Environmental Controls of phytoplankton in the river dominated sub-tropical coastal ecosystem of Bangladesh

Abstract

Sub-tropical coastal ecosystems are considered as the hotspot of species diversity. However, due to challenges regarding data collection, these regions are inherently data-limited and thus hindering informed species conservation strategies. Considering this, the present study aimed to assess the ecological characteristics of a river dominated sub-tropical coastal ecosystem of Bangladesh. More specifically, in this study we examined the spatial variability in environmental conditions and phytoplankton in the coastal ecosystems of Bangladesh. We also examined the causal relationship between environmental variables and phytoplankton species abundance. Data on environmental variables, including temperature, salinity, dissolved oxygen, turbidity, and macronutrients, was gathered in the north-east monsoon season between 2020 and 2022. We performed multivariate statistics to identify the variability in phytoplankton community dynamics. Results showed strong spatial variability in environmental conditions. Salinity, turbidity, nutrients concentrations and chlorophyll were found as the major drivers of this spatial heterogeneity. Central coast was characterized by low salinity, and high turbidity and silicate concentration while south-east coast was characterized by high salinity and low nutrient concentrations. In contrast, the south-west coast was characterized by high nitrate and phosphate, and moderate level of turbidity and salinity. Furthermore, our study identified 121 phytoplankton species. Maximum species diversity was observed in the south-west coast followed by central and south-east coast. Whereas 66 species occurred simultaneously in the three coastal zones, Coscinodiscus, Odontella and Nitzchia were the dominant phytoplankton genus in the central coastal zone. Skeletonema, Ditylum and Proboscia were found as dominant species in the south-east coastal zone. Coscinodiscus, Odontella, Nitzchia, Proboscia, Ditylum and Thalassionema were found as the key species in the south-west coastal zone. Seven explanatory variables (i.e. temperature, salinity, turbidity, silicate, nitrite, phosphate and zooplankton abundance) jointly explained about 44% to 73% variability in phytoplankton species abundance. We identified salinity and macronutrients as the major drivers of phytoplankton variability. This study is the first systematic report on phytoplankton community dynamics in-relation to environmental variables considering data from the entire coastal ecosystem of Bangladesh.