Microniche shape phytoplankton community structure in inter-connecting creeks within a mangrove ecosystem

Globally mangroves constitute an important coastal biotope and aquatic production is largely driven by phytoplankton communities within this ecosystem. Sundarbans Biosphere Reserve is a part of Sundarbans mangrove forest, the world’s largest contiguous mangrove wetland, which is currently facing hydro-ecological as well as severe impacts from climate change. Two major estuaries, Matla and Thakuran, were extensively studied during monsoon and post-monsoon seasons of 2019 and 2020 as part of a time-series program, Sundarbans Seasonal Time Series (SuSTS), which was established in 2012. SuSTS is represented by 250 stations and was monitored over June 2019, November 2019, and November 2020 to map the environmental conditions, including temperature, salinity, pH and dissolved nutrient concentrations. Phytoplankton abundance was mapped along these estuaries. Salinity, Electrical Conductivty, Total dissolved solids and pH showed distinct seasonal and spatial gradients. Phytoplankton communities showed strong site specificity and only Cyclotella was present in all stations across all sampling seasons as also confirmed by Spatial Abundance Distributions analysis. The formation of microniche harbouring selective phytoplankton genera with high abundance appeared to be controlled by environmental factors. However, overall distribution of phytoplankton across the studied estuaries, resulting in microniche formation, may be controlled by localized water dispersal patterns. It appears that microniche defined phytoplankton community structure does not alter under the broad seasonal patterns and thus, such sites could potentially become hotspots of high productivity, or harmful algal blooms. Understanding phytoplankton distribution patterns can provide key insights into niche partitioning in mangroves with consequences for overall production.