Variability in nitrogen sources for new production in the vicinity of the shelf edge of the East China Sea in summer

摘要

The relative importance of new nitrogen sources for primary production was investigated in well stratified subtropical waters in the vicinity of the shelf edge of the East China Sea by concurrent determinations of upward turbulent nitrate flux across the pycnocline, nitrate assimilation, N_2 fixation and primary production. On the coastal side of the Kuroshio jet close to the shelf edge, strong vertical diffusivity K_ρ was observed. The half-day mean K_ρ at the top nitracline was 3.9×10~(-5)m~2s~(-1). Consequently, a higher turbulent upward nitrate flux of 445μmolNm~(-2)d~(-1) across the nitracline was observed than that of 82μmolNm~(-2)d~(-1) off the shelf near the Kuroshio jet and 146μmolNm~(-2)d~(-1) on the slope at the outer side of the Kuroshio jet. This high upward flux supported the highest rates of primary production (27.3mmolCm~(-2)d~(-1)) and nitrate assimilation (764μmolNm~(-2)d~(-1)), and the contribution of the upward nitrate flux to new nitrogen was 5.7 times higher than that of N2 fixation. In contrast, N_2 fixation was a major new nitrogen source at stations off the shelf, where the upward nitrate flux was less evident due to less vertical diffusivity and weaker vertical nitrate gradients than near the shelf break in the Kuroshio jet. These observations demonstrate that new nitrogen sources were highly variable according to relative locations at the shelf edge and Kuroshio jet in the East China Sea and the adjacent waters during summer.