Turbulence and vertical nitrate flux adjacent to the Changjiang Estuary during fall

Abstract

Turbulent microstructure and nitrate measurements from one repeated sampling station and two transects were analyzed focusing on the internal-wave (IW) induced turbulence and vertical nitrate flux adjacent to the Changjiang Estuary during fall. The repeated sampling station has a subsurface nitrate maximum zone locating within the stratified interior layer. The subsurface nitrate maximum is clearly separated from the periodic hydrographic changes at the bottom boundary layer which is induced by tidal advection of the intruded Taiwan Warm Current. Turbulence in the stratified interior layer is largely driven by the local velocity shear which shows a semidiurnal cycle. The driving mechanism is suggested as the periodic occurrence of IWs with a period of ~2.5 h and an amplitude of ~8 m. Consecutive turbulence casts demonstrate the occurrence of large turbulent kinetic energy dissipation rate ε (10−6 Wkg−1) at the trailing edge of the wave which is the first observational evidence of the IW-induced internal turbulence in this region. The calculated turbulent vertical nitrate flux at the base of the surface boundary layer reveals a daily-mean value of 1.0 (0.4, 2.9) mmolm−2d−1, capable of supplying about half of the nitrate required for new production there during fall. Calculations also suggest that the vertical supply of nitrate is dominated by episodic strong mixing events driven by IWs which have increased the eddy diffusivity and thus the nitrate flux by one order of magnitude. The presented results provide important implications for understanding the nutrient cycling and new production off the Changjiang Estuary.