Stable isotope study on nitrogen and sulfur impact to coral ecosystem of Yoron, Japan

Abstract

In Yoron Island, cultivation of Sugarcane makes up 52 % of the agriculture on the island, and it comes with extensive use of chemical fertilizers with ammonium sulfate. Moreover, as the main island is geographically formed from karst by the raised coral reef, precipitation smoothly flows to the sea through the underground. In this process, sulfate is brought to the sea as well. It is believed that potential hazards are then caused to the coral reef ecosystem. Considering this background, we aimed to evaluate the influence of sulfate on coral reef by analyzing the stable isotopes ratio of nitrogen ( δ 15 N) and sulfur ( δ 34 S). Since the δ 34 S of sulfate is 0 ‰ , the same value would also be shown in coral skeleton if there were significant effect of inflows. Our result reveals that δ 15 N value in coral skeleton is similar to that of the chemical fertilizer. A contrary, δ 34 S value obtained did not show similarity with that of the sulfate. However, from the analysis of the δ 34 S in growth ring, we witness a decrease of 2 ‰ to 5 ‰ from 1979 to 2013 and from 1990 to 2013. The conclusive result is that the δ 34 S value in coral skeleton is not significantly affected by that of the value of sulfate fertilizer. Nonetheless, the consistent decrease of δ 34 S from 1979 to 2013 may possibly be related to the increase of inflow of land-substance with low δ 34 S value to the sea.