Moritaka Shimo

Characteristics of Surface Sediments along a Creek in a Mangrove Forest

The mangrove habitat is referred to as an advancing coast, where the land advances toward the sea due to soil sedimentation. In the present study, physico-chemical and mineralogical characteristics of surface sediments in a mangrove forest were investigated to evaluate the mechanism of accumulation of surface sediments from the viewpoint of colloid chemistry. A mangrove forest in the Fukido River basin, Ishigaki Island was selected as the study site. Six locations along the river in the forest were selected, and samples of surface sediments (0.5 cm thick) were collected at 4 points (a point on the water side of the bank directly in the river water (hereafter referred to as “0 m from the river water”), a point on the bank at a distance of 0.5 m from the river water and points at distances of 5.5 m and 10.5 m into the forest from the river) of each location (total 24 points). The soil texture of the surface sediments considered of HC at nearly all the sampling points, and more clay tend to accumulate on the water side of the bank of about 200 m upstream from the downstream end of the study area. The silt fraction of the surface sediments contained quartz at a rate of about 70% along with feldspar and a small percentage of cristobalite. In the clay fraction, the contents of kaolinite and illite were slightly higher than the contents of vermiculite and vermiculite-chlorite intergrade. The mineral composition was similar at all the points, suggesting that the minerals were carried from the upper reaches of the river and were deposited in the study area. Both the pH and EC values of the surface sediments tended to decrease toward the upper reaches of the river and to be higher on the bank than at the points farther into the forest. The total carbon content, organic carbon content, total nitrogen content and total phosphate content tended to be higher on the bank at about 200 m upstream from the low end of the study area, indicating a larger accumulation of organic matter at this site. High clay and high organic matter contents were observed on the bank at this location. From the above evidence, it was suggested that organo-mineral complexes were likely to accumulate in mangrove surface sediments.