[ACG43-P05] Tempo-spatial variations in the concentration of dissolved inorganic carbon (DIC) exported in mangrove-dominated river in Ishigaki Island
Keywords:Carbon cycle, Dissolved inorganic carbon, Mangrove, Organic matter decomposition
This study was conducted in the mangrove forest around the Fukido River, Ishigaki Island. In March and August 2016, river water was sampled with one hour interval for 24 hours on six points of river within the mangrove area and one point on the mouth of river. In addition, fresh water and sea water samples were also collected from the upper rivers located on the out of mangrove area and adjacent sea, respectively. Water sampling was also conducted in January 2015 (one point of river) and August 2015 (one point of river and mouth of river). Salinity and pH were measured using electrodes. DIC concentration was determined by open-flow system using infra-red gas analyzer after gasification of DIC to the CO2 by adding the phosphoric acid to the sample.
Salinity and pH increased during the high tide, whereas DIC increased during the ebb tide in all sampling points within the mangrove area. Salinity and pH showed positive correlation with water level at each point, suggesting the inflow of seawater by high tide because sea water had higher salinity and pH than fresh water. DIC concentration also should be affected by the mixing of sea water. Then, the DIC concentrations based on the mixing ratio were calculated using the simple two component mixing model based on the salinity and DIC concentration of fresh water and sea water and they were compared to the observed DIC values. Observed DIC was higher than estimated DIC during the ebb tide and it reached to two-hold of estimated values in maximum (the difference was 1.6 mmol C L-1 for this case). These results suggest that great amount of DIC should be transported to the river water in the mangrove area but we have to consider DIC outflow as a flux by considering the water volume.
Differences in observed and estimated DIC concentration differed even in the same sampling points depending on the sampling period; they tended to be smaller when there was much precipitation before the sampling or tidal change was small (middle tide or neap tide). In addition to the seasonal changes in the factors directly affecting the microbial decomposition (e.g., temperature), temporal changes in hydrological regime (influx of freshwater from the upper rivers or inflow of seawater by tide) would be important in estimating the long-term DIC efflux.
DIC concentration was spatially different even in the points with similar distance from the mouth of river. In addition to the difference in the water volume, DIC production rate (decomposition rate) or transportation rate to the river water maybe differed among the small water sheds within the mangrove area. As future perspectives, clarifying the decomposition activity in the mangrove sediment and its regulation factors and detail mechanisms of DIC transportation from the sediment to the river water are needed.