Drivers of Phytoplankton Community Dynamics across Tropical Coastal Habitats of Bangladesh

A N M Samiul Huda, Salma Akter

Phytoplankton regulate aquatic food webs and biogeochemical cycling, yet cross-habitat comparisons from tropical coastal systems remain limited. This study examined phytoplankton community structure and its environmental drivers across three contrasting coastal habitats of Bangladesh: the Meghna River estuary, the Teknaf mangrove coast, and the coral reef waters surrounding Saint Martin's Island. Samples were collected during the 2020-2021 dry season at 45 stations (15 per habitat). Temperature, salinity, dissolved oxygen, density, turbidity, chlorophyll concentration, silicate, nitrate, phosphate, phytoplankton abundance, and zooplankton abundance were assessed. Community patterns were summarized using Shannon diversity and non-metric multidimensional scaling, while generalized linear models were used to evaluate environmental drivers after multicollinearity screening and Akaike information criterion-based model selection. The three habitats showed distinct hydrographic and nutrient conditions. The estuary had the lowest salinity and highest turbidity and nutrient concentrations, whereas coral reef waters were more saline and comparatively nutrient-poor. Phytoplankton diversity was highest in the mangrove habitat (H = 2.89), followed by the estuary (H = 2.56) and coral reef habitat (H = 2.01). Diatoms dominated the estuary and mangrove habitats, while dinoflagellates dominated the coral reef habitat. Six selected predictors jointly explained 61%, 60%, and 63% of phytoplankton variability in the coral, mangrove, and estuarine habitats, respectively. Phosphate explained the largest share of variability in coral and mangrove waters, whereas silicate was the strongest explanatory variable in the estuary; zooplankton also contributed substantially in the coral habitat. These results indicate that nutrient gradients, modified by freshwater influence and habitat-specific trophic interactions, are major determinants of phytoplankton community dynamics across tropical coastal habitats in the northern Bay of Bengal.