Soft-sediment seafloors, the world’s largest biome, present major challenges, both for experimentation and inventory and for fossilization critical to paleohistorical analysis. Post-mortem, out-of-habitat transport of skeletal remains is rare, but rates of sediment accrual and burial are typically slow, and bioturbation is intense under aerated waters, promoting vertical mixing of shells over 10s of cm. Dead-shell assemblages are thus dominated by calcifying animals (especially mollusks, which also dominate total macrobenthic biomass) and are strongly time-averaged over past centuries to millennia. Here, we turn the prolonged ecological memory of dead-shell assemblages on the urban continental shelf of Southern California to advantage.
Results/Conclusions
Comparing the species composition of dead shells with a ~40-year biomonitoring time-series, we find an ‘anthropogenic ratchet’ in seafloor conditions that has created increasingly novel communities. Radiometric dating of shells shows that, for ~6,000 years until ~100 years ago, much of the mainland shelf of Los Angeles and Orange Counties was coarse-grained, shell-gravel habitat that supported diverse suspension-feeding epifauna, including large scallops and permanently attached terebratulid brachiopods, barnacles, and erect bryozoans. Within the last 40 years, these species have been sampled alive on the mainland shelf only rarely, in localized deep-water patches along the shelf-slope break; they remain in the regional pool owing to the still-sandy shelves of the sparsely-inhabited Channel Islands. The mainland shelf today is almost exclusively soft muddy habitat whose shelled fauna is dominated by small burrowing bivalves, including chemosymbiontic, deposit-feeding, and suspension-feeding species. Extirpation of the shell-gravel community was almost certainly by siltation, signaling a dramatic loss of habitat heterogeneity at site and landscape scales and an implicit decline of associated finfish and macrobenthic beta diversity. Channelization of rivers for flood control probably drove the initial transformation to muddy seafloors; 20th-Century shells are infaunal everywhere. The later shunting of wastewater to ocean outfalls, peaking in the early 1970s, created a new, anthropogenic habitat mosaic based on nutrient hot-spots dominated by abundant, late 20th-Century shells of hypoxia-tolerant chemosymbiontic bivalves. Ecological succession on increasingly healthy seafloors near outfalls, evident in biomonitoring data, is now reducing spatial variation, but the widespread muddy veneer on the mainland shelf, a legacy of the urban 20th Century, is here to stay, unlikely to be swept clean by natural processes. Faunal changes associated with secular warming will occur in the context of this new version of the mainland shelf, permanently ratcheted from conditions that had existed for millennia.