Seasonal and Spatial Variations of Total Mass Flux around Coral Reefs in the Southern Ryukyus, Japan

Abstract

Total mass flux, size distribution of sediment particles and some chemical compo- nents such as total carbon (TC), total nitrogen (TN) and calcium carbonate (CaCO 3 ) were monitored monthly using a multi-cup sediment traps at seven coral reef sites (6 reef flat and 1 reef slope) of the Marine Protected Areas around Ishigaki, Kohama, Kuroshima and Iriomote Islands in the southern Ryukyus, Japan from September 2000 to September 2001. The size distribution of trapped sediments revealed mostly uni-modal fine sand to mud in the reef flat and gravelly to coarse sand in the reef slope. The total mass flux ranged between 0.54 to 872 gm -2 d -1 , and showed a pro- nounced seasonality (high in summer-autumn and low in spring) at each site, which was consistent with the rainfall and typhoon regime. Exceptionally high values were observed on the reef slope (Iriomote) in February-March 2001 (1533 gm -2 d -1 ) owing to a large amount of bottom sediment re-suspension. On the reef flat (Todoroki South and North; Ishigaki), values obtained in July-August 2001 (872 gm -2 d -1 ) and Au- gust-September 2001 (800 gm -2 d -1 ) indicate the high terrestrial discharge from Todoroki River. Trapped sediment particles consist of CaCO 3 (1.2-27.1%) and a non- carbonate fraction (98.8-72.9%), which contains total carbon (4.9-26%), carbonate carbon (CO 2 -C) (0.2-3.1%) and non-carbonate carbon (NC-C) (7.9-25.6%). Total nitrogen content was in the range 0.02-0.48%. TN is contained mainly in the carbon- ate fraction and NC-C may be contained in the non-carbonate fraction. The low TN/ OC ratio of the trapped sediments suggests that they were mostly of terrestrial origin and that both fractions migrated. The high total mass flux derived from Todoroki River exceeded the threshold at which a lethal effect on coral community is caused. The results stress the importance of conducting seasonal studies of sedimentation over more than one year and at more than one location in south Japan coral reef ecosystems to gain an understanding of the processes controlling the total mass fluxes and their nutrients content, also to develop an awareness of how to prevent the dam- age of coral reef ecosystems and, if it does occur, to allow mitigation measures to be undertaken.