Scleractinian corals play important roles in the biogeochemical cycles of the coral reef ecosystem through coral metabolic activities. In particular, the cycling of dissolved organic matter (DOM) in coral reefs has often been focused on in recent years because of the improvement of the DOM analytical technique. This article summarizes recent findings on DOM in coral reefs and compares the chemical characteristics between DOM released from coral colonies and DOM produced on ecosystem scales. The present review shows that the carbon to nitrogen ratios (C:N ratios) of DOM are in a similar range between coral- and ecosystem-scales in Shiraho Reef, Japan, indicating that the coral colonies might be one of the major DOM producers. The efficient nitrogen recycling in coral-algal symbiotic colonies has also been demonstrated in recent years and this explains why corals can release nitrogen-rich DOM, even though the ecosystem is filled with oligotrophic seawater. A series of these biogeochemical and ecophysiological studies would provide a better understanding of the mechanisms which have been maintaining coral reef ecosystems.

The characteristics of near-inertial internal waves (NIIWs) induced by the passage of a low-pressure system on October 2007 were investigated using current data obtained from acoustic Doppler current profilers mounted on the seafloor of three mooring sites northeast of the Noto Peninsula, Japan. The observed near-inertial fluctuations showed a characteristic of NIIWs. We found that current fluctuations associated with the NIIWs were almost in-phase at three stations, and its amplitudes were larger in the middle layer (34-58 m depth) than in the surface layer on the continental slope. Because it was difficult to explain the characteristics of the observed NIIWs in terms of the elementary solution of internal wave theoretically, we performed a numerical experiment using a three dimensional level model that adopted realistic bottom topography and wind stress interpolated from Grid Point Value data (Japan Meteorological Agency). The model had a good reproduction of the amplification of NIIWs energy at the middle layer. Thus, we investigated the formation mechanism of strong and in-phase current fluctuations by a detailed analysis of the model results. The NIIWs, generated in the offshore region, propagated southward and reached the coastal area and Noto Peninsula. Scattering of the NIIWs occurred at the shallow region around the Peninsula, and the scattering waves were simultaneously induced and interfered around the mooring sites. To investigate the effect that the bottom topography contributed to the interference of the scattering waves, we performed two types of verification experiments in which shallow regions near the Noto Peninsula were deleted. These experimental results showed that the scattering waves that occurred at the continental shelf off the eastern coast of Noto Peninsula were important for the amplification of NIIWs at the middle layer, and the interference by the superposition of scattering waves from different regions around the mooring sites involved the formation of in-phase current fluctuations.