Coastal intertidal zones are important sinks for anthropogenic trash, but the seasonal fate, transport and bio-ingestion dynamics of microplastics (MPs) in abiotic and biotic matrices are inadequately studied. The spatio-temporal dynamics of MP contamination in sediment, surface water and intertidal hermit crab Clibanarius ransoni were examined at three different hydro-climatic seasons, Pre-Monsoon, Monsoon and Post-Monsoon along the rocky shores of the Saurashtra coast, Gujarat, India in the present study.  The MP extraction followed conventional techniques of digestion and flotation, and the polymer types were validated using Attenuated Total Reflection Fourier-Transform Infrared (ATR-FTIR) spectroscopy. MP abundance in abiotic matrices displayed high geographical and temporal heterogeneity. Water column concentrations showed a distinct seasonality with minimum concentrations during pre-monsoon baseline, moderate dispersion during monsoon (0.25–0.85 MPs L^-1), and maximum accumulation in post-monsoon season (0.40–1.25 MPs L^-1; p < 0.01). Sediment samples also revealed spatial differentiation (p < 0.05) with concentration ranges from pre-monsoon stability to highest values (230 MPs kg^-1) in the post-monsoon period.  Importantly, the analysis of tissue contamination in C. ransoni was significantly seasonal (p < 0.01), with increases from baseline ingestion levels pre-monsoon (<1.0 MP Ind^-1) to 1.0–1.8 MPs Ind^-1 during the monsoon, with a maximum of 2.0 MPs Ind^-1 post-monsoon. Bio-ingestion selectivity was significant as shown by the morphometric screening: C. ransoni swallowed only fibres for the whole year whereas sediment and water comprised varied shapes (fibres, fragment, films and foams). Further, the polymer composition shifted from a basic Polyethylene (PE) and Polypropylene (PP) dominance during pre-monsoon and monsoon periods to a very complex multi-polymer landscape (containing PET, Nylon, PVC and HDPE) during the post-monsoon season.  These findings emphasise the necessity of multi-seasonal monitoring of land-based runoff surges and bio-accumulation concerns in benthic fauna in coastal waters.
 

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