Diatoms in Sub-Surface Sediment Cores from Mangrove Forest Floors of Deltaic Islands in Sundarbans, India

Species constituting diatom assemblages in sediments provide important information regarding the past and present environmental conditions related to soil and water physico-chemistry and also biodiversity dynamics data. This information can reveal the developmental history of the region. Diatoms are sensitive to stochastic changes in the environment, making them extremely reliable bio-indicators. The purpose of the present study was to decipher such biodiversity related information provided by the diatom assemblages from the Indian Sundarbans. Diatoms were identified in the top 50 cm layer of two sediment cores: a newly silted up deltaic landmass with mangroves in early succession stage, and an island within the Protected Area Network with mangroves in late succession stage. Fifteen diatom species, with no past records in the sediments of Indian Sundarbans were identified and described. Four species among them are sporadically mentioned in previous phytoplankton records fi-om the Sundarbans, but these are not confirmed taxonomically. This paper provides identification and nomenclatural notes on all 15 species. High abundance and species richness of the centric forms Coscinodiscus spp., Cyclotella spp., and Thalassiosira spp. at both the sites is attributed to their proximity to estuarine rivers and their regular inundation, leading to settling of planktonic forms within sediment cores. Many pennate forms, namely Amphicampa eruca, Amphora holsatica, Diploneis spp. (except Z). smithii), Epithemia turgida, Eunotiapectinalis, Giffenia cocconeiformis , and Rhaphoneis rhombus which are being reported for the first time fi-om this region have never been part of described planktonic communities in estuarine waters, validating their' soil diatom' status. Typical fi-eshwater diatoms, namely Aulacoseira granulata and Epithemia turgida, in the sediments indicate lower salinity in these areas, not much earlier than present times.