MEPS 615:101-119 (2019) - DOI: https://doi.org/10.3354/meps12918

Gauging benthic recovery from 20th century pollution on the southern California continental shelf using bivalves from sediment cores

Jill S. Leonard-Pingel1,5, Susan M. Kidwell1,*, Adam Tomašových2, Clark R. Alexander3, Donald B. Cadien4

1Department of Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA

2Earth Science Institute, Slovak Academy of Sciences, 84005 Bratislava, Slovakia

3Skidaway Institute of Oceanography, University of Georgia, Savannah, GA 31411, USA

4Sanitation Districts of Los Angeles County, Marine Biology Laboratory, 24501 Figueroa Street, Carson, CA 90745, USA

5Present address: School of Earth Sciences, The Ohio State University Newark, 1179 University Drive, Newark, OH 43055, USA

ABSTRACT: Urbanization of coasts creates stresses on adjacent marine communities, but the full impact is seldom known owing to scarce historical records. Paleoecological analysis of sediment cores can be a powerful means of reconstructing baseline benthic communities, but is particularly challenging for continental shelves where dead-shell assemblages are prone to greater time-averaging than in natural sediment sinks such as deltas, coastal bays, and estuaries. We compared temporal changes in the composition of bivalve shell assemblages collected from box cores on the Palos Verdes (southern California, USA) shelf to a 40 yr time series of annually collected living benthic assemblages in the same area in order to calibrate bivalve core assemblages to known changes in community composition during wastewater remediation. Older (pre-1970) core assemblages were then used to reveal the nature of bivalve communities from the early 20th century and the extent to which present-day communities match, i.e. have recovered to, early urban baselines. Deep bioturbation and only moderate sedimentation rates (0.2 cm yr-1) damp the magnitude and rapidity of changes in core assemblage composition. Despite the geological complexity, bivalve core assemblages (1) detect known late 20th century dynamics in broad outline, (2) reveal the undocumented rise of chemosymbiont-bearing bivalves during the early 20th century, and (3) establish that the present-day community is largely but not fully recovered to its pre-effluent, early urban (1900-1930) baseline. Thus, cores capture the nature, timing, and duration of macrobenthic response to 20th century wastewater, validating this approach for shelf settings with scarce or no historical data.

KEY WORDS: Benthic assemblages · Mollusks · Sediment cores · Urban pollution · Southern California · Functional groups · Nutrient loading