What happens to aquatic plants when there is ice on our lakes? Most of our native plants die back in the late fall and then sprout new growth in the spring from seeds, roots, or “winter buds”. However, a few plants, like native coontail and invasive curlyleaf pondweed, tough it out under the winter ice. These plants have the ability to endure long periods of low light and are adapted to living in cold water. However, the conditions under the ice in some winters can challenge even these winter survivors.
Sunlight has no trouble getting through lake ice. Snow on top of the ice blocks out more sunlight, but plenty of light still gets through when there is less than a foot of snow. However, things get dark quickly when there is more than 14 inches of snow – this can block out most of the light, making the under-ice world very challenging for plants and fish. Low light can lead to reduced oxygen in the water (less photosynthesis by plants), which can stress fish or even lead to winter fish kills.
Snow, Light, and Curlyleaf
For a long time, lake dwellers have noticed that invasive curlyleaf pondweed seemed to grow more densely in places where snow was plowed away for ice roads or where the wind blew snow off of the ice. This strongly suggests that the extra light that passed through these areas with little snow allowed curlyleaf sprouts to persist or grow slowly through the winter. Come spring, these curlyleaf plants would already have been a few feet tall and ready to explode toward the surface as water warmed.
Without sufficient light under the ice, curlyleaf sprouts are not able to grow or send out runners to make more stems. Come spring, this light-stunted curlyleaf may be more sparse and only a few inches tall. When the ice finally does disappear, it will likely take several weeks for the curlyleaf plants to catch up, meaning that if the spring weather does not allow them to grow quickly, they may have to compete for light with other plants, and may not reach the surface of lakes as quickly. This is the current thinking about why curlyleaf growth is noticeably less dense following snowy winters.
SCIENTIFIC SUPPORT: A recent study looked at the amount of curlyleaf in some Minnesota lakes over several years and found that snowy years did in fact coincide with reduced curlyleaf (Valley and Heiskary 2012; Lake & Reservoir Management)
Spring Curlyleaf Treatments
When treating curlyleaf with herbicides, the current strategy is to treat in the early spring when curlyleaf is actively growing, but before most of the native plants have sprouted new growth. This helps to protect the native plants from herbicide damage and kills the curlyleaf before it can produce new turions (reproductive buds). Right now, the Minnesota DNR issues permits for curlyleaf treatments that limit treatments to the period when water temperature is between 50°F and 60°F. The thinking behind this temperature range is that lab studies have shown that curlyleaf begins to grow actively at about 50°F (making it susceptible to herbicide damage), while some native plants sprout and begin to grow at around 60°F (only a few plant species tested).
However, this use of this 50° to 60°F water temperature range is less than ideal. I have tracked lake water temperature using automated probes for several years and have found that water temperature can change by more than 10 degrees over the course of one day in near-shore areas. Furthermore, water in shallow areas warms more quickly than in deeper areas, and springtime temperatures can fluctuate wildly from day to day.
So the question becomes – Where and at what time of day do we measure the temperatures? Should we focus on measuring temperatures in near shore areas, deeper spots, mornings, evenings? Each of these could give very different results. What if the lake has been cold for a long time, but then rapidly warms up to 65° due to a few days of very hot weather, only drop below 50 again in the following days? We need to remember that the temperature is being used as an indicator for what the plants are doing. Warmer waters are almost certainly driving the sprouting of native plants, but given the variability of spring water temperatures, that 50°-60° window seems like a moving target that may not be as practical in real lakes as it was in controlled laboratory experiments. I think there is an old idea that might just give us a better way to time these treatments.
What Can We Learn from Farmers
Long ago, farmers recognized that weather and soil temperature affected when different plants sprouted, grew, and reproduced. Over the years, scientist developed a simple method to track this cumulative effect of temperatures on different plants. This method uses weather records to calculate “Growing Degree Days” – really just a simple way to track the amount of heat that plants have experienced (one hot day would be the same as several warm days). This method is also used to calculate “Heating Degree Days” which tells us how much we had to run our furnaces to keep our house warm. A similar calculation for lakes may help to clarify the optimal timing for curlyleaf treatments and would allow for smoother planning by using weather forecasts to predict when key Growing Degree Day thresholds would occur.
Lake Minnetonka Pilot Study
I am currently conducting a small research project on two bays of Lake Minnetonka (Hennepin Co., MN) to track early-spring sprouting and growth of native aquatic plants (with financial backing from the Minnehaha Creek Watershed District and Dr. Mike Netherland from the University of Florida – Gainesville). In addition to monitoring plants, I have installed electronic probes that will collect temperature and light data (~1 foot off of the bottom and in the top few inches of sediment) across a range of water depths. I plan to correlate plant activity with water and sediment temperature. My hope is that we will find clear patterns between native plant activity and a calculated “Lake Growing Degree Day”. If we do find a clear pattern, this study may lead to additional data collection on more lakes next year to help refine how we time curlyleaf treatments and ultimately change how the DNR permits such treatments.