Linkage between watershed and estuary estimated from the stable isotope analysis of the intertidal snail, Batillaria multiformis

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Specimens of the intertidal snail, Batillaria multiformis were collected from 25 river mouths in the Kunisaki Peninsula and the Nakatsu and Usa districts in southwestern Japan to assess the relationships between their isotopic compositions (δ¹³C and δ¹⁵N) and watershed characteristics. Among 22 watershed parameters, which were classified into the land use, human population, topography, geology, and vegetation in the catchment area of each river, five parameters were significantly correlated with the δ¹⁵N of snails (Pearson’s correlation coefficient r: all p<0.05). The δ¹⁵N values increased with a decrease in the inclination of the ground, volcanic rock rate, and forested land rate and with an increase in the cultivated land and paddy field rates. The δ¹³C and δ¹⁵N analyses showed minor contributions of terrestrial plants to the diet of snails and their dependence on estuarine and marine producers, mainly on benthic microalgae, for food. These findings suggest that the δ¹⁵N values of intertidal snails were determined by the consumption of estuarine and/or marine producers, which assimilated nutrients derived from the watersheds.