The Nutrient and Carbon Dynamics that Mutually Benefit Coral and Seagrass in Mixed Habitats under the Influence of Groundwater at Bise Coral Reef, Okinawa, Japan

Abstract

The coral species, Montipora digitata and seagrass, Thalassia hemprichii , co-inhabit the southern portion of the reef moat in Bise, Okinawa, Japan. To elucidate the biogeochemical relationship between coral and seagrass in mixed communities of the coral reef ecosystem, the carbon metabolisms and the inorganic nitrogen flux rates were estimated in various reef habitats. We used benthic chambers to investigate sandy, seagrass, coral-seagrass mixed communities, coral, and acorn worm habitats. Relatively high concentrations of nitrate and nitrite ions (NO x ) were observed in all habitats due to coastal groundwater inflow. The uptake rate constant of NO x was the highest in the coral-seagrass habitat and was significantly different from the rate constant in the seagrass habitat, indicating that seagrass benefits from co-inhabitation with coral. Dissolution of CaCO 3 was observed in the seagrass and coral-seagrass communities. This decline in basal coral carbonate substrate may contribute to increased fragmentation and dispersal of the coral habitat. On a biogeochemical scale, the coral-seagrass relationship benefits the seagrass in terms of NO x availability and benefits the coral in terms of carbonate dissolution, increasing fragmentation, and furthering habitat development.