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

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 20^th 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 20^th century dynamics in broad outline, (2) reveal the undocumented rise of chemosymbiont-bearing bivalves during the early 20^th 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 20^th 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