Nitrogen supply in the upwelling of mid-west region of the South China Sea: The case of mesopelagic fishes

Coastal upwelling zone hosts most fishing grounds worldwide and nitrogen dynamics in those systems mainly have been addressed through the nutrients replenished the ecosystem. Compared with coastal and upper-layer ecosystems, relatively limited knowledge has been explored by nitrogen isotope imprints through the food webs of mesopelagic fishes and their links to upwelling characteristics. Here, we collected several species of mesopelagic fishes in the mid-west region of the South China Sea (SCS) and analyzed them by biochemical methods (fatty acids, δ¹³C, δ¹⁵N, and δ¹³C of fatty acids). Due to the occurrence of upwelling, we want to evaluate the variation of nitrogen isotope compositions among these mesopelagic species and their links to the upwelling nitrogen supply. The δ¹⁵N of mesopelagic fishes in the non-upwelling area was depleted by 2‰ when compared with those in upwelling area, which indicated different nitrogen sources impacting for fishes in the two areas. A combination of multi-biochemical proxies was used to divide mesopelagic fishes into six groups and indicated that the feeding behaviors and upwelling were the main factors to affect the biological composition of mesopelagic fishes, even in such small regions. The differences in δ¹⁵N values between the upwelling and non-upwelling areas allowed us to estimate that N₂ fixation supported about 37% of the N-demand of food sources in the non-upwelling area, which was larger than the 10% in the upwelling area. These results illustrate that nutrients from the deep layers in upwelling systems might be the main factors that cause differences in δ¹⁵N in mesopelagic fishes between upwelling and non-upwelling areas. Thus, more studies of N dynamics in mesopelagic fishes should be conducted in the upwelling systems to illustrate the impact of climate changes on fish biomass in the future.