A paleoecological reconstruction of oysters from Orange County, California: Understanding the past to help restore the future

The only native oyster, the Olympia oyster Ostrea lurida off of the coast of southern Califomiais on the brink of extinction. Presently restoration projects, run by California State University, Fullerton, aim to repopulate this species in southern California. To aid restoration, it is imperative to know the baseline community structure of oyster reefs before human disturbance. The modern history of oysters has been well documented by biologists, however the paleontological history is less well known and many questions pertaining to past southern California oyster communities remain unanswered such as: was Ostrea lurida always the only native species off of the coast, or have multiple species lived here in the past? How did oyster size change through time? How did diversity and abundance of the oyster communities change through time? This study aimed to answer these questions to provide biologists with information on how oysters and their communities have changed through time, which will benefit successful oyster reef restoration. This study encompassed two parts: 1) paleoecological analysis of Holocene oyster reefs in Newport Bay, and, 2) an analysis of museum-based oyster specimens through time. The Holocene analysis entailed collecting new specimen data. Radiocarbon dating was performed on shells and charcoal from Newport Bay to confirm that the fossil oyster reefs from each side of the bay were the same age, The museum research involved identifying oyster fossils that were collected from various housing development sites throughout southern California. All specimens, both Holocene and museum, were used to build a database that tracked diversity, abundance, and oyster size over 6 time intervals within the last 90 million years. All oyster specimens from Newport Bay and the museum were measured to analyze their length, width, and thickness changes through time.;Results indicate that oysters are generally larger throughout the Cenozoic, peaking in size during the Miocene, and decrease in size beginning in the Quaternary through the Holocene. There is evidence of multiple native species of oysters off of the coast since the Late Cretaceous. Diversity follows a similar pattern, that is, 4 species existed in the Late Cretaceous, 3 species within the Eocene, 4 species within the Miocene, 1 species in Pliocene, 2 species in the Pleistocene, and 1 species in the Holocene. Ostrea lurida did not appear in the fossil record until the Pleistocene and continues to occur in the Holocene through the present. Thus, oyster diversity declines through time. Oysters are overall becoming less diverse, less abundant, and smaller through time.;Unfortunately, abundance patterns are hard to assess due to poor sampling quality. However the Pleistocene and Holocene intervals included ample samples for community analysis. When these 2 intervals are compared, the Pleistocene exhibits higher diversity, oyster abundance, and over-all higher ecological abundance than the Holocene. This might indicate that greater oyster diversity and abundance lead to greater biodiversity overall; however, more data is needed to support this claim.