30 YEAR LIVE/DEAD COMPARISON IN PERTURBED VS. STABLE ENVIRONMENTS: DEATH ASSEMBLAGE RESPONSE TO A GRADIENT OF ENVIRONMENTAL CHANGE

Environmental assessments in coastal habitats rarely include pre-disturbance surveys, particularly given the pervasive and long-term alteration of nearshore marine habitats by human activities. Skeletal and sedimentary records can provide medium to long-term baselines with which modern environments can be compared. Time-averaged mineralized death or fossil assemblages can capture community states over longer time periods (decades or centuries) where the noise of short term spatial and temporal volatility is dampened.

However to use these data, it is necessary to determine the taxonomic and temporal fidelity of skeletal material. For over thirty years the impact of submarine tailings deposition has been followed in Rupert and Holberg Inlets, and Quatsino Sound, Vancouver Island, British Columbia. Included in these time-series data are biological abundances, sedimentation rates, and sediment chemistry. For comparison with this rich live-data source, natural death assemblages and sediment samples were obtained in conjunction with the final benthic sampling in 2000. As a result of the environmental changes, gradients exist in sedimentation rate (0.5-500cm/yr), organic content (0.1-7%), and copper levels (20-4000ppm) among the 14 sites. This provides the opportunity to test live/dead fidelity and the utility of natural death assemblages in environmental assessment.

Preliminary results indicate that the bivalve death assemblage captures recent changes in the life assemblage. Taxonomic data shows fair agreement between the most recent live surveys and the death assemblage, 40-100% of live species are also present in the death assemblage, with an overall average of 70%. This agreement is despite potential confounding factors of small mesh size (0.5mm), and a recent taxonomic shift at impacted sites. Taphonomic condition varies among the sites as a function of residence time and surface exposure. Results indicate that skeletal death assemblages can act as a useful source of environmental information in both impacted and reference areas.