The maintenance of the subsurface chlorophyll maximum in the stratified western Irish Sea

Abstract

The diapycnal flux of nitrate from the deep water provides a limit on new production in the subsurface chlorophyll maximum (SCM) during summer in stratified shelf seas. Here we estimate the diapycnal nitrate flux into the SCM in the stratified western Irish Sea (SWIS). Sampling took place immediately before neap tides when winds were light, so flux estimates reported provide a lower limit to nitrate supply to the SCM. Measurements of turbulent kinetic energy dissipation, chlorophyll a, and nitrate were used to estimate the flux of nitrate and chlorophyll through the SCM. Turbulent dissipation was low in the SCM (10^–9 to 10^–7 m² s^–3), driving a correspondingly low nitrate flux into the SCM (0.31 mmol m^–2 d^–1). The thermocline was marginally stable throughout sampling, and thus the addition of shear would likely result in shear instabilities and mixing. We show that although the SWIS is documented as having an energetic internal tide at this time, there was a low level of dissipation within the thermocline. We argue that the internal tide sets up background shear, which results in marginal stability. The addition of extra shear through the passage of nonlinear internal waves and/or the wind can trigger instability and mixing. We extrapolate our flux estimate over the summer and show that the nitrate flux is insufficient to sustain the documented summer production estimates for the SWIS. This suggests that episodic events are likely to be important for nitrate fluxes, or even largely responsible for the nitrate flux that sustains the SCM.