Effects of ecological factors on the growth of Sonneratia Apetala Buch.-Hum. in Rangabali coastal zone of Bangladesh

Abstract

A segment of the coastal zone of Bangladesh at Rangabali Upazilla, Patuakhali district was investigated to determine the growth of Sonneratia apetala Buch.-Ham. in relation to ecological factors to know whether the areas is under ecological risk or not. Vegetation cover, landscape dynamics and estuarine water quality were also studied. The dynamics of the landscape of the study areas and vegetation cover were estimated using satellite images (from 1989 to 2017). Water samples were collected from three locations during several sampling occasions and four replicates of water samples were collected from each location. For plant and soil samples collection, three chars were selected namely Char Motherbunia, Char Taposhi and Char Kashem on the basis of three different hydrological regimes. Plant height and diameter at breast height (dbh) were measured during sampling occasions. Biochemical and anatomical adaptation of S. apetala were also studied. Although all the three chars showed a decreasing tendency in land area from 1989 to 1991, there were different patterns in the three chars. The area of Char Motherbunia gradually increased from 3006.8 ha (1991 ) to 8305.8 ha in 2017. As a consequence the mangrove covers also increased. Char Taposhi showed a fluctuating tendency. The mangrove vegetation covers also changed in a fluctuating manner. The land area of the Char Kashem showed a gradual decrease due to severe erosion of the char. As a result the vegetation cover also decreased except in 2010. The water of the estuary was slightly alkaline with a decreasing tendency in pH. Slight acidifications of the studied estuarine waters observed in all the three locations till 2016 might be attributed to the rise in atmospheric CO2 content due to climate changes. Salinity showed temporal and spatial variations among the locations. The salinity of Location 1 was nil since May 2005 whereas Location 2 showed temporal variation and was nil in May 2017. Temporal variations were also observed in Location 3 where lowest value 0.62‰ was also observed in May 2017. Two replicates of this location also showed no salinity. Fresh water supply from upstream reduced the intrusion of the sea water on the study area. Dissolved oxygen (DO)also showed similar patterns of temporal and spatial variations where DO values (>5.0 mg/l )were above pollution level. The Pb, Mn and Fe contents of the study estuarine water were below detection level indicating that the areas are still free from these heavy metals pollution. Soils of three chars were found less heterogeneous especially during each sampling occasions with few exceptions such as iron (Fe )showed high variability during all sampling occasions. The Na and K contents of soil also showed very high variability during 2016. The overall values of pH during study times ranged from 6.0 to 7.9 indicating that soils of all three chars were slightly acidic to slightly alkaline. Salinity showed overall variation of 0.65 (in 2016) to 15.42 ‰ (in 2010). Both layers of soils of Char Motherbunia showed low salt content indicating that the soils of this char were non-saline which might be attributed to flashing of the char with freshwater of the nearby river. There were gradual decreases in soil salinity of three chars till 2016 when all three chars were in non-saline condition but in 2017 Char Taposhi and Char Kashem showed saline condition. Organic matter (OM ) content varied between 0.335 and 1.187% where both the minimum and maximum values were found during 2016. A gradual decrease in OM content in the upper layer soil was found in Char Kashem. Sudden rise in OM content in the soils of other two chars during 2016 were noticed. In the soils of the three chars, Pb was found in some soil samples. In Char Motherbunia the values ranged from 15 – 31 µg/g in 2009 and 11 to 24 µg/g in 2010. Lead was absent in Char Kashem during 2017 except in one sample. Enrichment factor for Mn was very high during 2017 in upper layers of Char Motherbunia (172.2) and Char Kashem (126.4), and in lower layer of Char Kashem (998.3); for Pb the value in Char Motherbunia was 251.7, for Zn the values were also very high in upper layers of Char Motherbunia (238.8) and Char Kashem (182.7 ), and in lower layer of Char Kashem (1658.5) indicating extremely severe enrichment of Zn. Potential contamination index (Cp)of the studied metals namely Mn, Fe, Pb and Zn indicated that the metals are in low contamination level. The geo-accumulation index (Igeo ) indicated uncontaminated state of all the three chars. The study also revealed that the three chars are at low ecological risk in terms of potential ecological risk factors and hence ecological risk index. The foliar mineral nutrient of S. apetala were also more or less similar and showed lesser variation in the sampling occasions in the three chars. Mean nitrogen content (% ) of the leaves increased gradually since 2009 to 2017 whereas P content (% ) showed fluctuation. It seems that the study area showed a change from N limited to P limited status over a period of 8 years time. The average height of plants growing in Char Taposhi in 2003 (5 years old saplings ) was 2.2 ± 0.23 m and the diameter in breast height (dbh) was 3.18 ± 0.71 cm. These parameters were increased to 16.27 ± 0.77 m and 33.29 ± 6.14 cm respectively in 2017. The relative growth rate (RGR ) in terms of height in 2008, 2010 and 2017 were 0.24 ± 0.02, 0.23 ± 0.04 and 0.07 ± 0.01 respectively in comparisons to previous sampling year. The plant showed some adaptation in terms of anatomical and biochemical properties. The amount of crude protein content gradually increased in the leaves of S. apetala in Char Motherbunia and Char Taposhi. Highest values were found in Char Motherbunia indicated that stress condition due to flooding caused higher crude protein content. S. apetala was found to produce proline in the leaves to withstand salinity stresses. Proline content of the leaves of S. apetala was highest in the plants growing in Char Kashem where salinity was also the highest. Higher accumulations of Mn during 2009 and 2017 and of Fe during 2010 were recorded which is clear reflections of the bioavailability of these metals are. The present study revealed that sea water intrusion due to sea level changes is not noticed in this coastal area of Bangladesh. The chars are still at low ecological risk. These chars should be protected as these will also act as barriers to safeguard from the sea water intrusion and provide the habitats for different mangrove species. The data provided here will be helpful for the management of the coastal areas and fisheries sectors also as these areas are considered as breeding ground of Hilsa fish.