Stable Isotopic Signatures Of Zooplankton From The Northwest Pacific Ocean And Applications For Isoscapes Construction

The western Pacific Ocean is the main traditional fishery production area in China.The Kuroshio and Oyashio within the scope regulate the distribution of Marine fish resources and dominate the Marine biophysical environment,with significant interannual fluctuation,which has an important impact on the fishery,ecosystem and even the whole climate environment of the Northwest Pacific Ocean.Zooplankton transfer the energy and matter fixed by phytoplankton to higher trophic levels through feeding.It plays a key regulatory role in the Marine food web as an intermediate link between the microfood web and the classic food chain.Due to their long life history,zooplankton can integrate phytoplankton isotopes characteristics into their own tissues,reflecting the long-term assimilation of carbon and nitrogen.Studying the trophic positions of zooplankton in the food web provide insight into the material cycles and energy flows of Marine ecosystems.Therefore,it is necessary to carry out stable isotope studies of zooplankton to grasp the isotope distribution and influencing factors in the Northwest Pacific Ocean and provide information for stable isotope ecology studies of other Marine organisms in the Northwest Pacific Ocean in the future.In this study,zooplankton samples were collected during the“Songhang”survey in the Northwest Pacific Ocean in March and September 2019 and June-August 2021.The stable isotope differences of dominant zooplankton groups in the Northwest Pacific Ocean were investigated by using stable isotope techniques combined with hydrological data.And the spatial-temporal variation of stable isotope ratios of copepods,a dominant group,and its influencing factors were studied.Based on this,isoscapes of the study area is constructed.Specific research results are as follows:（1）The stable isotope ratios of carbon and nitrogen(δ¹³C andδ¹⁵N)of five dominant zooplankton groups in the Northwest Pacific:Copepoda,Ostracoda,Amphipoda,Euphausiacea and Chaetognatha were determined.The results showed that theδ¹³C values of amphipod,euphausiacea and chaetognatha were similar to those of Copepods and Ostracodes,and were significantly higher than those of copepods and ostracodes,indicating that the carbon sources of amphipod,chaetognatha and euphausiacea were similar and different from those of copepods and ostracodes.The order ofδ¹⁵N values from small to large was:copepods<ostracods<amphipods<euphausiacea<chaetognatha,indicating that the trophic level may be increasing gradually.The nutritive niche of copepods and ostracodes was obviously separated from amphipods,euphausiacea and chaetognatha,indicating that copepods and ostracodes were different from the other three types of zooplankton in feeding,and the degree of competition for food resources and habitat between species was low.The high degree of trophic niche overlap between amphipods andeuphausiacea reflects the possible competition between amphipods andeuphausiacea.（2）Taking copepods collected in the Northwest Pacific Ocean in March（spring）and September（autumn）of 2019 as objects,combined with their morphological characteristics and environmental factors in the sampling area,the characteristics of stable isotopes of copepods and their influencing factors were discussed.The results showed that the morphological characteristics（body length,area and volume）of copepods in spring were significantly higher than those in autumn（P<0.05）,and theδ¹⁵N value and morphological characteristics of copepods in two seasons were significantly positively correlated,while theδ¹³C value was significantly positively correlated only in spring,which may be because the sampling point in autumn was close to the Japanese island and had similar carbon sources.The generalized additive model（GAM）analysis showed that the seasonal differences ofδ¹³C andδ¹⁵N values of copepods were mainly related to chlorophyll a concentration and water temperature in the 50 m layer.（3）Based on zooplankton samples collected in the Northwest Pacific Ocean in the summer of 2021,a large herbivore Daphnia was selected as the research object.Combined with environmental remote sensing data,the GAM model was used to explore the relationship between stable isotope values of zooplankton and environmental factors,and the isotope baseline of the whole study area was predicted and the baseline map was constructed.The results showed that the stable isotopes of zooplankton in the northwest Pacific were significantly different,and theδ¹³C values were higher in the southeast and lower in the northwest.Theδ¹³C values were lower in the tideophilic region,but higher in the Kuroshio Current and the Oyashio Current mixed region and the Kuroshio extension region.Theδ¹⁵N values appear in several hot spots in the study area.GAM analysis shows that the stable isotopes of carbon and nitrogen are affected by sea surface temperature and chlorophyll concentration,and the spatial distribution ofδ¹³C values is also affected by salinity and the depth of the mixed layer.The spatial distribution ofδ¹⁵N is also related to sea level anomaly and latitude.