Non-linear ecological responses of Ostracod communities to multi-metal pollution based on tolerance-weighted indices from the Vedaranyam shelf, Bay of Bengal, India.

Benthic ostracods serve as effective bioindicators of sediment quality and metal enrichment in coastal systems, but quantitative tools linking their community structure to multi-metal contamination are limited. This study develops and validates two ostracod-based biotic indices, the Ostracoda Assemblage Pollution Index (OAPI) and its reduced form, Mini-OAPI, to evaluate benthic ecological responses to metal contamination on the Vedaranyam shelf, Bay of Bengal. Twenty-eight surface sediment samples were analysed for Fe, Mn, Cr, Cu, Ni, Pb, and Zn concentrations along with ostracod assemblage data. The indices integrate species diversity, functional guild composition, and normalized pollution load to produce a tolerance-weighted ecological deviation measure. The OAPI includes diversity, evenness, guild shift, and pollution load, performs best in data-rich settings, while Mini-OAPI shows stable diagnostic behaviour under data-limited conditions and consistently captures ecological responses along contamination gradients. Normalization to a 0-1 scale and the use of standardized disturbance classes (Low = 0.00-0.33; Moderate = 0.34-0.66; High = 0.67-1.00) ensure comparability across marine and estuarine systems. A unimodal diversity-pollution pattern consistent with the Intermediate Disturbance Hypothesis and weak Cu-organic associations indicate complex metal-biota interactions. These indices provide transferable, tolerance-weighted tools for ecological assessment and understanding ecosystem responses to environmental change.

Keywords: Ostracoda Assemblage Pollution Index (OAPI); Mini-OAPI; benthic bioindicators; non-linear ecological modelling; Intermediate Disturbance Hypothesis (IDH); copper paradox; marine pollution assessment.