Potential impacts of land use change dynamics and submarine groundwater discharge on fringing reefs of Kuroshima Island, Japan

The land based sources of pollution are major threat to the reef ecosystems; therefore, it is important to ascertain the temporal and spatial change pattern of coral cover at local scale to understand the causal links between reef degradation and ground discharge impact with changing land use practices. The land use change dynamics and its impacts on surrounding reefs were investigated using aerial photographs of 1977, 1994 and 2005 of Kuroshima Island, Japan by integrating remote sensing and GIS (Geographic Information System) approach. And the impact of groundwater associated nutrient discharge was estimated using Rn-222(radon) as an SGD (submarine groundwater discharge) tracer. Findings indicate that the land used for grazing has increased (597%) during the study period and most of this increase has been at the expense of forest and shrub land. The landscape development intensity (LDI) index, was used to evaluate the impact of land driven activities on coral assemblages. LDI results infer that north-eastern watershed (2.84), south-eastern watershed (3.02) and north-western watersheds (2.75 to 2.88) are most critical watersheds with higher LDI in comparison to other watersheds due to the enormous increase in pasture land. The benthic cover change analysis indicates the overgrowth of seagrass (99.5%) and microalgal productivity (99.6%) especially in the northern, western and southeastern part of the Island during 1977 to 2005. The groundwater discharge with high concentration of land-derived nutrients was observed as the main source for the reduction in coral cover around Kuroshima island.