Thank goodness! February is over. It’s cold and getting colder as I write, but it’s March. The cold can’t last … can it? It’s pretty well established that upland wildlife like pheasants and quail are going to lose some of the recent gains their populations have seen in the last few years due to February’s storms and deep snow. That said, a decent breeding season (dry and warm enough) could go a long way toward replacing the losses in areas where there is at least some winter survival. Deer are experiencing some stress, but should pull through in pretty good shape.
What about the fish, though? We used to go into freeze-up around Thanksgiving years ago, but lakes and streams stayed at least partly open well into December and even January this winter. That’s good news. The open conditions allowed water to remain well oxygenated longer than usual. Lack of snow even after ice-up allowed light to penetrate into the water. That, in turn, allowed algae to continue to photosynthesize and release oxygen into the water. That’s good news, too. Ice cover prevents lakes from “breathing.” Dissolved gases in the water can no longer disperse into the air, and new oxygen can no longer be absorbed from the air. Aeration systems allow lakes to continue “breathing” by keeping a window of water open to fresh air.
Summer’s growth of plants and algae die and settle to the lake bottom as the water cools. Aerobic bacteria begin breaking down the organic matter and use oxygen just like higher life forms. They can deplete dissolved oxygen to a point where it can no longer support fish. Fish are cold-blooded. Their metabolisms slow in cold water and their need for oxygen decreases. They can survive low levels of oxygen when it’s cold that would kill them when the water is warmer. They must have some, though. Deep snow blocks almost all light penetration for photosynthesis, and thick ice blocks any means of gaining new dissolved oxygen. Anaerobic bacteria continue breaking down organic matter even when dissolved oxygen levels drop to zero. They don’t need free oxygen to thrive and their waste gases include poisonous hydrogen sulfide and ammonia. Those gases can build up in a lake during a long, dark winter to a point where fish are not only suffocating from lack of oxygen, but are being poisoned as well. Lethal conditions begin deep and work up as winter goes on. Sometimes there’s only a foot or two of livable water under the ice by spring.
Winter fish kills are fairly common in Iowa lakes. Nutrient levels are high. That fuels heavy growth of plants and algae each summer. Bacterial activity can lead to anoxic (no oxygen) and toxic chemical conditions if winter is long enough and snow cover is deep. Rapid spring runoff from melting and rain can dump even more organic loading into a lake and cause fish kills just when it looks like things are getting better. This winter’s late start favors better survival of fish, but February’s harsh weather has led to deteriorating conditions. March would do us a big favor if it brings on warmer conditions gently, rather than a rapid thaw. We could do without much precipitation, too. Warm rain with rapidly melting snow would almost certainly lead to flooding as well as fish kills. Deeper, larger lakes typically fare better than smaller, shallower ones. Even in lakes where there is a fish kill, it’s seldom complete. Surviving fish will grow more quickly with less competition and may reproduce more quickly. Fish kills aren’t welcomed, but they are a normal part of the ebb and flow of life here in Iowa.
Steve Lekwa is a former director of Story County Conservation.