Dissolved Organic Matter in Coral Reefs: Distribution, Production, and Bacterial Consumption

Dissolved organic matter (DOM) constitutes the largest organic matter pool in coral reef waters and is released and utilized by various coral reef organisms. In this chapter, we review the distribution and fluctuation of DOM concentrations in coral reefs around the world, with a special focus on Shiraho Reef, Ishigaki Island, Okinawa, Japan, where DOM fluxes have been studied most intensively since the late 1990s. Then, we review the DOM production rates from specific reef organisms and DOM consumption rates by bacteria. Previous studies have shown that both dissolved organic carbon and nitrogen (DOC and DON, respectively) generally have a higher concentration in most coral reefs than in the surrounding ocean. At Shiraho Reef, the average ratio of the net DOC production to the net primary production on the reef flat was 18%, and the C:N ratio of DOM that was produced on the reef flat was estimated to be 9.3. The abundance of heterotrophic bacteria was also higher in most coral reefs than offshore, which indicates that bacterial growth was enhanced by reef-derived DOM. Some of the DOC that was produced in coral reefs was persistent to bacterial decomposition in the long term, which suggests that coral reef ecosystems export some reef-derived DOM to the ambient ocean, irrespective of the water residence time in the reef.