Seasonal variability and elemental mapping of ostracoda carapace in deciphering recent environmental shifts: a case study from Mandapam Group of Islands, Gulf of Mannar Marine Biosphere Reserve, southeast coast of India

Seasonal variations in Ostracoda populations and the elemental composition of their carapaces were examined across diverse ecosystems of the Mandapam group of islands, on the southeast coast of India. Sediment samples from five islands-Shingle, Krusadai, Pullivasal, Poomarichan, and Hare-spanning coral reefs, seagrass meadows, sandy substrates, and mangrove environments, revealed 18 species across 16 genera with distinct shell ornamentations (smooth, reticulate, spinose, punctate, and ridged). Ostracod abundance peaked during the post-monsoon season, followed by the pre-monsoon and monsoon seasons. Elemental analysis of the carapaces showed the presence of Ca, C, O, Na, Mg, Al, Si, Fe, K, Nb, and Rb, with seasonal shifts—particularly Nb and Rb—highlighting the influence of monsoonal dynamics. Statistical assessments of adult/juvenile and valve/carapace ratios helped infer the sedimentation rates and the diagenetic processes. Smooth and porous forms were closely associated with coral habitats, while reticulate, punctate, and spinose forms dominated mangrove and seagrass environments. Overall, Ostracod distribution and shell chemistry reflect both ecological preferences and seasonal environmental changes.