Ponds: Managing Algae and "Weeds"

Some algae and aquatic vegetation are good and necessary for a healthy waterbody. For example, planktonic algae, a microscopic, free-floating algae that exist in the top few feet of a pond are an essential part of the food chain for fish and other aquatic creatures. They can sometimes give the pond a green or brown coloration, and tend to have rapid growth (or blooms) in the warmer months. If you want to provide good fish habitat, clear water should not be your goal. Approximately 15-25% of the water surface should have some form of vegetation.

Prevention: The Best Pond Management

Your pond is part of a watershed. That means that water (with sediment, nutrient, pollutants) from the surrounding area moves through groundwater and surface water into your pond, and downstream to other waterbodies. Due to the predominant land management in Iowa, many of Iowa’s waterbodies have high levels of nitrogen and phosphorus. You can learn more about Iowa’s efforts to limit nutrient export through the Iowa Nutrient Reduction Strategy, here (https://nrstracking.cals.iastate.edu/iowa-nutrient-reduction-strategy).

Excessive amounts of algae and other aquatic vegetation grow because of an overabundance of nutrients like phosphorus and nitrogen. The best way to manage algae is to limit the amount of sediment and nutrients going into your pond by planting a buffer area directly around the pond. Native perennials with deep root systems that do not lay down during big rainstorms are best. You can directly transplant native plants, or you can encourage native plant growth with reduced mowing. Limit mowing around the pond to every other year, and alternate mowing between spring and fall, to discourage unwanted annuals. The buffer should be a minimum of 15 feet wide, and up to 100 feet wide for steeper slopes. Unless the source of the nutrients entering your pond is dealt with, you will likely have annual problems with excessive algae and/or aquatic vegetation.

Other preventive recommendations:

  • Fence off the pond from livestock
  • Discourage geese around the pond
  • Avoid fertilizing the land around the pond
  • Aeration: provides oxygen to water, helps bind phosphorus to bottom sediments, pushes floating plants to edges of pond

Treating excessive algae or other aquatic vegetation

ID-ing your aquatic vegetation

The first step in determining if management is needed is to ID the aquatic vegetation. Texas A&M University has created a useful website for this purpose: https://aquaplant.tamu.edu/. Another useful website from Clemson University Extension leads you through IDing your pond plants: https://www.clemson.edu/extension/water/stormwater-ponds/problem-solving...

Harmful Algal Blooms

To create a hospitable environment for fish and other water creatures, 15-25% of the pond surface area in Iowa should have some sort of vegetation. Earlier we talked about planktonic algae, a microscopic, free-floating algae that exist in the top few feet of a pond. In general, these algae are beneficial to fish habitat and the pond ecosystem. However, if you notice an increase in fish kills, a strong odor or unsightly surface scums you may have a noxious form called blue-green algae which can cause harmful algal blooms. You can read more about harmful algal blooms and their management here: https://northcentralwater.org/wp-content/uploads/sites/317/2021/11/HABs-Prevention-and-Treatment-for-Landowners-FINAL_2.pdf.

Mechanical (raking) and biological (fish stocking) control

Another type of algae, filamentous algae, can form a mat on the surface that looks like wet cotton or wool. It is often called “pond moss” or “pond scum.” If you have a small pond you may be able to rake out the algae. Just make sure that you take the algae a distance from the pond so the nutrients do not flow back. The algae can be used as a garden mulch. Tilapia eat filamentous algae and small floating plants, but they are a tropical fish and cannot survive below 55 degrees, meaning that they will need to be re-stocked annually.

Other types of aquatic vegetation include submerged and floating plants. If the pond is small, some of these plants can also be raked out and deposited away from the water body. Stocking the pond with Grass Carp is another option, particularly if you have excessive submerged plants. However, Grass Carp are sort of like the goats of the underwater world and they do eat everything. This means that if you overstock, they may eat too much of the underwater vegetation, displacing nutrients and potentially causing an algal bloom. ISU Extension has previously recommended stocking 3-4 per acre. The Iowa Department of Natural Resources (DNR) does not recommend stocking Grass Carp in private ponds because of the risk of total elimination of all aquatic vegetation, difficulty in removing and the long life of the Grass Carp. If you plan on stocking the DNR recommends a much more conservative number of 1-2 Grass Carp per acre. Also, be sure to choose triploid or sterile grass carp regardless of stocking rate.

Nontoxic dyes

Another option for excessive submerged plants is nontoxic dyes, which act as light screens and inhibit submersed plant growth. For example, the blue dye Aquashade absorbs light that otherwise would be used for photosynthesis. Aquashade is applied easily, disperses readily, and reduces growth of plants at depths greater than two feet. Dye concentration must be maintained throughout the growing season, so its use is limited to ponds with no outflow. Also, it must be applied before weeds emerge in the spring; once weeds reach the water surface, the dye has little effect. In this region, an initial application and a midseason application are suggested.

Chemical (herbicide) control

Using chemicals such as herbicides is a last resort in pond management. Herbicide use should be done in the spring as water temperatures are beginning to increase. If you use herbicides in high summer, it can result in fish kills as the dying aquatic vegetation takes up much of the oxygen in the pond. Treat ~10% of the pond at a time, wait 10-14 days and then treat another similar area and continuing with small sections. Herbicides like copper sulfate can help temporarily, but if they are used repeatedly, they will become less effective. Before using chemicals determine all possible uses of the pond such as swimming or fishing, follow label directions, and only use chemicals designed and approved for aquatic use. Additional ISU Extension publications about herbicide application can be found here:

Other useful resources: