Response of green mussels (Perna viridis) subjected to chlorination: investigations by valve movement monitoring.

Abstract

Perna viridis Linnaeus (1758) is a major foulant in the cooling water systems of electric power stations located on the East coast of India. Though chlorination is considered an effective fouling control measure, the strategy may fail in the case of bivalve mussels, due to the ability of the mussels to close their shells and still survive for extended periods of time. In a given power station, continuous low dose (exomotive) chlorination (0.2±0.1mgl-1) is practiced to control biofouling. Laboratory experiments were carried out to assess the mortality and valve movement response of Perna viridis exposed to chlorine, using a Mosselmonitor®. All size groups tested showed progressive reduction in valve opening upon chlorination. However, continuous dosing of chlorine concentration as high as 1.0mgl-1 was required for sustained and complete valve closure response in this mussel. At lower concentration (0.7mgl-1), the mussels were able to open their shells and feed. Sustained valve closure resulted in physiological stress to the mussels due to reduced feeding, subsequently leading to death. Time to 100% mortality was dependent on the size of the mussels. At 1.0mgl-1 chlorine residual, smaller size group (30-50mm) mussels showed 100% mortality in 79.3h, while larger groups (50-70mm and 70-90mm) took 152h and 243h, respectively. Frequency of valve opening was high in smaller size group mussels (30-50mm), compared with larger groups (70-90mm). Even though the time taken for killing was size-dependent, frequency of valve opening and time period between successive openings were found to be characteristic of individual mussels. The observations provide new insight into the response of bivalve mussels to continuous chlorination in the context of biofouling control and point to the need to adopt pragmatic strategies to prevent mussel spat settlement rather than killing of adult mussels, thereby reducing environmental burden due to chlorine residuals. Usage of target-specific biocidal strategies (intermittent/pulse dosing) or alternative biocides (chlorine dioxide) may help mitigate green mussel fouling in tropical cooling water systems.