Responses of native freshwater mussels to remediation to remove polychlorinated biphenyl-contaminated sediments in the upper Hudson River

The Hudson River, New York, was contaminated with polychlorinated biphenyls (PCBs) from two manufacturing plants over a period of approximately 30 years, and PCBs are still present in sediment and biota today. The river provides habitat for a variety of plants and animals, including native freshwater mussels. A remediation programme, consisting of dredging followed by the placement of backfilled sediment and/or a subaqueous cap, was initiated in 2009 to remove PCB-contaminated sediments, and managers were concerned about how remediation could affect the mussels.Quantitative surveys were conducted across nine reaches (pools) to estimate species composition, density, population size, population structure, and ecosystem services of mussel assemblages, pre- and post-remediation. Surveys were stratified into remediated areas (targeted for dredging) and non-remediated areas (not targeted for dredging).Pre-remediation, the river contained a low diversity, and high, but variable, density of mussels that were strongly dominated by Elliptio complanata. The presence of few fresh dead mussels indicates low natural mortality. Frequent recruitment by many species, varied length classes, and high biomass also support robust assemblages that perform important ecosystem services, such as biofiltration.Remedial activity removed most of the mussels and the associated services that they perform: their density was reduced by 83% and their filtration capacity was reduced by 93%, relative to non-remediated areas. Areas that were remediated were almost devoid of adult mussels, but the presence of small mussels indicates that natural recolonization is occurring. However, it is unknown whether these assemblages will persist. As mussels are generally long lived and slow growing, natural recovery is anticipated to be slow, in the order of decades.