Microniche selects phytoplankton community structure in the interconnecting creeks of the Indian Sundarbans

Sundarbans Biosphere Reserve is a part of the Sundarbans mangrove forest and is currently facing several hydrological climate challenges. Two rivers, Matla and Thakuran, were extensively studied during the monsoon and post-mon season of 2019 and 2020. A time-series program, Sundarbans Biosphere Reserve Time Series (SBRTS), was established in 2019, to monitor seasonal trends along the Matla and Thakuran estuaries. SBRTS has 250 stations and was monitored over June 2019, November 2019, and November 2020 to map the hydrological conditions, including temperature, salinity, pH and dissolved nutrient concentrations. Phytoplankton abundance was mapped along these estuaries. Salinity, EC, TDS and pH showed distinct seasonal and spatial gradients. Phytoplankton communities showed strong site specificity and only Cyclotella was present in all stations at all sampling seasons as seen by SpAD analysis. The formation of microniche harbouring selective phytoplankton genera in high abundance appears to be controlled by environmental factors. However, the overall distribution of phytoplankton across the studied estuaries, resulting in microniche formation, may be controlled by localized water dispersal patterns. Understanding phytoplankton distribution patterns can provide key insights into niche partitioning in the Sundarbans Biosphere Reserve.