Benthic foraminiferal assemblages elucidate ecological succession and test mineralogy in a newly colonized mangrove of the Subarnarekha estuary in the northeastern Bay of Bengal.
Abstract
The Subarnarekha estuary, along with its associated mudflats and mangrove ecosystems, represents a relatively recent geomorphological development on the eastern coast of India, where mangroves have naturally colonized newly formed mudflats during the last decade. This study aimed to assess how benthic foraminiferal assemblages and test composition respond to mangrove colonization by comparing an open shore (OS), a newly colonized mangrove (NM) and an old mangrove (OM). Surface sediments (n = 27) were sampled in the pre-monsoon, monsoon and post-monsoon seasons of 2025-26, together with grain size, total organic carbon (TOC) and pore-water variables. Twelve taxa were recorded, and total abundance ranged from 43 individuals/10 cc at OS in the pre-monsoon to 345 individuals/10 cc at NM in the post-monsoon. Habitat explained 67% of the variation in assemblage composition (PERMANOVA, p < 0.001), compared with 11% for season and 15% for the habitat × season interaction; dissolved oxygen, salinity and dissolved inorganic nitrogen were significantly correlated with the nMDS ordination. SEM was used for species identification, ICP-OES to measure element concentrations in pooled tests of Miliammina fusca, sediment and water, and Raman spectroscopy to characterize test-wall composition. Relative to the host sediment, M. fusca tests were enriched in Si (Si/Ca = 34.1) and Zn and depleted in Fe, Mn and Al, indicating selective incorporation of siliciclastic grains, while sediment Cr, Cu and Zn concentrations were below sediment quality guideline values. Raman spectra distinguished the carbonate band (∼1087 cm-1) of calcareous tests from the quartz and organic signals of agglutinated tests. The results show that foraminiferal assemblages of a decade-old mangrove already differ markedly from those of the adjacent open shore and provide a baseline for monitoring newly colonized mangrove habitats.