USING AQUATIC HERBICIDES UNDER CERTAIN CONDITIONS may cause unwanted consequences that have nothing to do with an herbicide's toxicity. For example, the sudden death of large numbers of plants in the water may cause fish-kills or algae blooms.

By careful consideration of the local environment where the herbicide is to be used, by careful selection of herbicide and equipment, and by careful application and timing, unwanted consequences may well be avoided.

Ideally a research study to evaluate the long-term ecological impacts of a herbicide application would have four replicated treatments:
1) The herbicide application;
2) an alternative method of weed control that completely removed the same amount of vegetation from the waterbody as the herbicide application;
3) an un-managed control treatment in which the target weed continues to grow; and
4) an un-managed control area which has never had the target weed.
(The last condition is least likely to be available in a similar area, unless the target plant is caught early in its invasion.)

It is important that the impacts of any management program be compared with the consequences of doing nothing.

Managing an aquatic weed infestation could result in the following impacts:

1) changes from populations of organisms that utilize the vegetation as substrate, food, or shelter to organisms that are adapted to open water;
2) reduced production of dissolved oxygen and changes in pH;
3) changes in patterns and velocity of water movement which could redistribute sediments; and
4) reduced competition from the target plant which would allow regrowth of desirable non-target vegetation.

The extent of habitat dominance by the target weed and the selectivity of the removal methods will determine the severity of changes between treatment and no-treatment.

Any ecological differences found between the herbicide management and alternative weed management methods (mechanical, biological) are likely to be impacts that are attributable to the direct effects of the herbicide such as toxicity, sub-lethal behavior changes, and consequences of the decay of the vegetation. The rate and magnitude of the latter impacts, which would include reductions in concentrations of dissolved oxygen, alterations in pH, release of nutrients, and deposition of dead vegetation on the substrate, would depend upon speed of action of the herbicide, with glyphosate and low-rates of fluridone causing much more gradual changes than fast-acting herbicides such as endothall, diquat and copper. Most of these types of impacts have been extensively reviewed by Murphy and Barratt (1990).

To compare the condition of a herbicide-treated site with an un-invaded area is to ignore the detrimental impacts of the un-managed target plant. Evaluation of the long-term impacts of control and long-term impacts of weeds left untreated is necessary if an area is being restored after a long-term invasion.

For fast-acting herbicides, partial-lake applications that are staggered over several weeks and which are applied from the shoreline out to open water, will allow fish to move away from potentially adverse conditions. Such treatments are recommended on the labels of herbicides that can be toxic to fish (such as endothall amine formulations and copper compounds), but they are also advisable whenever rapid plant death in heavy weed infestations or hot weather are likely to cause a sudden reduction in dissolved oxygen. An awareness of the possibility that thermal stratification might limit the dispersion of a herbicide application would be important for herbicides that have a narrow margin of safety for fish toxicity.

Even for herbicides that are very unlikely to have direct toxic impacts on fish, anglers frequently perceive that herbicide applications are disrupting fishing success. Results of field studies indicated that the catchability and movements of largemouth bass (Micropterus salmoides) were not directly influenced by applications of diquat or 2,4-D, nor was their feeding behavior affected by the dousing of bait-fish in 2,4-D (Sweatman et al. 1993, Boyer 1994). Catches of bass after the use of these herbicides on floating vegetation may be influenced by the relocation of fish following the loss of shaded habitat or by localized depressions in dissolved oxygen if significant plant decomposition occurs.

Public anxiety over herbicide use can be alleviated by the display of explanatory signs around treated areas, cooperation and education with local fishing communities, and sensitivity in timing treatments to avoid fishing tournaments and spawning seasons.

Part of the preceeding was excerpted from, Role of Herbicides in Aquatic Plant Management Programs, by W.T. Haller and A.M. Fox, 2002.

A BIT MORE...
Here is what field applicators are told (and tested on) about the environmental effects of aquatic herbicide use.