Onshore–offshore trends in community structural attributes: death assemblages from the shallow continental shelf of Texas

• Published in: Continental shelf research Vol. 19; no. 6; pp. 717 - 756
• Main Authors: Staff, George M., Powell, Eric N.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.05.1999
• Subjects: Marine; ASW, USA, Texas; ASW, USA, Texas, Corpus Christi
• ISSN: 0278-4343; 1873-6955
• DOI: 10.1016/S0278-4343(98)00108-3

Death assemblages were compared at three sites on the inner continental shelf of Texas using the community attributes of taxon richness, taxonomic composition, habitat tiers, and feeding guilds, by means of three descriptor variables, numerical abundance, paleoproduction (biomass-at-death), and paleoingestion (lifetime ingestion, a measure of energy flow). Each death assemblage was compared to the co-occurring life assemblage and to six other death assemblages covering a transect from the estuary to the continental slope. Analysis of death assemblage composition is increased by at least a factor of two if fragments are included. In each of these nine assemblages on the bay-to-slope transect, the whole-shell component of the assemblage was adequate for a thorough analysis of community guild, tier, and taxon structure. The assemblage types were each unique in a combination of key abundance, paleoproduction and paleoingestion-derived community attributes. At least as important, however, were the resemblances between certain assemblages. All shelf and heterotrophic slope assemblages were characterized by predator dominance of paleoingestion. Deposit feeders and chemoautotrophs increased in importance numerically offshore, but not when evaluated by energy flow. All offshore assemblages were characterized by 40% or more of the individuals being infaunal. On the whole, tier structure was more variable than guild structure within habitat. On the whole, paleoingestion was more variable than numerical abundance or paleoproduction within habitat, probably because of the reliance of paleoingestion on long-lived taxa that are normally relatively rare. Greatest similarity was seen in paleoguild structure (a simplified guild structure) within and between habitat. Using paleoguild structure significantly limited the discrimination of assemblage types. The relative proportion of predators in the shelf and slope heterotrophic assemblages was striking. Comparing predator lifetime ingestion with prey lifetime production reveals that the slope cold-seep assemblages and bay assemblages have a large surplus of primary consumers, whereas the normal slope assemblage and the three inner shelf assemblages are overrepresented by predators. Assuming that the proportions of predator and prey indicate the relative importance of non-preservable prey, non-preservable prey were relatively more abundant on the shelf and slope, and, in fact, nonpreservable species contributed about 90% of the individuals to the life assemblage at the three inner shelf sites.