This section was developed using information and original text from the Zebra Mussel (Dreissena polymorpha) Control Handbook for Facility Operators.
Exposure to Freezing Conditions
Target Life Stage: Veligers and adults
Recommended uses: Hydroelectric plant external structures, locks, dams, vessels
Control Objective: 100% mortality of adults and juveniles
Control Procedure: Expose mussels to temperatures less than 0°C for a predetermined period of time. Temperature tolerance will be directly correlated to both acclimation temperatures and immersion time. Clark et al. (1993) report 100-percent mortality of all mussels, even in clumps, at -10°C in 4 hr. One hundred-percent mortality of immersed single individuals occurs in 15.1 hr at less than 1.5°C, and in 1.3 hr at less than -10°C (Clark et al. 1993).
· For further information on freezing, see Freezing or Desiccation in the Reactive Control Methods section of Management and Control.
Exposure to High-Temperature Conditions
Target Life Stage: Veligers and adults
Recommended uses: Raw water systems, hydroelectric plant internal structures, locks, dams, vessels
Control Objective: 100-percent mortality of adults and juveniles
Control Procedure: This type of thermal treatment uses heated water to control zebra mussel infestations. In hydroelectric facilities, heated condenser discharge is circulated back to the water intake and recycled through the system. With each pass through the system, additional heat is provided by the steam turbines, increasing the temperature.
Exposure to temperatures above 32.5°C for more than 5 hr is lethal (Jenner 1983, Jenner and Janssen-Mommen 1989).
· For further information on thermal treatment using heat, see below.
Thermal treatment is a cost-effective and efficient method for zebra mussel control. Most regulatory authorities regard heat treatment as a more environmentally safe and benign method than chemical treatment; however, restrictions on the discharge of heated water have to be taken into account.
For the zebra mussel and other macrofouling species, the upper lethal thermal limits on which thermal mitigation strategies are based have generally been determined as either the acute upper lethal temperatures or the chronic upper lethal temperatures (McMahon et al. 1995). (For further information on these two strategies, see either Acute Thermal Treatmentor Chronic Thermal Treatment.) The reduced thermal tolerance of zebra mussels relative to other North American biofouling species makes the mollusc more susceptible to thermal mitigation.
The zebra mussel is capable of extensive temperature acclimation, affecting both its acute and chronic lethal temperature limits. Thus, regardless of the thermal treatment strategy employed, a raw water system will need to be heated to higher temperatures to achieve 100-percent eradication of mussel infestations during summer months (when source water temperatures are elevated) than in winter months. Initiating either chronic or acute thermal treatments during periods when water temperatures are below maximum summer levels may significantly reduce both the exposure time and treatment temperature required to achieve 100-percent kill of zebra mussels. Also important is the fact that smaller zebra mussels have greater thermal tolerance than larger mussels. Because of their higher thermal tolerance, infestations consisting primarily of smaller individuals (the usual case if a raw water system is subjected to annual or biannual mitigation treatments) will require higher treatment temperatures and/or longer exposure times to induce the desired mussel kill (McMahon et al. 1995).
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