Ecology and management of natural resources of the island, Nijhum Dwip in Bangladesh

Abstract

The present study has been carried out to estimate the land cover and land area change over the last forty years, determination of productivity and carrying capacity of a coastal island for management, food security and livelihood earning and physicochemical properties of soil and mineral nutrients of leaf of different mangrove species in Coastal Island namely Nijhum Dwip. Nijhum Dwip is situated in the Noakhali district in Bangladesh. The area is about 14050 ha. The islands are managed by Forest department by planting of trees in this area with species such as Sonneratia apetala Buch.-Ham (Keora), Avicennia officinalis L. (Bain) and Ceriops decandra (Griff.) Ding Hou (Goran) since 1972. Forest department introduced deer in Nijhum Dwip and other coastal islands which is fed on the Porteresia coarctata (Roxb) (locally known as Uri grass) and leaves of Sonneratia apetala Buch.-Ham.plants. This helped in the rapid buildup of new ecosystems in the coastal areas. But the rapid growth in number of deer and ban on killing them created an ecosystem service problem in this area. Carrying capacity was determined of an area of the Dwip to people exact number of deer the island can support. Soil and leaf samples were collected during four field visits i.e. 2013, 2014, 2015 and 2016 from six locations (six quadrats of 25x25 meter) of the Nijhum Dwip. Four soil samples (which contain upper and lower layers) from each quadrat i.e. total forty eight soil and four leaf samples of plants (Keora, Goran, Bain and Hargoza) from each quadrat were collected. Then these forty eight soil samples and twenty four leaf samples were analyzed in Ecology and Environment Laboratory, MS Laboratory of Department of Botany, Soil Chemistry Laboratory of Soil, Water and Environment Departments, CARS of Dhaka University and Bangladesh Council of Scientific and Industrial Research (BCSIR) laboratory. The mean value of some important soil variables such as moisture, pH, salinity, conductivity, organic carbon, nitrogen and phosphorus are 42.413 ± 5.58 %, 7.686 ± 0.53, 11.583 ± 2.203 ‰, 12.52 ± 4.28 mS/cm, 0.481 ± 0.221 %, 0.670 ± 0.118 % and 0.0130 ± 0.0154 % respectively in 2013. In 2014 and 2015, the values of moisture and salinity were in decreasing trend but organic carbon and nitrogen showed in increasing trend which represents the good condition of the island. In 2016, the mean value of moisture, pH, salinity, conductivity, organic carbon, nitrogen and phosphorus are 22.090 ± 3.808 %, 6.70 ± 1.233, 3.259 ± 2.270 ‰, 8.296 ± 2.541 mS/cm, 1.372 ± 0.480 %, 1.874 ± 0.302 % and 0.017 ± 9.01. The values of organic carbon and nitrogen in 2016 were more than that of 2013 which indicates that Nijhum Dwip is good for vegetation. The values of heavy metals of soil (Na, K, Ca, Pb, Mg, Fe, Mn, and Zn) were also determined of the samples. In 2013, the value of Na, K, Pb, Mn, Mg, Fe, Ca and Zn were 44.05 ± 11.45 μg/g, 41.84 ± 10.16μg/g, 0.493 ± 0.160 μg/g , 180.74 ± 49.02 μg/g, 229.4 ± 90.7 μg/g, 7297.8 ± 677.1 μg/g, 299.4 ± 90.9 μg/g and 188.81 ± 56.57 μg/g respectively. The value of Na, K, Pb, Mn, Mg, Fe, Ca and Zn in 2014 and 2015 showed fluctuating tendency and the mean value of Na, K, Pb, Mn, Mg, Fe, Ca and Zn in 2016 were 54.04 ± 17.97 μg/g, 24.81 ± 8.43 μg/g, 0.328 ± 0.07 μg/g, 394.3 ± 82.2 μg/g, 819.1 ± 186.9 μg/g, 10074 ± 720 μg/g, 1239.8 ± 440.1 μg/g and 325.18 ± 56.84 μg/g. All the heavy metals in 2016 showed increasing trend except K and Pb in comparison to the values of 2013. Principle component analysis (PCA) was carried out of the values of different variables of soils in different years. The mean value of different mineral nutrients of leaf samples has the other mangrove species throughout the study period. N content of leaves of four species was calculated in four sampling times. Avicennia officinalis maintain a higher amount N content in the leaves during 2013 than those of others species and the chronological order was as A. officinalis> S. apetala> C. decandra> A. ilicifolius. In C. decandra, N maintained negative significant correlation with Pb (r = - 0.918, p = 0.028), Ca (p = 0.044, r = - 0.889). In 2016 the chronological order of the N content of leaves were as A. officinalis> A. ilicifolius > C. decandra > S. apetala. N maintained very strong negative significant correlation with Mn (r = -0.906, p = 0.034) in A. ilicifolius. Highest total P content was found in leaves of S. apetala throughout the study period. In 2013, the P content of leaves was in the following chronological order: S. apetala> A. officinalis> C. decandra > A. ilicifolius and the P content of leaves during 2016 was as S. apetala> C. decandra> A. ilicifolius> A. officinalis. There were variations found in lead content in different mangrove species studied. Avicennia officinalis showed highest concentration of lead during most of the study period except 2016 when highest value was found in A. ilicifolius. Magnesium content in the leaves of A. officinalis was found to be lowest in most of the cases where the value was higher in S. apetala in all cases except 2013. In carrying capacity for this island forage demand of the particular species of livestock (here spotted deer is considered with body weight of female is considered to be 45 kg and that of male 65 kg) is about 328.5 kg/y and 474.5 kg/y for female and male spotted deer respectively. These values showed that if we plan to have only female in the herd then the number will be 2341 and 3600 in 2014 and 2015 respectively and if only male is used then the number will be 1700 and 2500 in 2015 respectively. If we use 50:50 of female and male then the number will half in either case. But in case of commercial carrying capacity, i.e. meat is the goal, and then male spotted dear with higher body weight might be preferable to the authority. The present study will provide present status of land area and land cover, carrying capacity and mineral nutrients soil and leaf of Nijhum Dwip for its better management.