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

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, N2 fixation and primary production. On the coastal side of the Kuroshio jet close to the shelf edge, strong vertical diffusivity Kρ was observed. The half-day mean Kρ at the top nitracline was 3.9×10−5 m2 s−1. Consequently, a higher turbulent upward nitrate flux of 445 µmol N m−2 d−1 across the nitracline was observed than that of 82 µmol N m−2 d−1 off the shelf near the Kuroshio jet and 146 µmol N m−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 (27.3 mmol C m−2 d−1) and nitrate assimilation (764 µmol N m−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, N2 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.