Community reconstruction in benthic paleoenvironments : trophic structure in living and dead macroinvertebrate associations, Corpus Christi and Aransas Bay systems, Texas

A non-transported fossil deposit is a time-averaged accumulation of the preservable, hard-part-bearing portion of a community. A simulated fossil deposit can be created by combining repeated collections of the shelled part of a Holocene living community. This study is a comparison of time-averaged monthly collections of Holocene macrobenthos, made over a two and one half year period from 21 stations in the Corpus Christi and Aransas Bay Systems, with the potential fossil deposit represented by dead shell material collected from surface sediments at the same stations. Of 112 Biologically Important Species (B.I.S.) of living macrobenthos, representing 265,050 individuals, 20 were molluscs, the rest were soft-bodied members of nine other phyla, primarily polychaetes and crustaceans. The molluscs comprised 28.4% of the total fauna --0.8% gastropods, 27.6% bivalves. Of 64 dead shelled B.I.S., representing 252,305 individuals, 63 were molluscs and one a crustacean. The molluscs comprised essentially 100% of the fauna -- 19.9% gastropods, 79.9% bivalves. Application of normal-mode numerical classification (cluster analysis) to the B.I.S. living macrobenthic, living mollusc, and dead mollusc faunal data identified station groups (communities) based on the similarity of their faunal composition. The recognition of two station groups in each of the three sets of data suggests that the faunal components at these stations represent characteristic associations of species from a single community, each association carrying with it enough of the pattern or structure of the total community to be a reflection of the distribution of that community. The Open Bay Hard Sandy Shelly Mud Community is characterized by bioturbated sandy or shelly mud that forms a hard substrate. The Open Bay Soft Mud Community is characterized by bioturbated slightly shelly mud with a soft surface layer, probably caused by shrimp dredging. Application of inverse-mode numerical classification and constancy and fidelity nodal analysis of two-way tables of the data identified the characteristic species associations for each of these two communities. Subsequent trophic analysis arranged these associations into trophic pyramids of numbers according to the feeding modes of the component species. Neither the species composition nor the trophic structure of the living and dead mollusc associations resembles those of the corresponding living macrobenthic association. Mollusc-only associations tend to overemphasize suspension-feeding primary consumers and shelled carnivores; living macrobenthic associations have more soft-bodied interface- and deposit-feeding forms, and few shelled carnivores. The species composition of the dead-mollusc associations bear little resemblance to that of the corresponding living-mollusc associations, probably because the dead fauna represents a much longer time-averaging than the years represented by the living fauna. However, the observation that living- and dead-mollusc trophic proportions, especially at the primary consumer level, are closely similar suggests a constancy of mollusc feeding niches over time, even though the species filling the niches may be different. These findings signal extreme caution in extrapolating from a fossil assemblage to the total living community from which it was derived. It is suggested, however, that if a fossil fauna similar to that of the Open Bay Hard Sandy Shelly Mud Community were interpreted to be from a similar physical environment, inferences could be made about its non-preserved soft-bodied fauna, based on the trophic comparisons in this study