Difference in Water CO₂ Dynamics between Riverine and Fringe Mangroves–A Case Study in Subtropical Mangroves

Abstract

Mangrove creeks adjoining typical riverine mangrove forests is known to be a net source of CO2 and dissolved inorganic carbon (DIC) to the ocean due to the decomposition of rich organic carbon in the soil; such information is limited in fringe mangroves. This study aims to clarify the difference in carbonate processes between the two types of mangrove forests. We conducted continuous monitoring of the partial pressure of carbon dioxide (pCO2) and measured carbonate chemistry parameters (total alkalinity (TA) and DIC) in two types of mangrove areas located in Okinawa, Japan. The result showed that the maximum pCO2 reached 5242 µatm in the Fukido River estuary (a riverine mangrove), whereas it was only 765 µatm on the Yubu coast (a fringe mangrove). The measured maximum TA and DIC values were 3285 µM and 3283 µM in the Fukido River estuary and 3162 µM and 2977 µM on the Yubu coast. These indicate that pCO2 on the Yubu coast was maintained at low values by the carbonate buffering capacity even when TA and DIC increased rapidly. The mangroves on the Yubu coast grow on dead corals and coral soils. It was suggested that the DIC/TA ratio was constantly kept at approximately 0.9 due to TA production by the dissolution of calcium carbonate (CaCO3), which resulted in lower pCO2.