Concentration and exposure time. Control of zebra mussel depends upon chlorine concentration, contact time, and water quality and temperature, where the relationship between concentration and exposure time is usually an inverse one. Van Benschoten et al. (1993, 1995) give models for predicting 95 percent mortality as a function of temperature and total residual chlorine concentration. Generally, the contact time required decreases as concentration increases; however, reducing chlorine concentrations by half results in half the mortality in less than twice the contact time, and this may be a way to reduce chemical costs (Van Benschoten et al. 1993). Water temperature is an important factor in effectiveness of chlorination in zebra mussel control; since chlorination is usually held at ambient temperature at treatment sites, the seasonal timing of chlorine application is important (Claudi and Mackie 1994). Higher concentrations of chlorine are required at lower temperatures in order to be equally effective. Chlorination may not be practical at less than 50 ºF (10 ºC), due to the longer contact time required.
Chlorine demand. Chlorine demand of incoming water should be established and included in calculating the application concentration. A guide to these calculations is given in Claudi and Mackie (1994). Consider effects of pH, organic and inorganic nitrogen content, temperature, and physiological status of zebra mussels on concentrations required for effective treatment. Water with large amounts of organic and inorganic compounds has high chlorine demand. Most of this information given here comes from experience with Great Lakes water; results may differ with other water qualities (Claudi and Evans 1993).
Mortality. Generally, 0.5 mg chlorine L-1 (ppm) for 2 hr gave 100 percent mortality in veligers (Klerks, Fraleigh, and Stevenson 1993). Fatality in adult mussels occurred at concentrations of 2.0 mg L-1.
End-of-season treatment. While chlorination is not the most efficient method where a single annual application is used to eliminate established adult mussels, it can be used in this way by applying high doses continuously for 2 to3 weeks. Results from tests in the field (Claudi and Mackie 1994) are shown in the following tabulation.
Periodic. Similar results will be produced by the treatment levels used in single end-of-season applications (Claudi and Mackie 1994). Other research (McMahan and Tsou 1990; Jenner and Janssen-Mommen 1993) is summarized in the following tabulation.
Intermittent. Intermittent treatment, used throughout the breeding and settle-ment period at rates of 2 mg L-1 chlorine (TRC) for half-hour periods at 12-hr intervals, has been effective in preventing primary veliger settlement but not in removing established adults (Claudi and Mackie 1994). Such a treatment can be combined with an out-of-season continuous chlorination treatment for 2 to 6 weeks at 2 mg L-1 TRC to eliminate accumulated adults (Claudi and Evans 1993). Treatment combinations are shown in the following tabulation.
Semicontinuous. On/off cycling of chlorine over short intervals takes advantage of the lag time of mussel shell opening after treatment is discontinued to mimic continuous treatment while reducing total residual oxidant loading significantly. It is expected to be comparable to continuous treatment in effects, but with lower chemical use and discharge, and subsequently lower cost. Tests showed that a cycle of 15 minutes of 0.5 ppm chlorine followed by 30 minutes without chlorine reduced settling to levels found with continuous treatment (Claudi and Mackie 1994). Results are summarized in the following tabulation.
Continuous. Low, sublethal levels of chlorination may be effective against zebra mussel by eventually producing chronic toxicity or preventing macrofouling. Speed of water flow may affect how readily mussels are detached and swept away. Continuous treatment at 0.5 mg L-1 TRC has been successful, preventing new settlement and killing adult mussels regardless of speed of flow (Claudi and Mackie 1994). The following tabulation shows several rates.
