Dynamic of Tridacna spp. population variability in northern SCS over past 4500 years derived from AMS ¹⁴C dating

The marine bivalve, Tridacna spp. is an iconic invertebrate of the Indo-Pacific coral reef communities from Eocene to present. However, field observations found that the population of Tridacna spp. has declined in recent decades and some species are now endangered in the northern South China Sea (SCS) of western Pacific, which are speculated to be connected with the human overfishing and/or climate changes. Thus distinguishing the impacts of human activities and climate changes on Tridacna spp. populations is essential for understanding the dynamic of Tridacna spp. population variability. Such effort will be important in launching conservation policies and restoring population. Here, extensive sampling was applied on sub-fossil Tridacna spp. shells at the North Reef of the northern SCS, and the long-lived (with a lifespan more than 30 years) Tridacna spp. population index (LTPI) over the past 4500 years was obtained based on the AMS¹⁴C dating method. The results show that LTPI has experienced several short-term collapses (shorter than 200 years) over the past 4500 years, which may be associated with excessive cold winter temperatures. Remarkably, LTPI usually recovered rapidly after the rewarming of temperatures, indicating a robust self-recruitment mechanisms in response to natural climate changes. However, the last catastrophic collapse of LTPI that occurred at around ~1820 CE - ~1900 CE didn't rebound despite the significant rise in temperature over the recent 100 years. The decoupling between LTPI and climate changes in recent hundred years was probably induced by the increased commercial fishing in the SCS, which has overwhelmed and exacerbated the self-recruitment mechanisms between Tridacna spp. population and climate changes.