Trophic relationships in early spring along the Okhotsk coast of Hokkaido, Japan, as traced by stable carbon and nitrogen isotopes

Abstract

We measured stable carbon and nitrogen isotope ratios (¹³C/¹²C and ¹⁵N/¹⁴N) of organic matter sources and consumers, including fishery resources, along the Okhotsk coast of northern Hokkaido, Japan, in early spring to characterize the food web. Phytoplankton and benthic detritus, including ice algae, fecal pellets, and benthic microalgae, constituted the main food sources for primary consumers (zooplankton and small benthic invertebrates, respectively); seagrass and terrestrial plant debris served no trophic function for consumers. Planktivorous and benthivorous fish species could be differentiated using stable carbon isotope ratios. Differences in stable nitrogen isotope ratios revealed species-specific feeding habits of crustacean groups: copepods were separated into three groups as herbivorous, omnivorous, or omnivorous-carnivorous, and benthic amphipods were divided into two groups as either omnivorous or omnivorous-carnivorous. We distinguished four levels of both pelagic and benthic trophic linkages. The δ¹³C values were enriched by 0.9–1.3‰ at the linkage between each pelagic trophic level except between the 1st and 2nd levels; δ¹⁵N values were enriched by 2.2–2.9‰ at each level. For benthic trophic linkages, the enrichment for δ¹³C and δ¹⁵N values between each level ranged from 0.6‰ to 1.0‰ and from 1.6‰ to 2.6‰, respectively. Trophic positions of copepod, mysids, and sand lance appeared between pelagic and benthic linkages. This suggests that those species have fed on both pelagic and benthic diets.