High-latitude benthic bivalve biomass and recent climate change: Testing the power of live-dead discordance in the Pacific Arctic

Abstract

Time-averaged molluscan death assemblages sampled from tropical to temperate open continental shelves commonly disagree in species composition with local living communities only in areas that have changed in response to anthropogenic eutrophication and other locally intense human stresses, providing a means of recognizing shifted baselines. In contrast, the ability of live-dead discordance to resolve the spatially heterogeneous effects of human-induced climate change has not been tested in high-latitudes, where climate change entails substantial changes in nutrient cycling with consequences for benthic biomass and where cold waters are antagonistic to carbonate shell preservation. North Pacific Arctic and Subarctic seabeds offer ideal conditions for testing the resolving power of molluscan live-dead discordance, using well-documented ecologic changes in nutrient cycling and benthic biomass in response to reduced sea ice. Ecosystem monitoring since 1980 has established that the boundary between the Arctic and the Subarctic on the Bering Sea continental shelf, maintained by ice-influenced bottom water, shifted northward between 1998 and 2001. The benthic community in the transitioned area now experiences new pelagic predators, more variable quantity and quality of deposited food, and altered sediment grain size, and macrofaunal dominance has shifted from diverse communities of specialized suspension or deposit feeders to facultative deposit feeding guilds. We find that in habitats where either Subarctic or Arctic conditions have persisted, bivalve death assemblages agree closely with counterpart living communities in taxon and guild composition and are not subject to significant post-mortem bias. Significant live-dead discordance occurs only in areas with documented changes in carbon delivery, sediment grain size, and community composition over the last several decades; there, death assemblages are mixtures of shells from pre- and post-transition communities, as confirmed by monitoring data. This spatial pattern is robust to both numerical abundance- and biomass-based measures of community composition. In fact, biomass is especially powerful in revealing fine, station-level discordance that is strongly tied to known sites within habitats where new carbon deposition levels, grain size, or benthos have occurred since 1980. Live-dead discordance can thus reliably differentiate between stable and rapidly changing habitats in cold, high-latitude settings, relevant to evaluating climate change, and biomass-based currencies of community composition are as robust as numerical abundance data, and in fact, improve spatial resolution.