Tracking ecological health status of a tropical coastal ocean- applicability of benthic foraminifera coenosis

Coastal oceans offer multifaceted ecosystem level services and represent a key part to achieve UN SDG Goals including SDG14. The northeast coastal Bay of Bengal (BoB) in South Asia is interspersed with several coastal biotopes influenced by the Ganga-Brahmaputra-Meghna delta among others and contribute immensely to sustainable blue economy. The Sundarbans mangrove, world’s largest continuous mangrove, a UNESCO World Heritage Site and a RAMSAR site along with long intertidal zones represent characteristic features of this region. These sites are increasingly reeling from numerous anthropogenic stressors and the scales of anthropogenic disturbances were characterized through benthic foraminifera coenosis over three seasons along with robust measurement of organic carbon in sediment as well as integrating new technologies such as eDNA approach. The studied sites showed overwhelming abundance of calcareous taxa Ammonia spp. and Quinqueloculina spp. as well as low taxon level diversity. There was notable high abundance of dead and degraded tests indicating potential taphonomic alterations that indicated changes influenced by lowering of porewater oxygen content, N:P stoichiometry and possible changes in freshwater flow. The values of total organic carbon of sediment exhibited wide variability and the strong influence of anthropogenic forcings in closer proximity to human influences. There was resulting evidence of anaerobic degradation of sediment organic matter resulting in changes in sediment pH and taphonomic alteration of benthic foraminifera tests. The eDNA based high-throughput sequencing exhibited signs of low diversity of benthic foraminifera and signals influenced by anthropogenic stressors such as forms of nitrogen. The study based on spatio-temporal mosaics of mapping of coastal health of northeast coastal BoB indicates increasing human pressure will have long-term effects on SDG Goal 14 and beyond.