Controlling Algae in Ponds and Lakes

One of the perennial problems faced by pond owners in rural and suburban settings is excessive algae growth, also called algae blooms in the water. Algae are divided into three classifications. Planktonic (single-celled), filamentous and Chara. Planktonic algae remain diversely suspended in the water and turn the water a uniformly green or blue-green color. Filamentous algae species string together, becoming floating mats of “pond moss.” The third type of algae, called Cara or muskgrass, are large green algae that are anchored to the bottom but do not extend above the surface. Chara is stem-like, with thin, leaf-like structures and is often confused with seed-bearing aquatic plant species. When crushed, chara produces a musky odor.

For maximum production, all plants need adequate water, sunlight and nutrients. Algae is no exception. In a pond, water and sunlight are a given, the limiting factor is plant nutrients. The first step in algae control is to reduce the movement (loading) of nutrients into the water. Whenever, I get a call about an algae problem in a pond, I try to identify the source of nutrients that is causing the problem. The two primary nutrients that must be controlled are nitrogen and phosphorus, with phosphorus being the larger concern when it comes to algae control.

If applications of commercial fertilizer or animal wastes applied to the soil surface are followed by a hard rain, they may be carried directly into the pond by runoff water. In addition to movement of surface-applied products carried in the runoff water, nutrients can be carried to the pond attached to soil particles that erode from slopes and end up in the pond as silt.

Nitrogen and Phosphorus are water soluble nutrients. Nutrients incorporated into the soil by tillage or surface applied and later dissolved by rainwater and carried into the soil with the water may also find their way into the pond. Most of our southeastern Nebraska soils can be classified as silty clay loam topsoil overlaying heavier clay subsoil. When water that is percolating down through the soil profile encounters the clay layer, its downward movement is impeded. It then moves downslope along the boundary and may emerge as a spring in a creek bottom or in the pond itself. In addition to applied fertilizer or animal waste, another source of potential nutrient loading can be domestic wastewater. Effluent entering the soil from septic absorption fields also becomes part of the soil water matrix and can move downslope as described above. Once nutrient loading has been reduced to the extent possible, chemical treatments can be used to control algae in a pond. Copper compounds such as copper sulfate and various chelated copper products are both safe and effective when used according to directions. Some aquatic herbicides that are used for seed-bearing aquatic plant species are effective against certain algae species as well. For more information on chemical control methods, go to the Aquaculture page in the Lancaster County Extension Agriculture and Acreage Resources section of their web site. Http://lancaster.unl.edu/ag/aquaculture.shtml

Source: Tom Dorn, NEBLINE