Behavior of Dissolved Organic Matter in Coral Reef Waters in Relation with Biological Processes

Abstract

Behavior of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in coral reef waters in relation with biological processes was studied with incubation experiments and field observations in May 2008 and 2009 at the fringing reef of Sesoko Island, Okinawa, Japan. Reef sea water (RSW) and coral mucus added RSW collected from Acropora digitifera (AcrRSW) and Montipora digitata (MonRSW) were incubated for one day in situ and then for 77 days in the laboratory under dark condition. The results indicated that the behavior of DON was different compared to that of DOC in RSW and mucus added (AcrRSW and MonRSW) during dark incubation. Concentration of DON increased from 8.3 µM to 11.8 µM for AcrRSW and 4.0 µM to 15.4 µM for MonRSW during dark incubation period. The increasing rates for DON in AcrSRW and MonRSW were 0.05 µM day-1 and 0.1 µM day-1 respectively. On the other hand DOC concentration decreased from 129.0 µM to 75.0 µM for AcrRSW and 75.1 µM to 64.7 µM for MonRSW, with decreasing rates of 0.7 µM day-1 and 0.1 µM day-1 respectively. We assume that the increase of DON may be determined by difference between rates of inputs of organic matter mainly from mucus and rate of degradation of dissolved organic matter in the water column. These results suggest that recycling of DON is slow than that of DOC in coral reef ecosystem.