EVALUATING THE INFLUENCE OF PRODUCTIVITY REGIME ON THE ENERGETIC STRUCTURE OF FOSSIL ASSEMBLAGES: A COMPARISON OF SUBFOSSIL MOLLUSK ASSEMBLAGES FROM THE CARIBBEAN AND PACIFIC SHELVES OF PANAMÁ

A growing body of palaeoecological studies delineate long-term trends in the energetic structure of marine ecosystems, based primarily on the taxonomic and trophic composition and size-frequency distributions of local fossil assemblages. Changes in primary productivity are often invoked as drivers of these trends but the relationship between productivity and the energetic structure of benthic assemblages is not well studied at the shelf depths that dominate the marine fossil record. Here we examine the taxonomic and trophic structure and size-frequency distributions of subfossil mollusk assemblages from the Caribbean and Pacific shelves of Panama, taking advantage of natural gradients in upwelling intensity and primary productivity. We studied 32 dredge and grab samples from soft substrate environments between 20 and 55 m depth from several regions in the oligotrophic Caribbean and from the mesotrophic Golfo de Chiriquí and eutrophic Golfo de Panamá in the Pacific. All samples were sieved using a 2 mm mesh. Using both manual and automated measurements we estimated biovolumes for 7163 bivalve shells and 1418 gastropod shells, all identified to family level. Because it is rarely possible to measure absolute abundance in the fossil record, we focus on per-capita energetic metrics: for each sample we calculated mean per-capita production, mean per-capita standing metabolic rate, and mean per-capita consumption of primary production. All three of these metrics show qualitatively similar patterns, with the lowest median values in the Caribbean, intermediate median values in the Golfo de Chiriquí, and highest median values in the Golfo de Panamá. These differences are largely driven by variation in the size and frequency of predatory gastropods, with differences in the sizes of suspension-feeding bivalves and gastropods playing a secondary role. Regional differences in sedimentation rate, degree of time-averaging, and destruction rate of different size fractions could in principle bias size-frequency distributions but there is little evidence that they are major contributors to observed patterns. Our analyses suggest that the size-frequency distributions in addition to trophic structures of fossil assemblages may provide valuable constraints on ancient productivity gradients.