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. 2024 Apr 11;15(1):3136.
doi: 10.1038/s41467-024-47548-2.

The deepwater oxygen deficit in stratified shallow seas is mediated by diapycnal mixing

Tom Rippeth  1 , Sijing Shen  2 , Ben Lincoln  2 , Brian Scannell  2 , Xin Meng  3 , Joanne Hopkins  4 , Jonathan Sharples  3
Affiliations

The deepwater oxygen deficit in stratified shallow seas is mediated by diapycnal mixing

Tom Rippeth et al. Nat Commun. .

Abstract

Seasonally stratified shelf seas are amongst the most biologically productive on the planet. A consequence is that the deeper waters can become oxygen deficient in late summer. Predictions suggest global warming will accelerate this deficiency. Here we integrate turbulence timeseries with vertical profiles of water column properties from a seasonal stratified shelf sea to estimate oxygen and biogeochemical fluxes. The profiles reveal a significant subsurface chlorophyll maximum and associated mid-water oxygen maximum. We show that the oxygen maximum supports both upward and downwards O2 fluxes. The upward flux is into the surface mixed layer, whilst the downward flux into the deep water will partially off-set the seasonal O2 deficit. The results indicate the fluxes are sensitive to both the water column structure and mixing rates implying the development of the seasonal O2 deficit is mediated by diapcynal mixing. Analysis of current shear indicate that the downward flux is supported by tidal mixing, whilst the upwards flux is dominated by wind driven near-inertial shear. Summer storminess therefore plays an important role in the development of the seasonal deep water O2 deficit.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1. A map showing the northwest European shelf seas on which the location of the measurements is shown as a △.
The map is contoured for daily averaged sea surface temperature at the beginning of the period of interest (19th June 2014). The areas with temperatures > 16∘C are the seasonally stratified Celtic Sea. The sea surface temperature (SST) is downloaded from NERC Earth Observation Data Analysis and Artificial-Intelligence Service (NEODAAS) Plymouth Marine Laboratory (https://data.neodaas.ac.uk/visualisation/). Source data are provided as a Source Data file.
Fig. 2
Fig. 2. Evolution of stratification, deep water oxygen concentration and turbulence at a seasonally stratified location in the central Celtic Sea (CTD on Fig. 1) over the summer of 2014.
a time series of net surface buoyancy forcing. b Evolution of water column temperature. c Deep water O2 concentration. Each point is the average deep water dissolved oxygen concentration. d time series of the rate of dissipation of turbulent kinetic energy (ϵ) at depths of 16 m (blue), 35 m (red), and 47 m (yellow). Source data are provided as a Source Data file.
Fig. 3
Fig. 3. Profiles of water column structure and distribution of dissolved matter.
a, f temperature (∘C), b, g chlorophyll ( mg m−3), c, h concentrations of dissolved NOx (nitrate + nitrite) (black star) (μ moll−1) and dissolved inorganic carbon (DIC) (open circle) (μ molkg−1), d, i concentration of dissolved oxygen ( m mol m−3) and e, j oxygen saturation (%). Profiles a–e were collected on the 19th June and f–j on the 21st of August 2014. The dashed horizontal lines show the heights of the ϵ time series. Source data are provided as a Source Data file.
Fig. 4
Fig. 4. Time series of wind and tidal energy input.
a The cube of the tidal current speed at 47m depth (blue) and the wind speed multiplied by wind stress (red). A 24 h running mean is applied to the tidal energy. The spectral amplitude of the vertical shear in the horizontal currents, squared, separated into tidal (P m2S2 = 12.21 h) and inertial (PI = 15.80 h) frequencies, are plotted for 3.5-day windows at the depths of b 16 m c 35 m and d 47 m. Periods of; strong tides (U3 > 0.06 m3s−3) are highlighted blue, and strong wind (Wsτs > 3Wm−2) highlighted pink. Source data are provided as a Source Data file.
Fig. 5
Fig. 5. Schematic illustrating the water column structure and diapcynal fluxes.
A warm surface layer (pink) overlying cooler deep water (light blue) separated by a thermocline. The mid-water green band indicates the position of the subsurface chlorophyll maximum, and brown spheres represent sinking organic matter. The fluxes due to diapcynal mixing are shown with dissolved inorganic carbon (DIC) and limiting nutrients (NOx) as a purple arrow, downward O2 flux as a blue arrow, and upward O2 flux as a red arrow. The oxygen usage associated with water column respiration and sediment remineralisation of sunken organic matter is indicated by the brown arrow at the seabed.
Fig. 6
Fig. 6. An example of the shear and the spectral distribution of shear in the horizontal velocity.
The left-hand plot shows the measured shear at a depth of 16m for the 30th July 2014. The right-hand plot shows the corresponding shear spectrum separated into the clockwise (thick line) and anticlockwise (thin line) components. The local inertial period (I) is indicated by the red crosses and the effective tidal period (M2S2) by blue crosses.
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