HIGH-LATITUDE, BIOMASS, AND RECENT CLIMATE CHANGE: TESTING THE POWER OF LIVE DEAD DISCORDANCE IN THE PACIFIC ARCTIC

Time-averaged death assemblages sampled from tropical to temperate open continental shelves commonly disagree in species composition with local living communities that have changed in response to anthropogenic eutrophication and other locally intense human stresses. In contrast, the ability of live-dead discordance to detect the spatially diffuse effects of human-induced climate change has not been tested, nor has its resolving power in rapidly changing high latitudes, where climate change entails massive changes in nutrient cycling with consequences for benthic biomass. The North Pacific Arctic and Subarctic 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 continental shelf was maintained by ice-fed bottom water that started to move north and become spatially diffuse between 1998 and 2001. The benthic community in the transitioned area now experiences new pelagic predators, a new amount, and quality of food and a new mean sediment grain size delivered to the benthos. This range of climate-associated changes has the potential to create measurable discordance living and death assemblages, despite lower preservation of aragonite in Arctic water. I find that in habitats where either Subarctic or Arctic conditions have persisted death assemblages agree closely with living communities: cold waters apparently do not alone lead to notable post-mortem bias. Instead, significant live-dead discordance is found only in areas of recent documented changes in nutrient delivery and sediment grain size. This 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 particular sub-areas within habitats with a new nutrient level or a new dominant grain size since 1980. The N Pacific Arctic and Subarctic live-dead discordance can reliably differentiate between unchanged and rapidly changing habitats and the biomass of death assemblages provided added spatial resolution to live-dead analysis.