Drivers of summer oxygen depletion in the central North Sea

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Abstract

In stratified shelf seas, oxygen depletion beneath the thermocline is a result of a greater rate of biological oxygen demand than the rate of supply of oxygenated water. Suitably equipped gliders are uniquely placed to observe both the supply through the thermocline and the consumption of oxygen in the bottom layers. A Seaglider was deployed in the shallow (≈ 100 m) stratified North Sea in a region of known low oxygen during August 2011 to investigate the processes regulating supply and consumption of dissolved oxygen below the pycnocline. The first deployment of such a device in this area, it provided extremely high-resolution observations, 316 profiles (every 16 min, vertical resolution of 1 m) of conductivity, temperature, and depth (CTD), dissolved oxygen concentrations, backscatter, and fluorescence during a 3-day deployment. The high temporal resolution observations revealed occasional small-scale events (< 200 m or 6 h) that supply oxygenated water to the bottom layer at a rate of 2 ± 1 μmol -3 day-1. Benthic and pelagic oxygen sinks, quantified through glider observations and past studies, indicate more gradual background consumption rates of 2.5 ± 1 μmol -3 day-1. This budget revealed that the balance of oxygen supply and demand is in agreement with previous studies of the North Sea. However, the glider data show a net oxygen consumption rate of 2.8 ± 0.3 μmol -3 day-1, indicating a localized or short-lived (<200 m or 6 h) increase in oxygen consumption rates. This high rate of oxygen consumption is indicative of an unidentified oxygen sink. We propose that this elevated oxygen consumption is linked to localized depocentres and rapid remineralization of resuspended organic matter. The glider proved to be an excellent tool for monitoring shelf sea processes despite challenges to glider flight posed by high tidal velocities, shallow bathymetry, and very strong density gradients. The direct observation of these processes allows more up to date rates to be used in the development of ecosystem models.

Figures

  • Figure 1. Bathymetry of the study area in metres (GEBCO, 2010). Depth contours have been added for the 20, 40, 80, and 160 m isobaths. The region where the glider was deployed is indicated by the large white dot at approximately 57◦ N, 2◦30′ E. Major landmarks are indicated as follows: A and B – Shetland and Orkney isles; C – Humber estuary; D – the Channel; E – Dogger Bank; F – Norwegian Trench; G – German Bight; H – Skagerrak. The general circulation of the North Sea (adapted from Turrell et al., 1992 and Hill et al., 2008) is overlaid as arrows for FIC (Fair Isle Current), DC (Dooley Current), SCC (Scottish Coastal Current), CNSC (Central North Sea Current), and SNSC (Southern North Sea Current). Hatch marks cover areas not subject to thermal stratification in summer.
  • Figure 2. BML oxygen saturation (%) and with the 70 % saturation contour during the August 2010 (a) and August 2011 surveys (b), mean summer bottom oxygen saturation values (%) from 1900 to 2010 from the ICES database (c), and mean summer bottom oxygen saturation values (%) from 1958 to 2008 from the General Estuarine Transport Model–European Regional Seas Ecosystem Model (GETM-ERSEM) output using European Centre for Medium-Range Weather Forecasts (ECMWF) European Reanalysis (ERA)-interim data (d).
  • Figure 3. Seaglider section of temperature (a; ◦C), salinity (b; PSU), potential density (c; kg m−3), chlorophyll a concentration (d; mg m−3), optical scattering as a volume scattering function at 650 nm (e; × 10−4β(θc)m −1 sr−1), apparent oxygen utilization (f; µmol dm−3) and oxygen concentration (g; µmol dm−3) sampled along the transect. Bathymetry as detected by the onboard altimeter is indicated along the bottom. Daytime chlorophyll a values showing signs of quenching are blanked. The vertical black line indicates the transition to a different water mass; data after this vertical black line are excluded from the oxygen calculations. The solid black contour indicates the 7 ◦C isotherm. The dotted black contour indicates the mixed layer depth (σ > 0.01 kg m−3).
  • Figure 4. Mean profiles as observed by the Seaglider of temperature (a; ◦C), salinity (b; PSU), potential density (c; kg m−3), chlorophyll a concentration (d; mg m−3), optical scattering as a volume scattering function at 650 nm (e; × 10−4β(θc)m −1 sr−1), and oxygen saturation (f; %) as height above the seabed. Daytime profiles were omitted for chlorophyll a.
  • Figure 5. Three-day composite of MODIS Aqua daytime sea surface temperature at 11 µm (◦C, a and b) and chlorophyll a concentration (mg m−3, (c) and d) across the entire North Sea (left) and around the Seaglider deployment area (right) from the 20 to 22 August 2011. The colour scale for the North Sea map of chlorophyll a is logarithmic to highlight chlorophyll a distribution in the central North Sea where concentrations are low. Northward Seaglider track is indicated by the black line.
  • Figure 6. A conceptual representation of the processes affecting oxygen supply and consumption to the bottom mixed layer during the glider survey. The water column is separated into two layers: the SML and BML (red and blue). The observed mean oxygen saturation profile (from Fig. 4) is overlaid on the water column to illustrate the position of the oxycline and deep chlorophyll maximum (indicated by the mid-water peak). ASE: air–sea exchanges; VMP: vertical mixing processes; Remin.: remineralization of organic matter.
  • Figure 7. Seaglider apparent oxygen utilization (µmol dm−3) along isobars at 1 dbar intervals from 45 to 71 dbar, coloured by pressure. Black vertical lines indicate the maximum difference in temperature at each time step as shown in Fig. 8. Binned data from the BML (45 dbar and below) are plotted against time and coloured by pressure to highlight any vertical gradients. The dotted vertical lines indicate when vertical mixing events were evident. The vertical black line indicates the transition to a different water mass.
  • Figure 8. Seaglider temperature (◦C) along isobars at 1 dbar intervals from 45 to 71 dbar, coloured by pressure. Vertical black stems indicate the difference between minimum and maximum values at each time step. This illustrates changes in vertical gradients; a small value indicates a homogenous BML, while a large value indicates a strong vertical gradient in the BML. We can clearly see events where the temperature gradient increases in the BML, showing injection of warmer surface water across the thermocline.

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APA

Queste, B. Y., Fernand, L., Jickells, T. D., Heywood, K. J., & Hind, A. J. (2016). Drivers of summer oxygen depletion in the central North Sea. Biogeosciences, 13(4), 1209–1222. https://doi.org/10.5194/bg-13-1209-2016

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