Concentrations of Dissolved Oxygen Near the Seafloor

Eutrophication is the result of excessive enrichment of water with nutrients. This may cause accelerated growth of algae and / or higher forms of plant life (Council Directive 91/676/EEC). This may result in an undesirable disturbance to the balance of organisms present and thus to the overall water quality. Undesirable disturbances can include shifts in the composition and extent of flora and fauna and the depletion of oxygen due to decomposition of accumulated biomass. Such disturbances then have other effects, such as changes in habitats and biodiversity, blooms of nuisance algae or macroalgae, decrease in water clarity and behavioural changes or even death of fish and other species. Identifying causal links between these disturbances and nutrient enrichment can be complicated by other pressures. Cumulative effects, including climate change, may have similar effects on biological communities and dissolved oxygen, further complicating efforts to demonstrate causal links.

The OSPAR Commission’s strategic objective, with regard to eutrophication, is to combat eutrophication in the OSPAR Maritime Area, with the ultimate aim that by 2020 the OSPAR Maritime Area is a healthy marine environment where anthropogenic eutrophication does not occur (OSPAR Agreement 2010-3). The OSPAR Eutrophication Strategy requires assessment of eutrophication to be based on the ecological consequences of nutrient enrichment and not just on nutrient enrichment alone, i.e. finding reliable evidence for accelerated growth of algae and / or macrophytes caused by anthropogenic nutrient enrichment, leading to undesirable disturbance. Eutrophication is diagnosed using OSPAR’s harmonised criteria of nutrient inputs, concentrations and ratios, chlorophyll-a concentrations, phytoplankton indicator species, macrophytes, dissolved oxygen levels, incidence of fish kills and changes in zoobenthos (OSPAR Agreement 2010-8). As there is no single indicator of disturbance caused by marine eutrophication, OSPAR applies a multi-step method using the harmonised criteria. Eutrophication is considered to have occurred if there is evidence for all of the stages shown in Figure a and of causal links between them (ECJ, 2009).

For OSPAR’s Intermediate Assessment 2017, five harmonised eutrophication criteria have been assessed at a regional scale: nutrient inputs, nutrient concentrations and ratios, chlorophyll-a concentrations, concentrations of the nuisance algae Phaeocystis, and dissolved oxygen levels. These are highlighted in Figure a. The individual assessment results of any one of these five common indicators do not diagnose eutrophication by themselves. However, the assessments provide useful information about trends and are important for informing management measures.

The concentration of dissolved oxygen was assessed as an indicator of indirect effects of nutrient enrichment (Figure a). Depletion of oxygen may be caused by decomposition of accumulated organic material produced by algae (phytoplankton) or seaweed communities during their growing seasons, and may result in behavioural changes or death of fish and other species. However, oxygen depletion is not necessarily a consequence of nutrient enrichment and may be influenced by several other factors, including changes in water temperature and salinity. Seasonal oxygen depletion can be a natural localised process, particularly where the water column stratifies seasonally.

Available data were used to calculate the mean concentration and percentage saturation of dissolved oxygen in the lowest quartile (25%) of the data within 10 m of the seafloor in marine waters (salinity ≥30) during the stratification season (1 July–31 October).

Dissolved oxygen was assessed in the OSPAR Quality Status Report (QSR) 2010 as part of the Common Procedure for the identification of eutrophication. There is no comparable regional indicator assessment of dissolved oxygen.

For temporal trend analyses, longer-term datasets (1990–2014) were used. Mann Kendall analyses were considered to provide a robust estimate of significant trends. Downward trends indicate that oxygen levels are decreasing over time and may be a cause for concern. Where Mann Kendall analyses showed significant trends, weighted linear (least squares) regression lines were used to show the trends in oxygen concentration and percentage saturation.

Analyses of longer-term data in the Greater North Sea, Celtic Seas, and Bay of Biscay and Iberian Coast regions showed that oxygen concentrations (1990–2014) varied from year to year in each of the assessment areas. Analyses by ICES rectangles showed similar interannual variability in the data.

For the period 2006–2014, far fewer ICES rectangles had sufficient data (>5 data points) for an overall assessment (Figure c). In the northern North Sea and southern North Sea, five rectangles had mean values of <6 mg/l (compared to 16 using the 1990–2014 time series). In the Skagerrak and Kattegat, approximately half of rectangles had mean values <6 mg/l.

Northern North Sea and southern North Sea

For the large-scale regions of the northern North Sea and southern North Sea, mean near-bed dissolved oxygen concentrations for 2006–2014 were >6 mg/l. Stratified-season means in the lowest quartile were >6 mg/l and ≥60% saturation in all years in the northern North Sea, and above 5 mg/l and ≥60% saturation in all years in the southern North Sea (Figure d). In both the northern North Sea and southern North Sea, 95% confidence levels indicate high confidence in the data. Mann-Kendall analyses using the annual mean data showed no significant trends in oxygen concentration and percentage saturation of dissolved oxygen (Table aand Table b).

Within the southern North Sea, ICES rectangles with mean dissolved oxygen values of <6 mg/l in the lowest quartile of the data were observed mainly in the eastern areas (outlined in bold in Figure e), and indicate localised areas (‘hotspots’) with lower oxygen levels (mean 4–6 mg/l, Figure 3 and Figure e; minimum = 2.8 mg/l, Table d). These results broadly agree with previous findings (e.g. Topcu and Brockmann, 2015; Große et al., 2016), although the ICES dataset did not include the earlier data (1980–1990) and local datasets used in those previous studies. The hotspots are located in regions that show a high degree of hydrodynamic variability. Most are relatively shallow (less than about 40 m), with competing influences from riverine outflow (such as the Elbe), wind mixing (e.g. in shallow areas along the Danish coast), and localised influences such as in the Elbe channel where seasonal stratification occurs in warm years and low oxygen conditions may occur near the bed if there is low through-flow (van Leeuwen et al., 2015).

Several researchers have shown that water column stratification represents a major process regulating the seasonal dynamics of bottom oxygen levels (e.g. Topcu and Brockmann, 2015; Queste et al., 2016). However, it has been shown that the lowest oxygen conditions in the North Sea do not occur in the areas of strongest stratification (e.g. Große et al., 2016; Queste et al., 2016). Although stratification is an important prerequisite for oxygen deficiency, the changes observed represent the end result of a complex interaction between hydrodynamic and biological processes. Oxygen consumption may be affected by biological processes such as pelagic and benthic remineralisation of organic matter, zooplankton respiration, nitrification and denitrification. Große et al. (2016) analysed oxygen dynamics under stratified conditions in the southern North Sea. They found that biological consumption processes (mainly by bacteria) control the sub-thermocline oxygen conditions. There is a strong link between near-surface primary production and the export of organic matter into the bottom water layer, enhancing bacterial remineralisation. These processes in combination with the size of the sub-thermocline water volume, control the evolution of the oxygen concentration. Regions with high surface net primary production and a small sub-thermocline volume are particularly susceptible to oxygen deficiency.

Mann Kendall analyses indicated no significant trends in dissolved oxygen concentration or saturation in 12 of the 13 ICES rectangles or in the aggregated dataset for the south-eastern part of the North Sea (Figure f). ICES rectangle 40F7 was found to have a significant downward trend in dissolved oxygen concentration (Table c). Summary statistics for the aggregated dataset (Table d) indicate variability in the data, with measurements ranging from a minimum of 2.82 mg/l to a maximum of 12 mg/l, and mean values in the lowest quartile of the data of 3.55–7.5 mg/l in years when more than four data points were available.

Skagerrak

In the Skagerrak, mean near-bed dissolved oxygen concentrations in the lowest quartile (2006–2014) over the large-scale assessment region were 4–6 mg/l. Annual means were generally above 3–4 mg/l and 30–40% saturation (Figure g). Variability in the data was high and is largely attributable to the complex oceanography of the region, and high environmental variability (Erlandsson et al., 2006; Hansen and Bendsten, 2013a,b; Lehmann et al., 2014). The Skagerrak is well known for having strong haline stratification (van Leeuwen et al., 2015). Water depths reach 200 m close to the coast, and the environmental conditions that lead to oxygen deficiency in the Skagerrak are different to those influencing the eastern part of the southern North Sea and the Kattegat. The deep water is open and well ventilated horizontally, resulting in oxygen deficiency being largely restricted to fjords, which were not included in this assessment (SMHI, 2016).

Mann-Kendall analyses using the annual mean data showed no significant trends in oxygen concentration or percentage saturation in the Skagerrak (Table a and Table b).

Analyses by ICES rectangle showed that regions with lower oxygen concentrations (i.e. <6 mg/l) were located in coastal regions. However, these values are not considered to indicate oxygen deficiency in this region.

Kattegat

For the period 2006–2014, annual stratified-season means in dissolved oxygen concentration were lower than for the other assessment areas of the Greater North Sea, but generally above 2 mg/l and 20% saturation (Figure g). Analyses by ICES rectangle showed that regions with the lowest oxygen concentration (2–4 mg/l) were located in the southern parts of the Kattegat. These values are not considered to indicate oxygeHeading 3n deficiency in this region. Variability in the data was high due to complex topography and oceanography (Hansen and Bendsten, 2013a,b; Lehmann et al., 2014; SMHI, 2016). The Kattegat and Sound are particularly vulnerable to hypoxia and anoxia (absence of oxygen or severe oxygen depletion), because the halocline is strong and tends to lie at 15–30 m depth. In some areas, the halocline may at times be near the seafloor. During these events, the volume of bottom water is low and oxygen levels depend on replenishment by horizontal advection.

For the offshore Kattegat, 2002 was a particularly hypoxic year (i.e. <2 mg/l, Figure g). This was attributed to several weeks of extremely calm weather, which prevented the horizontal advection of oxygen across the seafloor (Ærtebjerg et al., 2002; SMHI, 2007).

Mann-Kendall analyses showed a significant upward trend in the Kattegat for dissolved oxygen concentration (Table a) and percentage saturation (Table b).

The Sound

Annual stratified-season means in dissolved oxygen concentration (Figure h) were highly variable, ranging from 1 to 5 mg/l and 10% to 55% saturation during the period 2006–2014. Variability in the data was high due to complex topography and oceanography (Hansen and Bendsten 2013a,b; Lehmann et al., 2014; SMHI 2016). Mann-Kendall analyses using the annual mean data showed no significant trends in oxygen concentration or percentage saturation (Tables aandb).

English Channel

In the English Channel, mean near-bed dissolved oxygen concentrations in the lowest quartile of the data were >6 mg/l. Annual stratified-season means (Figure i) were generally >6 mg/l and 80% saturation, with relatively few data points in this area.

The Mann-Kendall analyses using the annual mean data showed no significant trends in oxygen concentration or percentage saturation in the English Channel (Tables aandb).

Celtic Seas

The mean near-bed dissolved oxygen concentration (2006–2014) in the lowest quartile of the data in the large-scale region of the Celtic Seas was >6 mg/l. Annual stratified-season means (Figure i) were generally >6 mg/l and 80% saturation, with relatively few data points in this area.

In one ICES rectangle (38E1) in the Celtic Seas, the mean near-bed oxygen concentration in the lowest quartile of the data was in the 4–6 mg/l range. Data in this rectangle are available from 2007 only, and the mean value in the lowest quartile of the data (5.8 mg/l) is the same in both the longer dataset (1990–2014) and the shorter dataset (2006–2014). Oxygen deficiency in this region is attributable to local issues within Killybegs Harbour, as reported on in the third application of the Common Procedure (see EPA and Marine Institute, 2016).

The Mann-Kendall analyses using the annual mean data showed no significant trends in oxygen concentration or percentage saturation in the Celtic Seas (Tables aandb).

Bay of Biscay and Iberian Coast

For the large-scale assessment region for the Bay of Biscay and Iberian Coast, mean near-bed dissolved oxygen concentrations for the period 2006–2014 were >6 mg/l. Data availability was poor in this region. Four ICES rectangles indicated lower oxygen concentrations (4–6 mg/l, Figure 3) but there were insufficient data for detailed analyses. Mann-Kendall analyses showed no significant trends in oxygen concentration or percentage saturation for the Bay of Biscay and Iberian Coast region.

Confidence ratings

Confidence in data availability is moderate-to-high due to temporal and / or spatial gaps in the data in many regions. The ICES database includes a large amount of data on dissolved oxygen, measured from all depths. Once these data were filtered by depth, season and salinity, the dataset was considerably reduced: from approximately 860 000 data points, including binned CTD data, to approximately 14 000 data points for the period 1990–2014, and approximately 7300 data points for the period 2006–2014. The largest dataset (see Table e) was available for the Celtic Seas; most data (2669 data points) were collected from coastal regions (Figure b) from 2006 onwards (Table e), indicating gaps in spatial and temporal coverage. Large datasets were also available for the English Channel (2256 data points) and Iberian Coast and Bay of Biscay (1515 data points), but they also represented largely coastal sampling (Figure b) from 2001 onwards (Table e), also indicating gaps in space and time. Confidence ratings for data availability are therefore moderate in these regions. Confidence ratings are higher for the Kattegat, Skagerrak and southern North Sea. Relatively large datasets were available in these regions (Kattegat 2417 data points, Skagerrak 1775 data points, southern North Sea 1505 data points, Table e); spatial coverage appeared to be sufficient (Figure b) and data were collected fairly consistently from 1990 (Table e), giving greater confidence in assessment outcomes. For the Kattegat and Skagerrak, this may be outweighed by issues relating to complex oceanography in these areas. The available dataset in the northern North Sea was relatively small (735 data points, Table e), although temporal and spatial coverage appeared to be sufficient for this region, which is not known to have low oxygen levels. Confidence ratings are therefore also high for this region.

Confidence in the methods used for assessing oxygen deficiency is moderate overall. The approach has been developed over many years, and applied in published international assessments. In this assessment, however, the method has been refined to focus specifically on measurements from near the seafloor, and to use the mean of the lowest quartile of the data, rather than the 5^th percentile of all data, which has been used in previous assessments. The methodology requires further development to address questions about the feasibility and practicalities of near-seafloor monitoring and assessment.