DIFFICULTIES WITH THE DEATH ASSEMBLAGE AS A RECORD OF RECENT ENVIRONMENTAL CHANGE IN A REEF LAGOON SYSTEM

Studies of ecosystem fidelity have shown that the time-averaged death assemblage of molluscs accumulating in the surface sediment is a good representation of the living community on decadal scales. The assumption then becomes that the death assemblage is preservable once buried beneath the upper surface sediments. Earlier results from this study have shown that this is not the case on the northeast shore of St. Croix, US Virgin Islands. Vibrocores through the entire sediment package (2-5m deep) across a lagoon revealed that the modern seagrass community is poorly represented in the subsurface below about 25cm. The lagoon is strongly affected by deep burrowing by callianassid shrimp that have reworked the sediment package such that the sediment is nearly devoid of molluscs. Instead, there is a thick shelly lag deposit atop the Pleistocene substrate at the bottom of each core. Taxonomic analysis of the shells in the lag assemblage do not match the surface seagrass death assemblage (dominated by larger epifaunal gastropods). The lags were strongly skewed toward small infaunal bivalve species, which is a better match to the surface assemblages found in open sandy environments on St. Croix.

The above study suggested one of two scenarios: 1) The typical seagrass community dominated by epifaunal gastropods and larger, shallow infaunal bivalve species is not easily moved into the subsurface by callianassids, but remains at the sediment-water interface where it degrades before it makes it into the subsurface; or 2) the seagrass ecosystem is a relatively recent environmental change that follows upon a 7,000 year history of primarily open bioturbated sand. This study builds on the previous work by comparing the taphonomic signatures. Taphonomy may better reflect the sedimentary environment of molluscs after death, which can avoid some problems with taxonomic data. Results for buried shells strongly suggest a bioturbated sand signature over a grassbed signature. This supports the hypothesis that the lagoon has undergone a recent change from open sandy bottom to seagrass beds. However, we also find that the size distribution of shells in the subsurface (smaller) differs from shells in seagrass beds (larger), so sorting by the shrimp may still be obscuring past times of seagrass dominance.