Ocean quahogs (Arctica islandica) and Atlantic surfclams (Spisula solidissima) on the Mid-Atlantic Bight continental shelf and Georges Bank: The death assemblage as a recorder of climate change and the reorganization of the continental shelf benthos

The degree to which evidence of range shifts in shelf biomass dominants, Atlantic surfclams and ocean quahogs, as a function of climate change is examined by comparing the dissimilarity in distribution of the living community and the record of habitation ensconced in the death assemblage. Comparison of the distribution of live animals and dead shells reveals for both species that live animals are rarely found where dead shells were not collected, but dead shells were collected at a substantively larger number of sites than where live animals were found. The geographic footprint resolved by the dead shells was much broader than that of live animals. Extensive spatially-coherent regions exist inshore and offshore of the present-day habitat of occupation where evidence of previous occupation exists. Sites where ocean quahog shells were found without live ocean quahogs included a portion of the continental shelf inshore of the present-day habitable range, an area only habitable under colder climatic conditions than observed today. Surfclam shells were found offshore of the present-day habitable region, in deeper water: additional warming of bottom waters offshore would be necessary for surfclams to occupy the region now occupied solely by dead shells. The death assemblage shows that these biomass dominants have repositioned themselves across the continental shelf during the past and this repositioning certainly is driven by changes in bottom water temperature. Moreover, the distribution of surfclam shells indicates that the death assemblage may provide a possible window into the future, as movement of the present-day population is towards these deeper-water previously-occupied regions. Overall, the death assemblage may provide an important source of information on range shifts sparing the need to carry out extensive and frequent benthic surveys and may permit reconstruction of ongoing shifts in community composition as a product of climate change when adequate survey data are not available.