LIVE-DEAD DISCORDANCE IN MOLLUSCAN ASSEMBLAGES INDICATES HUMAN IMPACTS, NOT NATURAL TAPHONOMIC BIAS

Mismatches between the composition of a time-averaged death assemblage (dead remains sieved from the upper mixed-zone of the sedimentary column) and the local living community are typically attributed to natural post-mortem processes. However, statistical analysis of 73 molluscan datasets from estuaries and lagoons reveals significantly poorer average “live-dead agreement” in settings of documented anthropogenic eutrophication (AE) than in areas of negligible AE. Taxonomic similarity declines steadily as a function of AE severity and, for datasets composed exclusively of adults, rank-order agreement in species abundance drops where AE is suspected. Live-dead discrepancies in trophic composition are consistent with eutrophication. Seagrass-dwellers are anomalously abundant and/or organic-loving species anomalously scarce in the death assemblage, suggesting compositional inertia to recent environmental change. This agrees well with – and constitutes an independent validation of – results reported in 2006 for 34 datasets from open shelves, and in a setting where live-dead mismatches might be suppressed (higher sedimentation rates and longer histories of AE) or where the effects of AE might be swamped by post-mortem mixing among habitats of smaller patch-size and higher energy. These results indicate that (1) actualistic fidelity estimates from non-pristine areas are erroneously pessimistic, (2) live-dead discordance in surficial grab samples provides valuable evidence for strong anthropogenic modification of benthic communities, and (3) death assemblages are a promising means of reconstructing otherwise elusive pre-impact ecological baselines.