PRESERVATION BIAS OR HISTORICAL SIGNALS: VARIANCE IN MOLLUSCAN LIVE-DEAD AGREEMENT WITH WATER DEPTH OFF OF THE OREGON COAST

Distinguishing natural variation in community composition from changes produced by anthropogenic stressors has become a major focus for live-dead (LD) analysis. At the habitat-scale, time-averaged death assemblages diverge significantly in community composition and structure from living assemblages only in regions of known human impacts. But not all stressed areas displayed strong LD mismatch in a global meta-analysis, and the reliability of LD discordance at the site- (point-) scale of cores is less certain. Sampling and postmortem bias should also be evaluated as drivers of LD mismatch in any study area before presuming previously unrecognized anthropogenic (or natural) ecological changes. Bulk-sediment samples processed for live and dead bivalves (1 mm mesh) were collected five times between April and December 2011 on the continental shelf of Newport, Oregon, using a 0.1 m² box core along four transects with stations at 30, 40, and 50 m water depth with a one-time sampling in June of stations at 60 and 70 m. Overall, when species are present both living and dead they share similar abundance patterns (positive rank correlation) but taxonomic similarity varies with depth. Deeper sites (≥ 50 m) tend to have fairly high LD taxonomic similarity per station (≥ 0.5 Jaccard-Chao), whereas shallower stations have larger counts of “live-only” and “dead-only” species and thus lower similarity. In the deeper stations, the highly gregarious small venerid Nutricola lordi and/or the chemosymbiotic Axinopsida serricata consistently dominate both live and death assemblages. At shallower stations, both live and dead species abundances are patchier. Some of this site-level LD discordance is probably preservational (e.g., thin-shelled tellinid and macomid bivalves tend to be less abundant dead than alive) and some is probably gear-related (e.g., active deep-burrowing pharid bivalves are more abundant dead than alive). However, disproportionately high and localized dead abundances of Axinopsida and a higher proportional richness of dead-only species in shallow versus deep water are interesting. This may suggest possible historical and/or mobile habitat patches resulting from either natural or human drivers, which we are now investigating.