakes that suffer from nutrient-loading are most susceptible to winterkills. During winter the overabundant algae and aquatic plants in these eutrophic lakes decompose and use precious oxygen. This causes oxygen levels in these lakes to decrease to dangerous levels long before the ice melts.
So the next time a harsh wind blows in from the north and sends a chill down your back, or when you resort to heroic measures to regain feeling in your fingers or toes, take a deep breath of that cold oxygen-rich air. You have it good.
Claire Polfus is a second-year ecological planner who is very happy with the plentiful oxygen in the winter air, especially when she is struggling up a steep hill on her skinny skis.