Variability of skeletal growth and δ13C in massive corals from the South China Sea: Effects of photosynthesis, respiration and human activities

The δ13C, skeletal density and extension rates were analyzed along same growth axes of massive coral colonies collected from the shallow sea to the east and south of Hainan Island, northern South China Sea. Seasonal changes of δ13C, extension and calcification rates of Porites lutea and Porites lobata are in phase and correlated with solar radiation. In addition, annual high δ13C values, extension and calcification rates occur in spring and low values and rates occur in late autumn or early winter. The annual highest density band of P. lutea formed in spring and the lowest ones in early winter, whereas P. lobata shows only a weak seasonal density change. We suggest that the seasonal features of the coral δ13C, extension and calcification rates are modulated by light-induced photosynthesis determined by solar radiation reaching the top of the atmosphere, and by cloud coverage. The results also support the suggestion that photosynthesis promotes both the extension growth and skeleton thickening for P. lutea, but that it mainly promotes the extension growth for P. lobata. Remarkable inter-decadal trends are evident in the coral records; these trends are progressive decreases of skeletal δ13C, density and calcification rate, and an increase of the extension rate over the last 30 years, which are marked by abrupt changes at about 1987. These trends are ascribed to intensified coral respiration arising from rising temperature, together with the effects of declining nutrient levels caused by the destruction of the coastal ecosystem in the sampling regions.