COASTAL ENVIRONMENTAL CONDITIONS DURING THE 20TH CENTURY INDICATED BY VARIATIONS OF SKELETAL PARAMETERS OF A PORITES LUTEA CORAL FROM EAST HAINAN, SOUTH CHINA SEA

Anthropogenic sources of CO₂, sediments, and nutrients have been identified as threat to coral reefs worldwide by changing the coastal physical and chemical conditions. Important information about historical environmental changes in a particular reef is recorded in the skeletal material of its massive reef building corals. The reconstruction of anthropogenic influences and natural disturbances, their impacts and interactions may reveal insights into possible future developments of the reef. This work focuses on the northeast coast of Hainan Island in the South China Sea, P.R. China. There, environmental conditions possibly have been modified by an altered land-use and a changing climate since many decades leading to present reef degradation. The skeleton of a massive Porites lutea coral colony has been sampled in form of a ‘drilling core’ of about 1 m length that revealed a data set completely covering the 20^th century. Basing on the annual banding pattern, skeletal growth characteristics over 110 years have been analysed along one corallite. Nearly 1000 geochemical subsamples, for each the ratios of δ¹⁸O, δ¹³C, and Sr/Ca, have been measured. The results indicate that the cumulative effects of the climate change (e.g. ocean warming and acidification, feed availability) favored the P. lutea colony until sampling (2007). The temperature rise calculated from the Sr/Ca ratios seems to have increased linear growth rates, particularly during ENSO events. An enhanced symbionts’ photosynthesis can be concluded from the δ¹³C ratios. The salinity reconstruction basing on the δ¹⁸O ratios argues for a global warming-driven upwelling intensification, potentially supporting coral nutrition and growth, thereby buffering against acidification effects. Thus, the observed decline of the local reef communities seems to be caused mainly by impacts from the land-use change and deforestation (e.g. nutrient and sediment input) and possibly over-fishing. Even though P. lutea corals cope with the stressful conditions for now, density and calcification seem to be adversely affected, reducing the resistance capacity of these key reef-builders. If the land-based disturbances continue, they urgently endanger the coral reefs of Hainan before climate change impacts worsen.