26 Jun 2023
 | 26 Jun 2023
Status: a revised version of this preprint is currently under review for the journal BG.

Vertical mixing alleviates autumnal oxygen deficiency in the central North Sea

Charlotte Williams, Tom Hull, Matthew Palmer, Claire Mahaffey, Naomi Greenwood, Jan Kaiser, and Matthew Toberman

Abstract. There is an immediate need to better understand and monitor shelf sea dissolved oxygen (O2) concentrations. Here we use high-resolution glider observations of turbulence and O2 concentrations to directly estimate the vertical O2 flux into the bottom mixed layer (BML) immediately before the autumn breakdown of stratification in a seasonally stratified shelf sea. We present a novel method to resolve the oxycline across sharp gradients due to slow optode response time and optode positioning in a flow "shadow zone" on Slocum gliders. The vertical O2 flux to the low-O2 BML was found to be between 2.3 to 6.4 mmol m-2 d-1. Episodic intense mixing events were responsible for the majority (up to 90 %) of this oxygen supply despite making up 40 % of the observations. Without these intense mixing events, BML O2 concentrations would approach ecologically concerning levels by the end of the stratified period. Understanding the driving forces behind episodic mixing and how these may change under future climate scenarios and renewable energy infrastructure is key for monitoring shelf sea health.

Charlotte Williams et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2023-100', Anonymous Referee #1, 04 Aug 2023
    • AC1: 'Reply on RC1', Charlotte Williams, 25 Aug 2023
  • RC2: 'Comment on bg-2023-100', Anonymous Referee #2, 07 Aug 2023
    • AC2: 'Reply on RC2', Charlotte Williams, 25 Aug 2023

Charlotte Williams et al.

Charlotte Williams et al.


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Short summary
Oxygen (O2) is a key indicator of ocean health. The risk of O2 loss in the productive coastal/continental slope regions is increasing. Autonomous underwater vehicles equipped with O2 optodes provide lots of data, but have problems resolving strong vertical O2 changes. Here we show how to overcome this and calculate how much O2 is supplied to the low-O2 bottom waters via mixing. Bursts in mixing supply nearly all of the O2 to bottom waters in autumn, stopping them reach ecologically low levels.