DEATH ASSEMBLAGES AS RECORDERS OF RECENT CLIMATE CHANGE ON THE CONTINENTAL SHELF

Few routine surveys of continental shelf benthic biota are in place today and fewer still have extended time series. Benthic surveys are difficult, expensive, and routinely underestimate abundance and spatial patchiness. Consequently, the importance of ongoing reorganization of the continental shelf benthos in response to climate change is nearly impossible to ascertain. A comprehensive dataset for the Georges Bank region is used to compare the distribution of the death assemblage and the living community at large spatial scales and to assess the application of the death assemblage in tracking changes in species’ distributional pattern as a consequence of climate change. Focus is placed on the biomass-dominant clam species of the northwest Atlantic continental shelf: the surfclam Spisula solidissima and the ocean quahog Arctica islandica. The distribution of dead shells, in the main, tracked the distribution of live animals relatively closely for both species. Out of habitat transport was inconsequential. Thus, the presence of dead shells was a positive indicator of present or recent past occupation by live animals. The regional distribution of dead shell differed from the distribution of live animals in a systematic way indicative of range shifts due to climate change. In each case the differential distribution was consistent with warming of the northwest Atlantic. Present-day overlap of live surfclams with live ocean quahogs was consistent with the expectation that the surfclam’s range is shifting into deeper water in response to the recent warming trend. Locations where living surfclams sans shells were collected measures the recentness of this event. The presence of dead ocean quahog shells at shallower depths than live ocean quahogs offers good evidence that a range shift has occurred in the past, but prior to the initiation of routine surveys in 1980. Growth rates of aged living ocean quahogs document rising temperatures since the early 1800s, suggesting that these animals are very sensitive indicators of the warming trend since the end of the Little Ice Age and that the death assemblage may permit reconstruction of temperature change on geographically large spatial scales.