BIVALVE DEATH ASSEMBLAGES DETECT DECADAL-SCALE VARIABILITY IN COMMUNITY COMPOSITION ON AN URBAN CALIFORNIA SHELF

One goal of conservation paleobiology is to develop reliable historical baselines for evaluating the current state of natural systems and efficacy of management practices. To reach this goal, we must first evaluate whether successive down-core assemblages can detect ecological changes with the decadal resolution and age accuracy needed for conservation efforts. Here, we use new box-cores from the continental shelf of southern California to test the extent to which a well-known history of pollution and benthic community change is captured. Wastewater from Los Angeles County was first discharged offshore of the Palos Verde peninsula (60 m) in 1937, but declined sharply with enactment of the Clean Water Act in 1972. Biomonitoring since then found large populations of the nutrient-loving lucinid bivalve Parvilucina tenuisculpta and chemosymbiontic Axinopsida and Thyasira in the 1970s to early 1980s (up to ~1500 individuals per 0.1 sq-m grab, >80% of living bivalves) that, with improved wastewater treatment, declined strongly in the late 1980s and 1990s to the <10% observed today. Suspension feeders are consistent at low numbers and proportions (10-20%) since the 1970s, mixed-feeders were highest in the 1980s, and deposit-feeding nuculanids have increased since the early 1990s. Both 24-cm long box-cores from 50-m water depth, located roughly 4 km ‘upstream’ of the discharge point, preserve a distinct but damped (20%) peak of the nutrient-loving species at 8-12 cm core depth, which dates to the 1980s based on Pb-210. These species decline to ~5% in the core-top (0-4 cm) and to 10-15% in the core-base, which dates to the 1950s. Suspension-feeders have a higher proportional abundance (30-40%) throughout the core than seen in counterpart live-data, as do deposit-feeding Nuculana (~20%), at least in part because of a preservational bias against thin-shelled mixed-feeders. Thus, although we find good capture of trends in indicator species of late 20^th century nutrient over-load and remediation, functional groups are the most robust level of taxonomic resolution for ecological reconstruction in this system, and indicate that benthic communities today have recovered a state comparable to that of the 1950s. This provides a firm basis for extracting pre-urban baselines from longer vibracores.