Algaecides are chemicals used to directly kill algae. The Latin root is simple – algae, or microscopic plants, and cide, a killing agent. By far the most common algaecide is copper, in use for over 100 years and very effective against a wide range of algae. There are many formulations, with differences mostly intended to improve effectiveness or duration of activity under various environmental conditions, but the key ingredient remains the copper ion itself. After reactions are complete, the copper remains, and is usually deposited in the sediment. This can’t be good for the lake, but there are few studies that have demonstrated any measurable negative impacts. With repeated treatment, the sediment may be considered hazardous waste if ever dredged, but for the most part the reacted copper appears to be inert. Doses of copper in New England waters rarely exceed 0.2 mg/L as copper, and are often <0.1 mg/L. Larger doses are used in some other parts of the USA, mainly to overcome interference by high suspended or dissolved solids, and these are poor examples to compare with New England applications. Some zooplankton and some trout species may be susceptible to toxic effects at applied doses, but the vast majority of non-target aquatic organisms are not threatened by copper doses used in New England.

A more recent algaecide is peroxide, formed from sodium carbonate peroxyhydrate when added to water. It is an oxidant that impacts cell walls of algae, with groups like cyanobacteria being generally more susceptible than stronger walled forms like some greens and diatoms. It leaves no potentially hazardous residues. The primary drawbacks are that sometimes we want to kill green algae, especially mats of filamentous forms, and peroxide-based algaecides are more expensive than copper alternatives. Still, the generally positive environmental profile of peroxide-based algaecides makes them attractive. Peroxides seem to be more effective than copper on cyanobacteria mats, which are often sources of taste and odor in reservoirs.

There are a few other manufactured algaecides that have specialized applications, but copper and peroxide represent nearly all the market for this type of treatment. It is preferable to limit nutrients to control algae, but this is much easier said than done, and having algaecides as a management option helps make our drinking water safe and our recreational lakes swimmable. Excessive use of algaecides should be avoided, and control of nutrients should be pursued as a long term solution, but algaecide application is a valuable management tool that should not be rejected without careful consideration.

The biggest issue with treatment is the tendency to wait until there is a major accumulation of algae to treat. At that point treatment will lead to a lot of decaying organic matter, release of nutrients, and possibly release of toxins. This latter possibility has led many states to disallow treatment if potentially toxic algae are too abundant. The most effective way to use algaecides is to prevent a bloom, not get rid of one. This means tracking algae on a regular basis, typically weekly, and reacting when problem species start to increase, which is not an easy task.

One other important point about algaecides warrants attention. As noted at the start, these are compounds that directly kill algae. Some regulatory agencies, notably but not exclusively in New York, have defined algaecides as any additive that prevents algae from becoming abundant. Consequently, phosphorus inactivation with aluminum or lanthanum is considered to be algaecide application, and since these are not registered as algaecides with the federal government, treatments using them cannot be permitted. By this line of reasoning, addition of oxygen to the bottom of a lake to keep phosphorus sequestered is also an algaecide application, and addition of water to cause dilution or flushing in a lake would also represent application of an algaecide. This sort of regulatory foolishness hurts sound lake management and highlights why it is institutions that limit success far more than science or economics.