Under Ice

Winter brings to Lake Champlain, and to the many smaller lakes and ponds around the Lake Champlain Basin, a window into a world where the normal rules of life are suspended. People who can't swim can walk on the water instead. Roads do not end at the lake's edge, but extend out to ephemeral villages of ice fishing shanties.

Dramatic as these changes are, we are largely spared the full impact of winter's chill. At the end of a day of ice fishing or ice skating, we can return to the warmth of our furnaces and stoves, cover ourselves in blankets and bed down in near-tropical temperatures.

The animals and plants that live in Lake Champlain must cope each day with cold temperatures, failing light, ice and decreased currents. And how well they cope has a great deal of influence on how we see the lake year-round.

Take, for example, the tiny floating plants, many of them comprised of just a single cell, that are the basis of the food chain in Lake Champlain. These plants, known as phytoplankton, convert sunlight to carbohydrates and proteins; they are food for nearly equally small animals, the zooplankton, which in turn are food for fish and other animals.

In summer, with its abundant light, phytoplankton grow and reproduce as long as there are enough nutrients in the water. In winter, light replaces nutrients as the major constraint on growth.

There are several factors that combine to make winter a time of darkness for the phytoplankton. First, shorter days mean fewer hours of sunlight. Even if the weather was always perfectly clear, our part of the country would receive only two thirds as many hours of sunlight during winter as during summer. In addition, because the sun is low in the sky, sunlight during winter is less intense than the hot, high sun of summer. Combined, the short days and low angle of the sun mean that less than half as much solar energy reaches the area in December, January and February as in June, July and August. Ice on the surface of the lake may compound the problem. Although clear ice, like liquid water, absorbs little light, milky ice or ice with many bubbles blocks much more light.

Prior to the 1950s, it was highly unusual for Lake Champlain not to freeze completely across in a given year. This freeze, known as "lake closing" is monitored by the National Weather Service (NWS). According to NWS records, from 1820 until 1950, there were only seven years when the lake didn't close. While Lake Champlain's shallower bays reliably freeze every year, since 1950 there have been 33 years without a lake closing, another regional indicator of global warming. The earliest lake closing was on January 7 in 1868 and the latest was on March 26 in 1894. In average years of lake freeze-over, the lake closes by mid-February and the ice sets the stage for the lake's equivalent of blackout shades -- snow.

Snow cover makes a big difference in light penetration. In one study in Massachusetts, a foot of snow on top of ice prevented more than 99 percent of the weak winter sunlight from reaching the water below the ice. Less than a half inch of new snow can block two thirds of the light from penetrating. Without sunlight to maintain photosynthesis, most phytoplankton die.

Different species of phytoplankton typically rely on one of four strategies to get through winter's darkness. Some, including certain blue-green algae and diatoms (a type of algae characterized by elaborate cell walls containing much silica), rely on what scientists call "resting stages", special cells that persist through the winter with little or no activity, using very little energy. Like hibernating animals, algae in resting stages essentially suspend the day to day efforts of living for the duration of the cold season.

Another approach is simply to persist through the winter without tremendous metabolic slowdown, living on stored food. Yet a third approach is used by a few algae that can live on dissolved organic compounds present in water -- essentially behaving more like animals than like plants, in that they are not creating their own food.

Finally, some types of phytoplankton, particularly those that are capable of adjusting their position in the water column, persist and even thrive under ice. When snow shuts down most others, these cluster near the surface of the ice where the light is strongest. Species using this strategy include some of the blue-green algae that can regulate their buoyancy by retaining or expelling gases, as well as some algae with flagella, long whip-like organs that allow the plants to move independently of water currents.

For these species, ice may even offer some advantages. In summer, wind-driven currents and turbulence from waves often sweep even mobile phytoplankton away from their preferred depths. Ice cover, by protecting the water's surface from the wind, eliminates turbulence and slows currents. This allows phytoplankton that actively regulate their depth more control. Species that simply float, however, may sink to the bottom without the turbulence of waves and currents to keep them suspended.

There is one way that winter is easier on all phytoplankton than on people or other largely land-bound organisms. While air temperatures may drop to twenty below, the water under the ice is never colder than 32 degrees. Near the lake bottom, temperatures will hover at around 40 degrees through the coldest of cold snaps. Thus, unlike their terrestrial cousins -- trees, shrubs, and grasses -- phytoplankton do not need special adaptations to prevent their cells from freezing.

If phytoplankton did not have adaptations to survive winter's chill and darkness, Lake Champlain would be a very different place. For instance, phytoplankton are the main food of zooplankton, which in turn are a primary food of smelt, which are the main forage fish for sport species such as lake trout and walleye. Fish-eating birds such as mergansers and cormorants are similarly dependent on phytoplankton. And without phytoplankton, phosphorus that pollutes the lake from sources such as agricultural runoff, stormwater, and wastewater treatment plants would be even more available as a fertilizer for other aquatic plants. The result might be a persistent thick coating of green slime on the rocky shores of the lake. With fewer fish, fewer birds and more slippery rocks, our year-round enjoyment of Lake Champlain would be greatly reduced.