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Spatial and Temporal Variations of Organic Matter Contents and Potential Sediment Nutrient Index in the Sundarbans Mangrove Forest, Bangladesh

Gazi Mosharof Hossain*
Associate Professor, Dept. of Botany, Jahangirnagar University, Savar, Dhaka 1342, Bangladesh

Mohammad Amir Hossain Bhuiyan**
Associate Professor, Dept. of Environmental Sciences, Jahangirnagar University, Dhaka-1342, Bangladesh

Abstract/ Executive Summary:

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 isolines 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.

Keywords: Organic carbon, Sediment nutrient index, Sundarbans, Mangrove forest, Bangladesh

Location:

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Manures, Sediment

Objectives:

To (i) study the seasonal variability of total organic carbon and potential sediment nutrient content and (ii) the spatial distribution of total organic carbon and potential sediment nutrients in different ranges of the Sundarbans mangrove forest area. Sediment nutrient data obtained here would be come an important baseline data for nutrient cycle for further research.

Methodology:

Geographical Location Geographically, the Bangladesh Sundarbans lies between21°31'N and 22°30'N latitudes and 89°01'E and 90°18'Elongitudes in a relatively flat deltaic swamp with an average elevation of 0.9 to 2.1 meters above mean-sea level(Bird, 1967). A total of 62% forest situated within the territory of the southwest part of Bangladesh, while the remaining 38% is in the West Bengal of India (Siddiqi, 2001). 3.1 Sampling and Analyses of Sediments For the present research 32 representative sample sites were selected on the basis of their spatial distribution,forest types, saline zones and other ecological variability. A total of ninety six sediment samples of the three seasons were collected using stainless steel sediment collector at a depth of 5cm from the surface of 32 different locations of the four major forest ranges (viz., Chandpai, Khulna, Sarankhola and Satkhira ranges) of the Sundarbans, Bangladesh. In order to investigate temporal effects on sediment quality,sediment samples were collected on three different seasons:winter or dry (November-January), pre-monsoon or summer (March-May) and monsoon or rainy (July-September) seasons from 2010 to 2012. Geographical coordinates of sample locations were recorded using a GPS device and processed for spatial analysis. Sediment samples were collected and preserved according to the guidelines of Simpson et al. (2005). After labeling samples were carefully stored in ice-filled containers and transported to the laboratory for elemental (Grain size, TN,TP and TOC) analysis. Electrical Conductivity (EC) was measured using EC-meter following the methods of SRDI (2012).Sediment grain size analysis was conducted using sieve method following the procedures of Gee and Bauder (1986). TN was analyzed using Kjeldahl method after Jackson (1962). TP was determined by Olsen’s method after Olsen et al. (1954). TOC was determination by chromic acid wet oxidation method following the procedures of Walkley and Black (1934).3.2 Statistical Analyses Sediments quality data were statistically analyzed using SPSS software (version 16.0). One-way ANOVA (DMRT) was used to test for significant differences (P< 0.05) of marginal means of sediment quality among the variables. Factor Analysis (FA) was applied on the data set to examine the possibility for developing a multivariate index for potential sediment nutrient associations (Field, 2009). The potential Sediment Nutrient Index (SNI) was calculated following the formula (Bui et al., 2013):SNI = aCTN + bCTP + cCTOC (1) where a, b, and c were the coefficient scores derived from the first component of FA; and CTN, CTP and CTOC were concentrations of TN, TP and TOC, respectively. Principal Component (PC) analysis was used for clustering nutrient quality considering the sampling sites. Ward’s method was used for clustering variables where Squared Euclidean distance was chosen for distance measurement between clusters (Palma et al., 2010). ArcGIS (version 10, ESRI) was applied for determine the spatial distribution of the total carbon content and potential sediment nutrients including SNI of the Sundarbans mangrove forest in Bangladesh.

Source of Information: KSCE Journal of Civil Engineering (2016) 20(1):163-174

Article Link (Online Journal): DOI 10.1007/s12205-015-0333-0

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

The assessments of total organic carbon content and potential sediment nutrient levels at mangrove forest floor of Sundarbans in Bangladesh were carried out in this study. The results showed that the estimated mean sediment organic carbon and total nitrogen in the northeastern Sundarbans were 11.30 mg/g and0.65 mg/g, respectively. 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. Hence, depending on the distributions of TOC and TN, the studied forest ranges could be exhibited the following order: Sarankhola and Chandpai ranges> Khulna range > Satkhira range. The southwestern part showed relatively lower values of TOC and TN but higher values of total phosphorus and salinity. The total organic carbon at the southwestern part was comparatively low probably because of least human activities. These results were clearly demonstrated from the cluster analysis where all sampling sites were divided into two major clusters and correlated with southeast and northeast regions. Carbon and nitrogen budget in the Sundarbans mangrove soils were mostly influenced by biological (vegetation types and density), chemical(salinity) and physical processes (waves, tides, erosion etc). Some anthropogenic factors also influenced the contribution of additional carbon and nitrogen (industrialization and nature of livelihood etc.) at the northeast part of this ecosystem. However, the elevated amount of soil nutrients (TOC and TN) in the Sundarbans is a clear signature of forest growth. However,TOC, TN, TP and C/N are closely associated with grain sized is tribution where fine grained soils contain higher amount of carbon and nitrogen. Conversely, large grained sediment in the coastal sites contain higher amount of TP and salinity. Low SNI value indicated the low nutrients value or oligotrophic status of the Sundarbans mangrove forest sediments. Hence, it was evident that potential nutrient levels changed from the high values at northeast sites to the southwest sites. This pattern was common across the four forest ranges which could provide essential information relating to forest growth and health as well. The assessment of sediment nutrient levels could provide essential information relating to environmental impacts over longer periods of time because changes of sediment conditions in natural aquatic ecosystems generally take much longer time than overlaying waters. The SNI developed in this study summarized neatly the biogeochemistry of surface sediments of mangroves and it could be used for the assessment and monitoring of sediment quality in the mangrove forest in Bangladesh. The use of GIS to monitor and present organic carbon content and potential sediment nutrient data, especially SNI, is essential for managers to manages wamp ecosystem and other anthropogenic activities surrounding the Sundarbans. Findings of this study, however, may further attempt to examine nutrient status and its effects on productivity.

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