Ice Out and Its Meaning for Lakes

The annual date of ice out for some lakes is fodder for prognostication and even wagers, but for aquatic plants and animals, that date has deeper ecological significance. Light and temperature are key cues in the aquatic environment, and ice cover keeps lakes cold and dark in late winter. As the air temperature warms, the ice melts, usually leaving open water around the edge and then falling apart over deeper water over a short time period. If that date is earlier, algae and rooted plants can get a head start on spring growth. If that date is later, growth is delayed. Temperature also affects when hibernating aquatic animals, like turtles and frogs, become active. Fish are active even under the ice, as any ice fisherman will tell you, but are more aggressive after ice-out and turn to spawning activities based on temperature cues.

While lakes may not actively manage time, it is a lot like it is for people; if you get up early, you can get a lot more done in a day, and you may not be able to finish your to-do list if you sleep in. As the water warms and light penetrates further without ice, lots of biological processes increase in lakes. Bacteria decompose organic bottom sediments, using oxygen and releasing various substances into the water column. Algae take up nutrients and use sunlight to photosynthesize and make more biomass. Zooplankton eat algae and reproduce more frequently, but small fish also eat zooplankton and limit that trophic level by early summer in most lakes. Fish spawn and make small fish that eat those zooplankton.  In the meantime, rooted plants are growing, either from seeds, various winter buds, or root stocks, anywhere that light penetrates to a hospitable bottom substrate. Benthic invertebrates, often dependent on those plants, grow, reproduce and are eaten by fish or each other. A lake waking up from what seems like a winter sleep is indeed a busy place!

With variation in ice out date from year to year, and weather variation once the ice does go out, the sequence and intensity of cues will vary considerably from year to year, making every year unique to some extent. General patterns of plant growth, algae succession, fish spawning and other biological processes are known, but small changes can make quite a difference. A cold snap or windy period in May can retard stratification or cause a downturn in fish spawning that is not recoverable in that year. A very mild winter like we had going into 2016 can let perennial plants like invasive species of watermilfoil get a very early start (some plants may not even have died back to roots and stems) that outcompetes native species and makes it hard for harvesting programs to keep up. Weather plays a big role, and is influenced by climate change.

Climate change is a popular topic and the subject of spirited debates, but the data clearly show that lakes have been experiencing earlier ice-out dates over the last century (see graph). We seem to be losing a day of ice about every decade, such that based on the period of record going back about 150 years ice-out is now occurring two weeks earlier on average. Just keep in mind that aquatic organisms do not live in the “average”, and lakes have experienced both very late and very early ice out dates in just the last few years.

Ice out dates for various lakes.