Phytoplankton are most frequently controlled by 1) nutrient reduction (e.g., aeration) or 2) the regular application of algicides. Less commonly, 3) colorants are added to reduce light needed for phytoplankton growth, or 4) densities of herbivorous zooplankton are increased to graze down phytoplankton abundance.
The most commonly used algicides are copper-based compounds, including copper sulfate (CuSO4) or chelated copper compounds like Cutrine-Plus® (a mixture of copper ethanolamines). Copper sulfate is less expensive, but is less effective in hard water and requires more frequent application; chelated copper compounds are less sensitive to hardness effects and remain active for a longer period. Algal death following treatment is quite rapid, and the decomposition of the settled, dead algal material by bacteria can cause anoxia at the bottom of the pond, sometimes causing fish kills. Bacterial decomposition further liberates a large portion of the nitrogen and phosphorus stored by the algae, making them available for new growth once toxicity levels have declined. The effect of copper build-up in the sediments is not fully understood, but may be a concern for long-term health of the pond. A permit is required to apply algicides in Pennsylvania. Licensed applicators are often contracted to provide a regular schedule of treatments.
The use of barley straw is sometimes viewed as an attractive alternative to commercial algicides. Barley straw, when allowed to decompose in pondwater, produces an "algistatic" effect, inhibiting algal growth but not killing already-existing algal cells. The suggested method of application is to loosely enclose the straw in mesh bags or tubular netting (e.g., Christmas tree wrap), then tether the material just beneath the surface. Application in early spring is recommended, as barley straw is presumed to have little effect on already-established algal blooms. The principal ecological concern regarding the technique is the largely unknown long-term impact of repeatedly adding large quantities of organic matter to the pond ecosystem. This concern can be reduced by making sure that the straw is removed from the pond at the end of the growing season.
Despite the widespread appeal of barley straw as a "natural" form of chemical treatment, the chemical reactions underlying the inhibitory effect on algae are not well understood, and the degree of response appears to vary widely. Because of uncertainties regarding its mode of action, barley straw is not a product that is currently registered by the U.S. EPA.
Colorants act to reduce the penetration of wavelengths of light needed for photosynthesis. The pond is typically treated at the beginning of the growing season as a means of reducing the growth of primary producers. While often effective and relatively inexpensive, the unnatural tint to the water imparted by the colorant may be aesthetically displeasing.
Enhancement of zooplankton densities to control algal growth, also known as biomanipulation, requires a substantial reduction in the abundance of smaller, planktivorous fish such as Bluegill and Golden Shiners that would otherwise reduce zooplankton densities. This is best accomplished by increasing the abundance of piscivorous fish (e.g., largemouth bass). In effect, the bass eat the small fish, thereby allowing the zooplankton to increase and reduce phytoplankton abundance, as shown in the diagram below.
While biomanipulation has frequently been effective in larger lakes, its usefulness in small ponds is not as well documented. One concern is that increased grazing by zooplankton may simply favor kinds of algae that are either inedible or too large to be eaten. Such shifts in the species composition of the phytoplankton community circumvent efforts at reducing phytoplankton abundance.
In order to be successful, the densities of planktivorous fish have to be maintained at low levels until rooted aquatic plants have become well established, a process which may require several years. Biomanipulation is thus a good example of a management strategy that affects the entire pond ecosystem and requires considerable long-term planning. A good reference to the theory and application of biomanipulation in ponds is Moss, B. et al. (1997) A Guide to the Restoration of Nutrient-Enriched Shallow Lakes.