Negative Contribution of Groundwater Input from Subsurface Spring to Coral Reef Bleaching

The bleaching in 1997 and 1998 left the largest damage to the coral reefs in Okinawa Islands, Japan. Among these coral reefs, some corals were recovered to 2000 and others were completely damaged and no further recovery is expected. The environmental changes, forced those damages, are still under discussion among the increase of water temperature, increase of sediments or turbidity, and other associated human activities. In this study, we discuss a possibility of groundwater input from subsurface springs along the coast line, which is contributing to the environment of coral reefs. As a capacity of groundwater is considered to be estimated by the amount of woods or forests over the region, the normalized vegetation index (NDVI) which is given as the contrast index between near infrared and red band may suggest the amount of water supply through the subsurface springs near the coast to the coral reef. To understand the historical disaster of the coral reefs due to the change of land activities, the temporal changes of NDVI along the coast are mapped to monitor a change of forests with considering the changes of underground water supply through subsurface spring along the coast line. Also, coral reefs are mapped with a classification being alive, recovered, and bleached. In-situ measurements were conducted over the coral reefs, where alive, recovered and bleached with CTD profiling, water sampling and analysis. From our in-situ measurements, the salinity and temperature, exhibited the clear difference over the coral reefs alive or bleached, could be the first parameters to determine the environment of coral reefs to keep in alive or bleached. The nutrients including nitrate, phosphate, and silicate could be the parameters to determine the environment, but it was difficult to discuss their contributions under the current measurements. We discuss a possible protocol to monitor the groundwater supply through the submersible layer and their contribution to the environment of coral reefs.