Stable isotope analysis of food-web system in subarctic to subtropical region of western North Pacific

Carbon and nitrogen isotope ratios of biota are controlled by two factors, a metabolic system and a lifestyle of predator and a regional variability of environmental parameters on food web system. To evaluate the environmental factors (i.e., nutrients and temperature) on the basic food web in the marine ecosystem. We studied the seasonal variation of nitrogen and carbon isotope ratios of zooplankton in the subarctic site of K2 (47ºN, 160ºE) and the subtropical one of S1 (30ºN, 145ºE) in the western North Pacific Ocean, both of which are JAMSTEC observation site. Biota were collected in eight depths (0-50, 50-100, 100-150, 150-200, 200-300, 300-500, 500-750, 750-1000m) at both sites using IONESS from February 2010 to July 2011. All samples were classified under a stereomicroscope into species or genus level and used adult stage for isotopic analysis. We also collected water samples from seasonal and vertical (from the surface to 1,000m depth) profiles in δ¹⁵N (NO₃⁺ + NO₂^-) at K2 and S1 in special reference to nitrogen cycles. The δ¹⁵N and δ¹³C values of amphipods and copepod, omnivorous zooplankton in the surface ocean, showed a large seasonal variation. The δ¹⁵N values of zooplankton and nitrate ions in the water at the S1 site tended to be lower than those at the K2 site, indicating that the basic food web could be affected by nitrogen fixation. We further compared the trophic fractionation of carbon and nitrogen isotopes (Δδ¹³C, Δδ¹⁵N). It was observed that a simple relationship exists in the Δδ¹⁵N/Δδ¹³C regardless of species and ecosystems. This contention suggests a unified regularity is maintained in the lower to higher trophic level in the marine ecosystem.