Coastal marine ecosystems are very complex and composed of myriad organisms, including offshore, coastal, and migratory fish occupying diverse trophic positions (TPs) in food webs. The illustration of trophic hierarchy based on the TP and resource utilization of individual organisms remains challenging. In this study, we applied compound-specific isotope analysis of amino acids to estimate the TP and isotopic baseline (i.e. δ15N values of primary resources at the base of food webs) for 13 fish and 1 squid species in a coastal area of Sagami Bay, Japan, where a large diversity in the isotopic baseline is caused by an admixture of ocean currents and artificial nitrogen inputs. Our results indicate that the TP of fish and squid varies between 2.9 and 3.9 (i.e. omnivorous, carnivorous, and tertiary consumers), with low variation within individual species. Moreover, the δ15N values of phenylalanine revealed the diversity of isotopic baselines between and within species. Low values (7.8-10.3‰) and high values (18.6-19.2‰), with a small variation (1σ < 1.0‰), were found in 2 offshore species and 3 coastal species, respectively. In contrast, highly variable values (9.8-19.7‰), with large variation within species (1σ > 1.0‰), were found for the remaining 9 migratory species. These results represent evidence of differential trophic exploitation of habitats between offshore and coastal species, particularly among individuals of migratory species, that were all collected in a single area of Sagami Bay.
Seasonal Nitrogen Baseline (δ15N) Variation of Sinking Particles in the Western Arctic Revealed by Compound-Specific Isotope Analysis of Amino Acids
