Effects of intermittent halogenation on settlement, survival and growth ofthe zebra mussel, Dreissena polymorpha

The effect of intermittent (2-4 h/day) treatments with chlorine or bromine at levels of 0.5 and 1.0 mg/L (total residual oxidant) upon settling and gr owth of veliger larvae of the zebra mussel, Dreissena polymorpha, was exami ned in two successive flow-through studies. The work was conducted in a fie ld laboratory which received a constant supply of water from western Lake E rie. Veliger densities in the water at the field site peaked at 530/L, whil e mussel densities on settling monitors reached 147,100/m(2) over the cours e of the two studies (early July to late September 1991). In Study 1, a 2-h daily treatment with 1.0 mg/L chlorine reduced mussel settling by 91% as c ompared with controls, although mussel densities of up to 6,000/m(2) still occurred. Treatment with 0.5 mg/L chlorine for 4 h/day produced a similar r eduction in mussel settling during the second study. Bromine was less effec tive than chlorine at reducing settling of veligers. Mu ssels which remaine d settled in the treatment tanks had growth rates similar to controls, reac hing 2-4 mm in length over a 30-day period. The intermittent halogen treatm ents similarly had no effect on whole body glycogen levels or growth of adu lt D. polymorpha. The intermittent chlorination regime used may therefore d elay, but would not prevent a mussel fouling problem.