Relative importance of marine and terrestrial organic matter in the trophic base of sandy beach communities revealed by a triple stable isotope approach.

Abstract

Sandy beach ecosystem is an ecotone receiving a variety of organic matter subsidized from both ocean and terrestrial areas. These organic materials should be trophic bases sustaining the beach communities composed of invertebrates such as crustaceans and insects. However, little is known about how much these communities relied on subsidized organic matter from the ocean and terrigenous organic matter. In this study, therefore, we examined the relative importance of marine and terrestrial organic matter as the trophic base of the beach communities on the Shinhama and Yuriage beaches of Sendai Bay, Japan, using the stable isotope ratios of carbon (δ¹³C), nitrogen (δ¹⁵N), and sulfur. (δ³⁴S) with a Bayesian statistical isotope mixing model. Using these data, we also assessed the effects of seawalls constructed on the sandy beaches 120～200 m away from the shoreline. The stable isotope analysis showed that arthropods on both Shinhama and Yuriage beaches exhibited a range of stable isotope ratios between terrestrial plants and seaweeds. The trophic contribution of the mixed isotope analysis revealed that the contribution of marine organic matter was high for beach hoppers, tenebrionid beetles and ghost crabs. The contribution of organic matter produced by the beach plants was also high for the arthropods collected on the beach but not so for those collected on the land side of the sea walls, where the arthropods mainly relied on organic matters produced by non-beach plants. These results indicate that the arthropods inhabiting sandy beaches strongly depend on, directly and indirectly, the organic matter subsidized from the ocean as a trophic base and suggest that seawalls act to block the spatial flow of such marine organic matter.