This has been a weird winter, even by Maine standards. There were cyclists on Mount Desert Island in December and robins in Bangor in February. Despite last week’s two snow storms, it feels like we may have gotten off easy with a mild winter and early spring.
Ice is already disappearing from many of Maine’s lakes. This not only signals the end to a disappointingly short ice-fishing season; this year’s early ice-out could cause significant changes to the biology of our lakes.
Shortly after ice-out, the water in a typical Maine lake mixes, powered by wind moving across the water surface. You can see a similar process by blowing across a cup of coffee or tea just after adding cream: a little breeze will quickly mix the beverage and cream. Most lakes also mix completely in mid-fall.
Between spring and fall mixing lakes stratify, or break into layers, like a cake. During this time, a warm layer of water sits on top of a cold layer. You can usually notice this by diving off of a boat or dock: if you dive deep enough, the water suddenly gets very cold. This is because colder, denser water sinks to the bottom, while warmer, less dense water rises to the surface.
So what does stratification have to do with ice-out? An early ice-out allows the upper reaches of a lake to warm faster, so lake stratification can happen earlier in the year. If an early spring is followed by a hot summer, lake stratification will be stronger and last longer than in an average year.
During times of strong stratification, the two water layers of a lake become very different from each other. Waves on the surface mix oxygen into the upper layer of water, providing a steady supply for fish and other aquatic critters. The lower layer, however, is essentially sealed off on its own with no supply of oxygen. Therefore, any living critters in the lower layer will slowly use up oxygen without it being replaced. During a long period of stratification, the lower layer can eventually run out of oxygen completely.
If a lake’s lower layer of water runs out of oxygen, an army of chemicals can move from the lake bottom into the water. One of these chemicals — phosphate — can cause plenty of trouble. Phosphate is a common chemical in fertilizer. If you fertilize your lawn, then you have used phosphates to help things grow. When a lake is full of oxygen, phosphates stick to iron in the lake bottom. But when oxygen is absent, phosphates are free to run amuck in the water.
Once phosphates have been released into the lower water layer of a lake, they are eventually spread throughout the lake whenever the lake mixes — usually in the fall or spring. Just like lawn fertilizer, phosphate stimulates growth, but in this case it stimulates algae.
If algae are stimulated, they can sometimes reach unusually high numbers in a lake, causing an algal bloom. Algal blooms usually make a lake look strangely green or cloudy. While some algal blooms are relatively harmless, others can produce some toxic chemicals or take even more oxygen out of the lake, harming fish.
An early ice-out after a strange winter does not mean disaster for our lakes, but it could cause some noticeable changes. Lakes are often less constant than they appear to be; an early ice-out now could cause our lakes to look a little different by the end of the year.
Zachary T. Wood is a Ph.D. student in the ecology and environmental sciences program at the University of Maine.