ABILITY OF DEATH ASSEMBLAGES TO TRACK KM-SCALE SPATIAL AND DECADE-SCALE TEMPORAL VARIABILITY IN LIVING COMMUNITIES: A TEST ON THE URBAN SOUTHERN CALIFORNIA CONTINENTAL SHELF

The modern continental shelf of southern California provides an excellent opportunity to evaluate the ability of time-averaged death assemblages – here, of shelled mollusks – to detect known spatial and temporal changes in key environmental variables. This shelf received increasing quantities of wastewater from point sources (treatment plant outfalls) through the 20th century until the 1972 Clean Water Act, when nutrient and toxic inputs began to decline. Biomonitoring since then has found both strong spatial gradients away from wastewater outfalls and declines over time in populations of pollution- and hypoxia-tolerant species, including the chemosymbiontic lucinid bivalve Parvilucina tenuisculpta, and increasing species richness (e.g., Stein & Cadien 2009). Comparing samples collected in 1975 (~peak urban emissions) and 2003-2004, we find that, despite the potential for spatial mixing of dead shells, death assemblages do as well and by some metrics better than single biological surveys in detecting gradients in water depth and distance to outfalls. Temporally, death assemblages are labile on decadal scales, despite considerable natural time averaging (2003-05 death assemblages have a median shell age of ~90 yr by radiocarbon-calibrated racemization, which is an order of magnitude greater than in modern estuaries). For example, focusing on the proportional abundance of the indicator taxon Parvilucina, the composition of death assemblages shifts between 1975 and 2003 at sites both proximal and distal to outfalls; the median age of Parvilucina shells in 2003 death assemblages is 36 yr, and thus date to populations associated with high emissions. These changes in death assemblage composition lag behind changes in the living, creating a mismatch with the living that correctly identifies population changes in previous decades. However, decadal changes in death assemblage composition are weaker than in the living assemblages that drive them, a damping of variability expected from time averaging. Death assemblages are thus conservative records of temporal variability. Coring planned for 2012 will test, among other things, the ability of buried fossil assemblages to detect the known 20th century rise and fall of urban nutrient input on this shelf, as well as less strong decadal shifts in natural productivity.