Detection of low bottom water oxygen concentrations in the North Sea; implications for monitoring and assessment of ecosystem health

Abstract

Abstract. This paper presents new results from high temporal resolution observations over two years (2007 and 2008) from instrumented moorings deployed in the central North Sea, at the Oyster Grounds and on the northern slope of Dogger Bank (North Dogger). The water column was stratified in the summer at both sites, leading to limited exchange of the water in the bottom mixed layer. Data from these moorings revealed the variable nature of summer oxygen depletion at the Oyster Grounds. The combination of in situ and ship-based measurements allowed the physical and biological conditions leading to decreasing dissolved oxygen concentrations in bottom water to be examined. In 2007 and 2008, the concentration of dissolved oxygen in the bottom water at both sites was observed to decrease throughout the summer period after the onset of stratification. Depleted dissolved oxygen concentration (6.5 mg l −1 , 71% saturation) was measured at the North Dogger, a site which is not significantly influenced by anthropogenic nutrient inputs. Lower oxygen saturation (5.2 mg l −1 , 60% saturation) was measured for short durations at the Oyster Grounds. The seasonal increase in bottom water temperature accounted for 55% of the decrease in dissolved oxygen concentration at the Oyster Grounds compared to 10% at North Dogger.Dissolved oxygen concentration in bottom water at the Oyster Grounds was shown to be strongly influenced by short term events including storms and pulses of particulate organic matter input. In contrast, dissolved oxygen concentration in bottom water at the North Dogger reflected longer seaCorrespondence to: N. Greenwood (naomi.greenwood@cefas.co.uk) sonal processes such as a gradual temperature increase over the summer and a more steady supply of particulate organic matter to the bottom mixed layer. The differences between the study sites shows the need for an improved understanding of the mechanisms driving these processes if the use of oxygen in marine management and ensuring ecosystem health is to be meaningful and successful in the future. These high frequency observations provide greater understanding of the nature of the depletion in bottom oxygen concentration in the North Sea.

A Novel Modeling Approach to Quantify the Influence of Nitrogen Inputs on the Oxygen Dynamics of the North Sea

“…Low simulated O 2 concentrations also occur directly north of Dogger Bank which matches with observations (Weston et al, 2008;Greenwood et al, 2010;Queste et al, 2016). The same applies to the Oyster Grounds site (see Figure 5, region 2) where simulated minimum near-bottom concentrations yield 6-6.5 mg O 2 L −1 .…”

“…The same applies to the Oyster Grounds site (see Figure 5, region 2) where simulated minimum near-bottom concentrations yield 6-6.5 mg O 2 L −1 . As the Oyster Grounds are well known being susceptible to low O 2 conditions (de Wilde et al, 1984;Peeters et al, 1995;Weston et al, 2008;Greenwood et al, 2010), this region is also used for a detailed analysis. At this site, the comparison with Figure 4 indicates a strong influence of the NL-1 and UK-2 rivers, and the NA (not shown).…”

“…Therefore, only the simulated volume transports are validated and spatial distributions of TN from selected sources are presented and compared to earlier TBNT studies. Finally, the results of the expanded TBNT method are presented with special focus on two regions regularly affected by O 2 deficiency -the southeastern North Sea (Topcu and Brockmann, 2015) and the Oyster Grounds (e.g., Greenwood et al, 2010).…”

Stability of dissolved and soluble Fe(II) in shelf sediment pore waters and release to an oxic water column

“…These observations enable an empirical assessment of the impact of oxygen concentration on the release of Fe(II) from seafloor sediments (Dale et al 2015). Parts of the UK shelf, other than our study site, seasonally undergo modest periods of reduced-oxygen concentration (e.g., 160-200 lmol l -1 , compared to 280-310 lmol l -1 at other times of the year; Greenwood et al 2010). To examine the likely impact of such changes in bottom water oxygen on the release of dFe from our study site, sediment cores and bottom water were sealed from the atmosphere, so that benthic respiration processes would draw down oxygen from the overlying water into the sediment.…”

Uncertainty and sensitivity in optode-based shelf-sea net community production estimates

“…The Cefas (Centre for Environment, Fisheries and Aquaculture Science) SmartBuoy network consists of autonomous data collection moorings placed at key locations in the UK shelf seas (Mills et al, 2005;Greenwood et al, 2010). The long-term, high-temporal-resolution multi-parameter data sets produced by the programme provide unique opportunities for observing biogeochemical processes in temperate coastal and shelf seas (Neukermans et al, 2012;Blauw et al, 2012;Foden et al, 2010).…”