Spatial and temporal variations of organic matter contents and potential sediment nutrient index in the Sundarbans mangrove forest, Bangladesh

This paper has investigated the total organic carbon contents together with total nitrogen, total phosphorous, electrical conductivity and grain size parameters of the surface sediments of the Sundarbans mangrove forest in Bangladesh. The results showed that the seasonal variations of Total Organic Carbon (TOC) and Total Nitrogen (TN) were significant at all sampling points. TOC and TN content at the monsoon were quite higher than that of the winter season. TOC and TN also showed strong positive correlations between them and negatively correlated with Total Phosphorus (TP). Depending on the distributions of TOC and TN, the studied forest ranges could be arranged at the following order: Sarankhola and Chandpai ranges > Khulna range > Satkhira range. In contrast, TP content showed the reverse order of TOC and TN, where the higher concentrations were generally distributed at the southern parts of Sundarbans. Besides, TOC and TN showed linear positive correlations with grain size distribution where fine grained sediments contain higher amount of TOC and TN. However, in terms of grain size variation, TP reversed the results of TOC and TN. Thus “grain size effect” was an important factor for influencing the distributions of sediment nutrients. Carbon-Nitrogen (C/N) ratios showed the highest contribution of terrestrial organic carbon near the Sarankhola and Chandpai ranges were iso-lines projecting towards northeast, indicating the influence of the upstream river water. Sediment Nutrient Indexes (SNI) were low, although reasonably variable, in forest floor directly receiving effluents from other anthropogenic sources/farms. It was evident that potential nutrient levels changed from the high values at northeast sites to the southwest sites, and this pattern was common across the four forest ranges.