SHELL FRAGMENTATION AS AN INDICATOR OF BENTHIC CONDITIONS

Post-mortem modification of marine skeletons records environmental conditions around the sediment-water interface. Understanding these modifications provides insight into environmental conditions and the formation of the fossil record. Fragmentation and disarticulation of shells are largely a function of biogenic activity by shell-crushing predators, thus we used fragmentation of shell death assemblages to evaluate benthic health in natural and anthropogenically impacted coastal marine environments. At well-oxygenated sites in Quatsino Sound (Vancouver Island, Canada), fragmented shells constitute 70% of the death assemblage. In the restricted fjord of Holberg Inlet (Vancouver Island, Canada), fragmented shells make up 50% of the total skeletal death assemblage. In Rupert Inlet (Vancouver Island, Canada), the locus of submarine tailings deposition, fragmented shells make up 25% of the total skeletal death assemblage. Shell fragmentation levels at these sites (all >70m water depth) are inferred to be a function of shell-crushing predator activity, or the exclusion thereof by benthic stress due to hypoxia or smothering. Similarly, along a 50 km west-east transect through Long Island Sound (New England, U.S.A.), fragmentation levels range from 44% at the western end where summer hypoxia is common, to 68% eastward where waters remain oxygenated. Potential confounding factors such as water depth, sediment characteristics, and taxonomic composition fail to explain this pattern. As such, levels of shell fragmentation in subtidal deposits could serve as a preliminary indicator of benthic health, permitting the identification of stressed environments. In the more general application of death and fossil assemblages, it may be perilous to disregard shell fragments if they are the bulk of the information from precisely the most diverse environments, or at least those environments that permit the greatest benthic activity among shell-crushing predators.