A nineteenth- and twentieth-century reproductive regime shift in benthic foraminifera from the Santa Barbara Basin, California

Long-term records that span the past several centuries and capture within-population variation are critical for distinguishing ephemeral ecosystem changes from regime shifts. Using an approximately 2 kyr record of reproductive life history from the central Santa Barbara Basin, we examined population trends in reproductive mode and accumulation rate (i.e. reproductive output) across four species in the biserial benthic foraminiferan genus <i>Bolivina. Bolivina</i> populations were consistently dominated by asexually produced individuals until the mid-nineteenth century, after which they exhibit an increase in variance and a decrease in the mean proportion of asexually produced individuals. At the same time, they underwent an order-of-magnitude decline in accumulation rate. The magnitude and persistence of these changes suggest that the nineteenth and twentieth centuries represent a life-history regime shift. The compounding effects of anthropogenic impacts and long-term trends in the California Current System (such as heightened deoxygenation and altered sedimentation regimes) may have pushed the Santa Barbara Basin towards increased investment in sexual reproduction.