Spring Tonic for Lakes and Ponds
Melting Ice Rejuvenates Large Bodies of Water in Spring
Feb 16, 2009
As ice melts in northern climates, the different physical characteristics of ice and water generate strong vertical currents in ponds and lakes that replenish oxygen, remove carbon dioxide, and add nutrients to the water. The survival of the plants and animals that live there depends on these changes in the quality of the water. Without these changes, the build-up of respiratory and excretory wastes and loss of nutrients and oxygen that occur over the winter would stunt the growth of all organisms living there.
How These Physical Properties Rejuvenate Lakes
Once a body of water has frozen, oxygen can no longer diffuse into it from the air, carbon dioxide no longer diffuses out of it into the air, and both carbon dioxide and nitrogenous wastes from metabolism accumulate under the ice. Although there may be some exchange of gases and nutrients as streams and rivers flow into and out of these lakes and ponds, these inputs and outflows alone are not sufficient to stimulate an active biotic community after the ice melts.
Ice usually melts most rapidly at the upper surface exposed to the air, rain, and sun. As water accumulates on the surface, it picks up oxygen from the air and becomes richly oxygenated. As this water melts through the ice, it is heavier than the ice and the colder liquid below. This heavier water plummets to the bottom of the lake, bringing its dissolved oxygen with it, and pushing the deeper water up to the surface. This initiates a vertical circulation of water in the lake with denser water moving to the bottom and less dense water moving to the surface. As the less dense water reaches the surface, it picks up oxygen and releases carbon dioxide, becomes colder at night, and again sinks to the bottom of the lake.
Nutrient Replenishment
After several cycles of sinking and rising to the surface, the movement of water removes much of the accumulated carbon dioxide and replaces it with oxygen from the overlying air. With replenished oxygen levels, bacteria in the water can aerobically digest accumulated metabolic wastes and convert them into nutrients. But the majority of the nutrients that enter the water during the spring turnover is as a result of the current flow of the water sinking and rising.
When the colder water sinks to the bottom, it moves the sediments off the bottom. These sediments are picked up and carried toward the surface as the less dense water rises. Eventually, these sediments become distributed throughout the water and the soluble portions dissolve into the water while most of the insoluble (and heavier) portions drop back to the floor of the lake.
The Lake Awakens
The disturbed sediments contain cysts and eggs of algae, seeds of plants, and eggs of animals. As these reproductive structures are exposed to light and warmer temperatures, they germinate or hatch and release the plankton (both plants and animals) that form the base of the food chain in the lake.
Enriched oxygen levels and warmer temperatures allow larger animals and plants to become active in turn. Insect larvae resume activity and metamorphose to adults, fish resume feeding, plants grow and the lake takes on its spring appearance. All of this is triggered by melting ice redistributing gases, nutrients, and wastes in the water.
Ice is not the only mechanism responsible for gas and nutrient distribution in lakes and ponds. In the deep South, ponds also awaken in the spring, but the degree of change is much less intense because depletion of oxygen and accumulation of carbon dioxide are lower with no ice cover. Additionally, the constant activity of animals breaking through the surface of these bodies of water help to provide year-long oxygenation and decarboxylation of the water and mixing of sediments.
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