It has been a tough winter – long and lots of snow. What this means for lakes is oxygen-replenishing sunlight is blocked for too long. In many cases, especially in shallow lakes, the residual oxygen from the previous fall is consumed by fish and other animal life as well as decomposing organic matter. When it is used up, fish suffocate and die. This is known as winter fish kill or winterkill.
In Minnesota, the rule of thumb is that lakes less than 15-feet deep are prone to winterkills. However, there are many exceptions to this guideline. Generally, lakes that are shallow have many rooted plants throughout their basin. With sufficient sunlight, these plants produce more oxygen than they consume, but when sunlight is cut off, that is reversed. Lakes with little water and lots of plants, like many shallow lakes, have too little water volume, therefore no they have too little reserve.
Some deep lakes experience winterkill too. Tamarack Lake, a lake I studied, had a winterkill following the winter of 2000/01. Tamarack Lake is 82-feet deep! [The photo is of the shore of Tamarck Lake in the spring of 2001]
It turns out maximum lake depth is not always the best indicator. The basin of Tamarack Lake has a deep spot, but most of it is shallow – think Champaign glass profile. Tamarack Lake may actually be too deep.
Very deep lakes with small surface areas may not turnover completely in the fall, so their deep waters, which normally have low oxygen during the summer, can start out winters with a deficit. Even in more typical winters, these deep lakes can winterkill. Lake McCarrons in Roseville is another example.
Why don’t all shallow lakes winterkill? It turns out this phenomenon is not as simple as it seems. Obviously, lakes devoid of fish will show no signs of winterkill even when they lose oxygen. In less obvious cases, there may be inflowing surface or groundwater that may aide in oxygenating a lake during the winter or provide a refuge for fish. Each fish species has different tolerances for low oxygen. For example trout and common carp have a low tolerance and tend to be the first to die. On the other band bullheads are very intolerant and are the last to die (there are some tales of bullhead even surviving out of water for extended periods).
Winter aeration is a common method to mitigate winterkills. Aeration is commonly provided in one of two ways. In some cases, water is pumped from the lake and run over an on-ground cascade, which agitates and aerates the water as it flows back into the lake. The other method involves pumping compressed air to one or several underwater bubblers, which keeps an area of water open and exposed to the air. The bubbles created by the “aerator” do not significantly add oxygen to the water.
Care must be taken when contemplating winter aeration. First, because it creates open water, there is an obvious safety hazard. Permits and safety measures are therefore required. Secondly, the agitation of the water may induce an increased oxygen demand, so if the aeration system is not appropriately sized and deployed, it may actually use up more oxygen than it creates and increase the risk of winterkill.
There is a relatively new area of inquiry and management dealing with shallow lake ecology. Shallow lakes have two “stable states.” Shallow lakes may have abundant, native rooted plants and clear water or they may have few rooted plants and turbid, algae-filled water. The former state is considered healthier and more natural.
The shift from one state to the other may be triggered in a number of ways. Increased nutrients may cause excess algae, which in turn shades the rooted plants. Alternatively, some fish, particularly common carp, root out the plants and can cause this shift.
The rooted plants are key. The plants physically stabilize the soft lake sediments and retard wind and waves from stirring the lake. Also, these plants provide shelter and refuge for small animals, crustaceans such as the waterflea, which eat algae. Removing the plants takes away their hiding places and visually-feeding fish can now find and eat them. Another way for the waterfleas to avoid being eaten by fish is if there are no fish in the first place.
Healthy shallow lakes commonly winterkill. Indeed, this frequent occurrence helps maintain and sustain shallow lakes’ health. Stocking fish following winterkill is not natural and tends to de-stabilize the health of shallow lakes.
An overarching theme of A Lake manager’s Notebook is that management actions almost always have downsides in addition to the intended benefits. In the case of stocking gamefish in shallow lakes, it clearly provided benefits for recreational angling, but there are downsides too. These include an ongoing investment in stocking to sustain this condition and the destabilization of the ecosystem – making the lake’s quality and condition more difficult sustain.
This has been a severe winter and we can expect to see many lake winterkills. This is life for a shallow lake and that’s not (necessarily) bad.