Blue Carbon in Shallow Coastal Ecosystems

Abstract

Coral reefs show high organic and inorganic carbon production and create unique landforms on tropical coastlines. The balance between organic and inorganic carbon production is determined by benthic organisms such as corals, macroalgae, and seagrasses, and also by reef hydrodynamics, which in turn determine CO 2 sinks and sources within the ecosystem. Many studies have shown that net organic carbon production in coral reef ecosystems is almost zero (balanced), with net positive calcification resulting in reefs acting as CO 2 sources. However, the relationships among productivity, benthic organisms, and hydrodynamics have not been well documented; more detailed information is required from both field observations and coupled physical-biological models. Reef sediments have low organic carbon content (median, 0.35% dry weight), approximately 50% those of tropical and subtropical seagrass beds (median, 0.67%) and 5% those of mangrove forests (median, 6.3%). Sedimentation rates do not vary significantly between these three ecosystems, so organic carbon input and decomposition in the surface sediments are key factors controlling organic carbon burial rates. Coral reefs provide calm conditions that enhance sedimentation of fine sediments, which facilitates the development of seagrass beds and mangrove forests. Seagrass meadows and mangrove forests in turn trap fine sediments from terrestrial sources and prevent high-turbidity water from reaching coral reefs. Coral reefs, seagrass meadows, and mangrove forests are thus interdependent ecosystems; to effectively store and export blue carbon in tropical coastal areas, it is necessary to maintain the health of these ecosystems.