Spatial and Temporal Variations in Contribution of Benthic Diatoms and Terrestrial Plants to Food Source of Benthic Animals in Imazu Tidal Flat

This study investigated the spatiotemporal dynamics of benthic diatoms and terrestrial plants as food sources for benthic animals in Imazu Tidal Flat, using fatty acid biomarkers and stable isotope analysis. The sediments of estuarine sites exhibited higher terrestrial fatty acids and depleted δ13C values, reflecting riverine organic inputs, while sediments of outer flats showed marine-dominated signatures linked to benthic diatoms. The decrease in benthic diatom contribution during winter may be associated with reduced productivity under lower water temperature. Benthic diatom-derived 20:5ω3 (Eicosapentaenoic acid) dominated benthic diets annually (82–100% contribution), but winter elevated the contribution of terrestrial plants due to plant decay and reduced tidal flushing. Food source of terrestrial plants for Polychaetes (e.g., Heteromastus sp.) increased significantly in winter, which underscored the changes in the consumption of some benthic animals due to the increase in the relative contribution of allochthonous food sources. The observed seasonal interaction between riverine organic matter and benthic diatom availability, influenced by temperature, suggests potential flexibility in the tidal flat food web. This highlights the importance of understanding spatial–temporal variation in food sources when considering land–sea ecosystem connectivity.