Meeting Abstract

45.5  Wednesday, Jan. 5  Dead-shell assemblages as a means of detecting ecological changes and baseline conditions: A test on the urban southern California continental shelf KIDWELL, SM*; TOMASOVYCH, A; Univ. of Chicago, Chicago; Univ. of Chicago, Chicago skidwell@uchicago.edu

Scientists in many disciplines are concerned with detecting and understanding community-level responses to human stresses and identifying appropriate baselines for restoration efforts: to what extent and in what ways have local systems been altered, and what did natural look like? These challenges are especially acute in coastal ecosystems. We are testing the ability of naturally occurring assemblages of dead molluscan shells to record the known environmental history of the southern California shelf. Despite the potential for spatial mixing of dead shells, death assemblages do as well or better than single biological surveys in detecting gradients in water depth and distance to wastewater outfalls. Moreover, despite natural time-averaging of shells over millennia (our amino-acid dating of hundreds of shells from three key species), the impacts of 20th-century urbanization can be recognized by contrasts with recent living communities. For example, death assemblages capture both the recent increase in abundance of the chemosymbiotic bivalve Parvilucina tenuisculpta (dead shells have a median age of only 36 years, and are especially abundant in death assemblages near outfalls where surveys have encountered the highest living abundances) and a historical decline in the suspension feeder Cyclocardia bailyi, which is well-represented in death assemblages across the shelf but is rarely sampled alive within the last 30 years. Dead-shell assemblages — the taxonomically identifiable debris from sieve residues that are usually discarded– are thus a promising means of filling the pre-monitoring data gap on the nature of seafloor communities, permitting study of long-term natural and anthropogenic trends.