Paleontologic Insights Set Restoration Aims and Priorities for an Urban Ocean

The timespan of scientific observation severely limits our understanding of recent ecological changes driven by human and natural pressures. However, the dead shells of mollusks – clams, oysters, scallops, snails – and other seafloor-dwelling animals such as barnacles and tube-secreting worms are remarkably durable, and have been shown to persist for decades to thousands of years in the top few inches of the seabed. We used these naturally accumulated archives of past populations to reconstruct the ecological history of the continental shelf off Los Angeles, a major urban watershed in southern California. Although this is one of the best-studied seabeds in the world, our paleontological approach revealed the entirely unsuspected loss of an extensive ecosystem founded on dense shell gravel, about 100 years ago. Unlike today’s shelf, which is muddy and inhabited almost exclusively by burrowing species that feed on organic matter on or below the seafloor, the vanished ecosystem included abundant scallops and other species that attach to shells or rocks on the seafloor surface and filter food particles from seawater, thereby enhancing water quality. These shellfish are notoriously intolerant of suspended sediment and deposition, and so the discovery of their abundant dead shells within the modern seabed suggests extirpation by ‘muddification’. Radiocarbon-based dating of shells indicates that the shell-gravel ecosystem started to decline in the 1820s-30s and disappeared entirely from the mainland California shelf by the early 20^th Century – the youngest shells date to ~1910 AD. Loss of this distinctive ecosystem, which had thrived for thousands of years, was thus complete before 20^th century urban wastewater input and seawater warming. Instead, the most likely driver of decline was intense livestock grazing in the Los Angeles watershed, starting with Spanish colonization in the 1760s, which would have increased sediment runoff 10-fold. Land use shifted to cultivation in the 1870s, but sediment delivery to the ocean would only have abated with soil conservation and storm-water control in the 1940s-50s. By then, the shell-gravel ecosystem had vanished from the mainland California shelf, placing a premium on protecting remnant populations on the still-sandy seabeds of offshore Channel Islands. Widespread mud, a legacy of long-ceased human activities, is the ‘new normal’ for the mainland shelf, which has been altered, probably irreversibly, from the mosaic of seabed communities that constituted ‘natural’ until cows arrived.