Skeletal Growth Response of PoritesCoral to Long‐Term Ocean Warming and Acidification in the South China Sea

Identifying the response of corals to anthropogenic ocean warming and acidification remains challenging, but is critical to understanding coral resilience in a changing world. Here, we report composite records of skeletal growth (1847–2014) of nine Poritesspp. colonies from the open ocean of the northern South China Sea (SCS), and paired reconstructions of sea surface temperature (SST) and seawater pH (pHsw) by examining skeletal Sr/Ca and δ11B from two of the cores. The reconstructions demonstrate that the open ocean of the northern SCS has experienced significant warming since the 1960s with an overall temperature elevation of ∼0.50°C, and persistent acidification since the 1920s with a decline rate of −0.0102 ± 0.0017 pH units per decade. Under such environmental changes, coral skeletal density represents a long‐term mild decline for the past 168 years, demonstrating significant responses to pHswand SST variation; whereas coral extension and calcification appear to be less affected, exhibiting overall increases over the studied period. However, persistent declines of both parameters are evident from the 1960s to 2014, which coincide with the sustained low pHswand elevated SST. Although the general response of coral calcification (mainly dominated by upward growth) to both warming and acidification falls just short of statistical significance over the decadal timescales, the significantly persistent decline in skeletal density and recent downtrends of calcification still underline a pessimistic situation for corals in the northern SCS. Coral reefs are particularly susceptible to climate change, but our understanding of how corals cope with it remains insufficient. Here we explore the responses of coral calcification to climate changes, by comparing paired reconstructions of coral growth and seawater environment variations. To this end, we collected nine Poritescorals from the Xisha Islands in the open ocean of the northern South China Sea (SCS) and examined their skeletal density and growth rate. Meanwhile, we adopted two of the drillcores to generate sea surface temperature and pH reconstructions from skeletal Sr/Ca and δ11B. The reconstructions show that the open ocean of northern SCS has experienced long‐term pH decline from the 1920s and significant warming since the 1960s. Under such environmental changes, we find that coral skeletal density is more susceptible to seawater pH and temperature variation, showing a long‐term decline for the past 168 years. However, it appears that coral upward growth bears certain resistance to environmental changes, showing an overall increase over the studied period. Nevertheless, during the recent decades characterized by sustained low pHswand elevated SST, we find that all the calcification parameters have begun to decline persistently, signifying a pessimistic situation for the corals in the northern SCS. Evident ocean warming and acidification over the past century in the northern South China Sea have been registered in PoritescoralsEnvironmental changes have led to a long‐term reduction in skeletal density of Poritesduring the past 168 yearsAll the calcification parameters of Poritesexhibit significant persistent declines from the 1960s Evident ocean warming and acidification over the past century in the northern South China Sea have been registered in Poritescorals Environmental changes have led to a long‐term reduction in skeletal density of Poritesduring the past 168 years All the calcification parameters of Poritesexhibit significant persistent declines from the 1960s