Stable carbon and nitrogen isotope analysis and environmental characteristics of benthic animals in Ushibashi Estuary, Miyagi Prefecture

Abstract

To clarify the effect of irrigation drainage from paddy fields on an estuarine tidal flat (Ushibashi Estuary, Miyagi Prefecture),we investigated the carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N) of organic matters within the paddy fields, agricultural drainage channels (rivers),and estuarine tidal flats, and those of 18 species of estuarine benthic consumers from March to December 2020. The tidal flat sediments had lower δ¹³C and δ¹⁵N at the riverward station (δ¹³C: -27.1 to -25.4‰; δ¹⁵N: 2.7 to 4.3‰) than at the seaward station (δ¹³C: -26.5 to -20.5‰; δ¹⁵N: 3.3 to 7.1‰).The total nitrogen to organic carbon ratio (C/N) was low in tidal flat sediments (3.3 to 7.5),and it was estimated that organic matter derived from freshwater and seawater microalgae was deposited at the estuary. However, the effect of direct supply from the paddy field to river particulate organic matter (POM) was not detected. The carbon sources of most benthic animal species residing in tidal flats were presumed to be of marine origin (marine- POM and microphytobenthos) by a δ¹³C-based stable isotope mixing model (simmr).The contribution of food resource for some bivalves and polychaetes were higher in terrestrial sources than in marine sources. The estuaries of the agricultural drainage channels serve as a carbon sink for storing organic matter from the rivers, providing a unique habitat for benthic animals.