After a long, cold winter we are finally seeing signs of spring. Soon will come the time when our lakes start to “turn over”, a natural mixing of the top and bottom layers of the water. This happens in the spring, as the ice melts and the lake surface heats up and decreases in density. Once the temperature (and therefore density) of the surface water equals that of the bottom water, very little wind energy is required to mix a lake completely.
Unlike most liquids, water is actually less dense as a solid than as a liquid. The result is that ice floats, and the densest cold water (at 4˚ F) sinks. This leads to lakes becoming “layered” -- capped with ice on top, and the water below stratified by density. This stratification is actually weaker in winter than in summer, but the ice cap keeps water layers from mixing. In the spring, when the ice cap melts and the surface heats up, mixing (or “turn over”) occurs. This turn over is vital to the health and well being of aquatic life in the lake, since the upper layers become infused with oxygen by the air and, upon mixing, bring oxygenated water down to the lower layers. The lower layers, in turn, bring sediment- and nutrient-laden waters up toward the surface.
Last year, according to the DNR, Minnesota had one of the latest ice-out seasons in years, with ice-out dates ranging from 10-21 days late across the state! Lake Minnewaska in Pope County actually set a record with an ice-out date of May 11 (breaking the previous record of May 5, 1950). Interestingly, 2012 had one of the earliest ice-outs on record. Many things influence ice-out dates, but the position of the jet stream is most important. Last April, the jet stream formed in such a way as to allow low-pressure systems to move straight down from northern Canada, bringing the storms that caused cold conditions to persist in Minnesota.
An icy Upper Mississippi River near Coon Rapids, Minnesota.
Photo: Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale International Project
This year, with thick snow and ice on lakes, we may experience another late ice-out year. How does this affect lake ecosystems? According to MN DNR Fisheries Specialist Joel Stiras, the main affect of a later ice-out is less oxygen in the water. This is largely due to a late mixing date, but involves other factors as well. For example, the deep snowpack we have blocks light from penetrating into lake water; light is needed by plants to photosynthesize, so reduced amounts of light also means reduced oxygen production by plants. Additionally, the lack of light causes plants to die, and their decomposition uses up even more oxygen due to bacterial metabolism. This means that a winter with heavy snows can actually result in fish kills due to reduced oxygen levels. The good news is that last year, even with the late ice-out, Mr. Stiras did not notice a marked difference in fish populations. He will be watching closely to see how fish fare if the ice is late two years in a row.
For our aquatic life, an ideal situation for turn over would be a gradual warm up with plenty of wind to help with mixing. Rain can be beneficial too, since it melts ice rapidly. There can actually be a danger of surface waters warming too soon and not fully mixing, but this rarely happens because water takes a longer time to heat compared to air.
Interestingly, road salt that runs off into lakes and rivers during snow-melt and spring rains causes solutes to build up in waters, which can lead to chemical stratification rather than temperature stratification. Invertebrates are most sensitive to such salt damage, and declines in their populations can affect the entire food chain, including fish, turtles, snakes, birds, and everything that eats or indirectly depends on aquatic insects.