Chlorine controls zebra mussels through the effects of oxidation, consisting of either direct toxic effects on the adult, inhibition of settlement and growth of the larval stage, or weakening of the byssal thread attachments. Toxicity of chlorine to zebra mussels is a function of concentration, exposure time, and the type and quantity of chlorine compounds formed in water following treatment (Claudi and Mackie 1994).
Chlorine or hypochlorite reacts with water to form hypochlorous acid (HOCl), which readily dissociates to hydrogen ions (H+) and hypochlorite (OCl-). The hypochlorite ion is reduced to chloride ions and hydroxide ions forming a basic solution as it accepts electrons:
OCl-+ 2e- + HOH ® Cl- + 2 OH-
The ratio of hypochlorous acid to hypochlorite ions depends mainly on pH and to a lesser degree on temperature. Together, the two make up freeavailable chlorine (FAC). The undissociated hypochlorous acid (HOCl) is a strong oxidizing agent and has the principal biocidal activity of these two chlorine species, damaging membranes, diffusing through cell walls, and disrupting enzyme activity, and perhaps affecting ion regulation (Claudi and Evans 1993; Claudi and Mackie 1994).
These FAC compounds react with ammonia and other nitrogen-containing compounds to make chloramines, which also contribute to disinfection and are known as combined available chlorine. These two types of available chlorines make up total residual chlorine (TRC) (Van Benschoten et al. 1993; Claude and Mackie 1994). The presence of organic nitrogen and other compounds reduces TRC because chlorine forms complex nitrogen compounds. Additional chlorine has to be added to obtain a specific TRC level, and this differential, called chlorine demand, varies with type of raw water and season.
Chlorination affects zebra mussels through toxic effects of free chlorine and chlorine products. It affects adults, inhibits settlement and growth of veligers, and weakens the byssal thread attachments that hold the mussels in place. Because mollusks sense chlorine at 0.04 mg L-1 total residual oxidant and close for extended periods to escape it, it is thought that oxidizing biocides can cause mussel mortality through asphyxiation or limited glycolysis over a prolonged period of constant chemical feed. In addition, the free chlorine and chlorine products have a chronic toxic effect. The toxic oxidant compound is also thought to accumulate as some siphoning goes on (Van Benschoten et al. 1993, 1995).