Benthic Foraminifera as Bioproxy for Tracking Environmental Variabilities in the Northeast Coastal Bay of Bengal

Mandal, Arkaprava (2025) Benthic Foraminifera as Bioproxy for Tracking Environmental Variabilities in the Northeast Coastal Bay of Bengal. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Arkaprava Mandal (19RS085)) 19RS085.pdf - Submitted Version Restricted to Repository staff only Download (4MB)

Abstract

Benthic foraminifera are single-celled eukaryotes enclosed in a test and colonizing sediment. Globally, their diversity and distribution are largely influenced by salinity, pH, and dissolved oxygen, mainly across temperate ecosystems. However, limited studies have investigated the role of mangrove vegetation, eddy currents, food availability, and inter-species interactions on the benthic foraminifera community structure, focusing on subtropical shallow coastal oceans. Extensive sampling (n = 292) was carried out to investigate the role of above mentioned parameters on the functional diversity of benthic foraminifera across a number of coastal biotopes located along the northeast Bay of Bengal. This included the Indian part of Sundarbans (Sundarbans Biosphere Reserve and Sagar Island), Junput and Tajpur (estuaries without mangrove vegetation) over monthly and seasonal intervals. A total of 18 benthic foraminiferal taxa (13 calcareous and 5 agglutinated) were identified, with seasonal variation in standing crop: highest during post-monsoon (120–150 individuals), followed by pre-monsoon (60–90 individuals) and lowest during monsoon (40–60 individuals). Microscopic enumeration (bright field and FE-SEM) and next-generation sequencing analyses revealed Ammonia sp.1, Ammonia sp.2, and Quinqueloculina seminula are most prevalent taxa in Sundarbans Biosphere Reserve (SBR) and Sagar Island; whereas Asterorotalia trispinosa, Ammonia sp.1, and Ammonia sp.2 were dominant in Junput and Tajpur. Entzia macrescens was found only in SBR and Sagar Island among the agglutinated taxa. Statistical analysis showed that eddy currents and nutrient stoichiometry influenced seasonal variation of benthic foraminiferal assemblages in SBR. During post-monsoon, strong eddy currents (0.8–2 m/s) (measured by ADCP) transported nutrient-rich waters that promoted diatom population, serving as food source and facilitating foraminifera standing crops. Porewater salinity, pH, dissolved nitrate, and TOC positively correlated with foraminiferal abundance in Sagar Island. Pneumatophore networks of mangrove vegetation are known to enhance sediment oxygen content and thus influence benthic foraminifera community structure. Porewater pH has been identified as a key factor influencing taphonomic alterations of foraminiferal tests. In Tajpur and Junput, seasonal nutrient loading influenced the community structure. Asterorotalia trispinosa dominated during the monsoon, while Ammonia sp.1 and Ammonia sp.2 standing crop increased in the post-monsoon season. This study provides critical insights into benthic foraminiferal assemblages and their responses to environmental cues, offering a crucial biological benchmark for ecological monitoring of subtropical shallow coastal ecosystems in a changing climate.

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