ALGAL DIVERSITY IN THE COASTAL WETLANDS OF COX‟S BAZAR IN RELATION TO ENVIRONMENTAL VARIABLES AND ECOLOGICAL NICHE

Abstract

Algae have amazing diversities in respect to their habit, habitat, and taxonomic characters. Coastal wetlands are semi saline water bodies of varying length and dimension and in Bangladesh are present in the vicinity of the Bay of Bengal. Among the diversities of algae of the coastal wetlands, phytoplankton are major occupants and contribute cellularly built vital food elements to the trophic cascades. In those wetlands, the faunal diversity is fully dependent upon the productivity of phytoplankton. The qualitative and quantitative aspects of phytoplankton community, on the other hand, reflect the water quality status of a habitat. In the present research, a two-year study (2018-2020) on the assessment of the water quality of two coastal wetlands namely, Bakkhali River and Reju Canal, of Cox’s Bazar city area was carried out. Cox’s Bazar, a significantly famous maritime touristic spot of Bangladesh, where the aquatic ecosystems are routinely threatened by strong anthropogenic activities. So, the goal of the present research was to accumulate field data on water quality governing parameters as well as the quality, quantity, and seasonality of algal diversity of the two major coastal wetlands. Relevant to this, data collection was done on air- and water temperature (AT and WT, respectively), Secchi depth, salinity, total dissolved solids (TDS), electric conductivity, dissolved oxygen (DO), pH, alkalinity, NO3-N, soluble reactive phosphorus (SRP), soluble reactive silicate (SRS), chlorophyll-a (chl-a), phaeopigment, and phytoplankton density. A total of six (3 in each wetland) sampling stations were fixed to collect samples. Due to the remoteness of the study habitats from Dhaka, the sampling was done monthly once. The collected samples were processed following the standard procedures as available. The digital database thus created based on the analytical results was used to perform multivariate statistical analysis for predicting the ecological niche of the two wetland ecosystems. Besides, the dynamics of phytoplankton density (PD) variable with respect to the study stations and seasons were done via box-plot diagram. Impact on PD by AT, WT, water transparency (Secchi depth), biomass (chl-a), phaeopigment, and NO3-N were made via simple linear regression. To see the water quality, trophic diatom index was calculated. To predict PD using all the variables, advanced machine learning model, Random Forest, Support Vector (SVM) was used. Taxonomy of phytoplankton community was worked out and the species as new reports for Bangladesh and some new species were screened and reported. All the nutrients like nitrate (NO3-N), phosphate (PO43-) and silicate (SiO4-4) showed seasonal as well as spatial variation. Higher values of nitrate were observed during the pre-monsoon period than the other times of the year. The DO content of the water exhibited high degree of variation throughout the year especially during post monsoon and winter. Results of multiple correlation analysis reveal significant positive correlation between phytoplankton and xv different physicochemical parameters. Phytoplankton biomass as chl-a is also compared to the study sites. Physicochemical variables of both the studied ecosystems are almost similar only exceptions could be observed in case of phytoplankton density. In Reju Canal the density of phytoplankton is nearly 5-fold higher than the Bakkhali River. The phytoplankton was found to be a function of temperature factor. Both the ecosystem has a dynamic equilibrium and therefore the ranges of the concentration of dissolved nutrients were wide. The upper limit of DO concentration in Bakkhali River and Reju Canal was 9.8 and 7.9 mg/L, respectively. The present hydrobiological condition is ideal for the growth of phytoplankton and species richness of Chaetoceros throughout the year for Reju canal on the other hand excessive nutrient load create negative impact on phytoplankton growth in Bakkhali river for some samplings due to higher conductivity and salinity. During monsoon, the dilution of nutrients promotes quality of phytoplankton for richness rather than quantity. Heavy precipitation favored the growth of phytoplankton as well as chl-a concentration. Among all the studied parameters, conductivity showed significant role for the growth and distribution of phytoplankton. Nitrate nitrogen was found as the limiting factor for phytoplankton growth. In the present study, Reju Canal habitiat showed 1.5 times higher NO3-N than that of Bakhkhali river. So, phytoplankton diversity is higher in Reju canal. On the other hand, microbial degradation and chemical pollution helps to retard the growth of the phytoplankton in Bakhkhali River. Different hydrobiological parameters and presence of Chaetoceros and Cyclotella differentiate two ecological niches of the studied wetlands. Trophic diatom values indicate a moderate to good water quality of the studied ecosystems. As coastal wetlands, the Bakkhali river and Reju canal supports a significantly large phytoplankton diversity dominated by diatoms. Its self-purification capacity might be still high to lead a fairly good water quality. The niche defining characters of two dominant centric diatoms namely, Chaetoceros and Cyclotella could be as those by water transparency, water temperature, salinity and other nutrients. The study may contribute 48 new reports of phytoplankton for Bangladesh, which awaits a further detail address on a preliminarily identified source-list as new contribution.