Onshore–Offshore Trends in the Size-Frequency Distribution of Death Assemblages: Northwestern Gulf of Mexico

Abstract

The size-frequency distributions of death assemblages were compared at three sites on the inner continental shelf of Texas by means of three descriptor variables, numerical abundance, paleoproduction (biomass at death), and paleoingestion (lifetime ingestion, a measure of energy flow). These death assemblages were then compared with six other death assemblages covering a transect from the estuary (Copano Bay, TX) to the continental slope. Typically, size-frequency distributions are based on abundance and size classes are set proportional to the largest individual in the collection. Restriction to this one analysis would have identified few of the important trends observed in this study. The evaluation of size frequency on the basis of species' maximum size as well as assemblage maximum size and the comparison of a suite of assemblages on the basis of the largest maximum size provide important new inferences into community dynamics. The distribution of measures of energy flow across the size-frequency spectrum provided an additional, valuable source of information on community structure and habitat optimality. Within-habitat variability was consistently less than between-habitat variability. The autochthonous continental slope assemblages were the most diverse in their size-frequency spectra. Comparison between habitats showed that the continental slope assemblages had the largest proportion of adult individuals. The continental shelf assemblages were dominated by juveniles. The chemoautotrophic and heterotrophic assemblages in Copano Bay and on the continental slope were similar in most respects despite substantial differences in their trophic structure. Similarity existed in the proportion of adults, in the tendency toward bimodality, and in the degree to which species reached maximum size. The shapes of the size-frequency spectra were controlled in large measure by (a) the relative loss of juveniles through taphonomy, (b) the degree of survivorship to adulthood, probably predominately determined by predation, (c) the food and space resources present that control species size, and (d) the optimality of the habitat that allowed animals to approach maximum size. The habitats on the continental slope had the highest proportion of individuals near maximum size. The Copano Bay assemblages were also characterized by a large proportion of adult individuals; however, these normally did not reach sizes above 70% of species' maximum size. The largest individuals were found at the petroleum seeps and in the heterotrophic assemblages from Copano Bay. Continental slope habitats should be temporally most stable, and our information supports that expectation. Food supply should be greatest in estuaries and in cold seeps where chemosynthetic