The recent rains have helped not only trees recover from the droughty conditions of last year, but ponds as well. Many ponds have water levels back to normal, water temperatures that are cooler than last year at this time, and in many cases algae growth that is less than normal for the end of June.
However, we have almost reached the July 1st rule of thumb date recommended by the Illinois Department of Natural resources to have completed aquatic herbicide applications. By this date many aquatic plants have reached maturity and seeded. Mature plants are more difficult to control and because they have produced seed are beginning to die on their own, so a herbicide treatment may not be necessary.
In addition, as pond water temperatures increase, the capacity to hold dissolved oxygen decreases. This reduced carrying capacity for dissolved oxygen occurs each summer and is usually not a problem for fish; unless other conditions occur which further reduce the available oxygen.
One condition which can further reduce the available oxygen for fish is large amounts of decomposing plant material. Decomposing plants use oxygen and increase the biochemical oxygen demand in the pond. Sometimes this increased use of oxygen for decomposition can lower dissolved oxygen levels to a point where a summer fish kill occurs. Therefore late summer herbicide treatments in ponds may cause problems.
During the rest of the summer, algae and other aquatic vegetation management leave us with two options: first tolerate the vegetation, or secondly apply herbicide to only a portion of the aquatic vegetation. The first option has some obvious drawbacks. For example, high populations of algae reduce the aesthetics of the pond and decrease the quality of the pond for swimming and boating.
However, during extremely high daily temperatures, even partial herbicide treatments can be risky. Therefore, tolerating the algae is probably the safest management option especially when planktonic algae blooms occur in late July and August. When planktonic or microscopic algae populations are high, the pond’s water appears to be green in color and is referred to as an algae bloom. In order to control planktonic algae, the entire pond must be treated with an algaecide, such as copper sulfate.
Although these plants are microscopic, the dissolved oxygen in the pond can be dramatically reduced when large numbers of these plants are killed at the same time. Planktonic algae will die naturally and slowly as water temperatures decrease and during cloudy days, usually not causing an excessive oxygen demand and fish kill.
Filamentous algae are more practical to control with a partial pond treatment because this type of algae floats on the surface in mats of green hair-like filaments. A small portion, less than one-fourth of the vegetation can be sprayed without affecting the remaining vegetation. At least seven to ten days should be allowed between applications to allow decomposition, and then another portion of the vegetation can be treated.
Remember that even partial herbicide applications will reduce available oxygen levels. Therefore, the condition of your pond and weather conditions should be carefully evaluated before making any herbicide applications.
— Dave Shiley,
University of Illinois Extension