Contemporary ecological threats from historical pollution sources: impacts of large-scale resuspension of contaminated sediments on sessile invertebrate recruitment.
Historical contamination continues to pose a serious ecological threat to many ecosystems across the world. Marine sediments have long acted as sinks for contaminants from surrounding industry and urbanization, and sediment-bound contaminants are known to affect the ecology of sediment infauna. When sediments are disturbed, however, contaminants are resuspended and potentially released into the water-column and dispersed to other environments. The threat posed by the resuspension of contaminated sediments has been a focus of substantial research in the geochemical and ecotoxicology fields, yet to date there has been no ecological assessment of the impacts of a real-world resuspension event involving contaminated sediments. We assessed the ecological threat posed by the resuspension of contaminated sediments by testing for impacts of a major dredging operation in an estuary with highly contaminated sediments. We sampled the recruitment of sessile invertebrates in this estuary and two external reference estuaries, before and during dredging. These invertebrates are filter-feeders that settle and live on hard substrata above the seafloor. Impacts of the dredging-related resuspension were tested using a Beyond BACI analysis. Dredging activities resulted in the large-scale resuspension of contaminated sediments. Concurrently, the recruitment of the dominant filter-feeders (e.g. barnacles and polychaete worms) was virtually extinguished for 4 months, despite being abundant prior to dredging. This pattern contrasted with the recruitment of the same invertebrates in the reference estuaries, which showed little change over the same period. Synthesis and applications. The severe decrease in the recruitment of sessile invertebrates within an estuary exposed to dredging and the deposition of contaminated sediments indicates that the resuspension of these sediments pose a real ecological threat to organisms in contact with the contaminated water-column. Containment measures (e.g. silt curtains) are, therefore, essential and further engineering innovations (e.g. dredging designs and operations) are necessary to reduce resuspension during the dredging and deposition of sediments. In addition, this study demonstrates that past pollution events can cause current ecological impacts that extend well beyond those habitats recognized as being contaminated.