Dynamics of dissolved organic matter in the mangrove area of the Fukido River, Ishigaki Island

Abstract

Mangrove forests are tropical forests with carbon-rich soil. Dissolved organic matter (DOM) is important for investigating the mangrove carbon cycle and ecological roles because the forests connect the river tributaries with the coast via rivers running through them. The ecological functions and turnover rate of DOM depend on its composition. Therefore, we characterized the DOM of the Fukido River in a mangrove forest by analyzing the percentage of humic substances (%HS) in DOM. Water sampling from the headwaters to the sea revealed that %HS declined going toward the sea due to mixing with low-%HS seawater and the possible selective precipitation of HS into mangrove soil. The headwaters had a higher %HS than most clear water rivers and may be an important supply of HS for mangrove and coastal areas. The sequential extraction of water-extractable organic matter from mangrove soil (sampling depth: 0–25 cm) using ultrapure water indicated a significant increase in HS concentration along with a decline in salinity. These results suggest that high salinity is responsible for the selective accumulation of HS in mangrove soil, which can be a key to elucidating the mechanism of organic matter accumulation in mangrove soils.