Winter Nutrient Concentrations in the Greater North Sea, Kattegat and Skagerrak

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. 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 2010-3). 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 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.

Elevated nutrient concentrations promote the growth of phytoplankton and other plant life. Long-term winter dissolved inorganic nutrient concentrations and ratios, namely of nitrogen and phosphorus, act as indicators for quantifying the pressures of human activities and for evaluating the success of measures taken. For the interpretation of winter inorganic nutrient concentration gradients, sources and sinks, influx of nutrients from long distance transport and local upwelling events are important (e.g. Weston et al., 2004; Blauw et al., 2006; Bristow et al., 2013). Winter dissolved inorganic nutrient concentrations do not represent the total nitrogen and phosphorus pool available for winter primary production. The uptake of nutrients by benthic micro- and macroalgae and macrophytes could locally be important in coastal areas.

This indicator on nutrient concentrations covers the nutrient parameters dissolved inorganic nitrogen (DIN), dissolved inorganic phosphate (DIP) and silicate (Si). Oxidised nitrogen (nitrate (NO₂^-) + nitrite (NO₃^-)) (NOx) is used in lieu of DIN (nitrate + nitrite + ammonium (NH₄⁺)) for the assessment of N (nitrogen) / P(phosphorus) ratios. Based on calculations from the data underlying this indicator assessment, NOx concentrations represent approximately 96% of DIN concentrations. The metrics used are spatial distribution, temporal trend, and ratio of nutrient concentrations.

Silicate concentrations are assessed for comparison as silicate is less influenced by anthropogenic activities. This nutrient compound can also have an important influence on the species composition of phytoplankton communities (a parameter used in diagnosing eutrophication status). Diatoms in particular need silicate to build their frustules (the silica-rich cell wall of a diatom).

The ratio between nitrogen (expressed as DIN or NOx depending on data availability) and phosphorus (expressed as DIP) is assessed because it can influence the phytoplankton community composition. A divergence of the N/P ratio from the Redfield Ratio (N/P = 16) can, in some circumstances, trigger the growth of harmful or toxic phytoplankton species. Reductions in inputs of one nutrient compound can alter nutrient ratios, which can affect phytoplankton composition, as recently demonstrated in the North Sea (Burson, et al., 2016).

Nutrient concentrations in coastal waters are mainly determined by riverine nutrient inputs and the mixing of these inputs with seawater. Riverine inputs are reflected by higher nutrient concentrations in shallow coastal areas with low salinity, such as on the continental coast of the southern North Sea, decreasing seaward with lower concentrations in the open sea with deeper and more saline offshore waters (i.e. exhibiting nutrient gradients). The influence of riverine inputs on nutrient concentrations is less visible in deeper coastal areas where they are diluted immediately by vertical and lateral mixing, such as on the east coast of the United Kingdom. The effect of these mixing processes on the assessment of nutrient concentrations needs to be taken into account. This is achieved by assessing nutrient concentrations within defined salinity bands.

Linkage to sources

Nutrients such as nitrogen and phosphorus enter the marine environment from the atmosphere, via rivers and land runoff, or by direct discharges into the sea. Inputs carried into the Greater North Sea by main rivers, such as the Humber, Thames, Scheldt, Rhine, Weser and Elbe, are clearly visible in elevated DIN and DIP concentrations in coastal areas. These nutrients are transported by the residual current system (Otto et al., 1990) and form belts of nutrient-rich waters along the coasts, especially in the southern North Sea (Figure 1and2). Silicate concentrations are also higher along the coasts of the northern North Sea and southern North Sea and decrease towards offshore areas, indicating the influence of riverine inputs (Figure n).

Northern and southern North Sea

Winter DIN and DIP

The mean (2006–2014) winter DIN and DIP concentrations in the southern North Sea show gradients with higher concentrations at the coast which decrease towards the open sea (Figures 1and2). These gradients reflect the influence of riverine discharges in the coastal areas (Figure b).

DIN and DIP concentrations in the southern North Sea show statistically significant decreasing trends since 1990 in all specific salinity-ranges assessed, except for DIP in waters with salinity >34.5 (Figures handi). In the period 2006–2014 there is a statistically significant increase in DIN concentrations in coastal waters (salinity 18–30) in the southern North Sea. Changes in DIN in offshore waters (salinity >30) and DIP in coastal and offshore waters in the period 2006–2014 reflect the variability of sampling and ecological processes, and are not statistically significant (Figure g).

In the northern North Sea there are no statistically significant trends in coastal and offshore areas for DIN or DIP, in either the long-term (since 1990; results not presented here) or the short term (2006–2014; Figure g).

N/P ratios

Mean (2006–2014) winter NOx/DIP ratios are around 14 in the northern North Sea and over 30 in the coastal waters of the southern North Sea (Figure j). Along the continental coast of the southern North Sea and in the estuaries of the Humber and the Thames, NOx/DIP ratios >18 have been observed. Locally, ratios exceed 70 in estuaries on the continental coast. Values around 160 are reached for example in the Ringköping fjord (west coast of Denmark) and the estuaries of the Elbe, Weser and Ems. High ratios in coastal areas may link to stronger past reductions in riverine inputs of phosphorus compared to nitrogen, as demonstrated by Burson et al. (2016).

In the northern North Sea there is no statistically significant trend in mean winter NOx/DIP ratios between 2006 and 2014 (Figure k). In coastal areas (salinity 18–30) mean winter NOx/DIP ratios have been variable since 1990 but show no statistically significant trend . Ratios have shown a slight statistically significant increase in offshore areas (salinity >30) (Figure l).

In the southern North Sea mean winter NOx/DIP ratios during 2006–2014 have been around 20 in the offshore areas (salinity >30), with no statistically significant trend (Figure K). In this period there is a statistically significant increase in coastal areas (salinity 18–30) (Figure k). There is no statistically significant trend of NOx/DIP ratios for the long-term (1990–2014) in either coastal or offshore areas (Figure l).

The assessment of the data for the specific salinity range areas of the southern North Sea resulted in a more differentiated picture (Figure m). In estuarine waters with salinities of 0–5 ratios increased significantly while there was a statistically significant decline in ratios in waters with salinities of 32–34.5 (Figure m).

Winter silicate concentrations

Mean (2006–2014) winter silicate concentrations show similar gradients to winter DIN and DIP concentrations in the southern North Sea, with high levels (>40 µM) in coastal areas, which decrease towards offshore areas (<4 µM) (Figure n). Locally, concentrations exceed 80 µM in estuaries on the continental coast. In the inner Elbe estuary, for example, concentrations increase up to 185 µM. Long-term time series (1990–2014) for silicate show no statistically significant trends in coastal areas (salinity 18–30) or offshore areas (salinity >30) in the northern North Sea and southern North Sea (Figure o). Trend assessments for the specific salinity-range areas of the southern North Sea show no pronounced peaks or trends for silicate except for a statistically significant decrease in silicate concentrations in estuaries (salinity 0–5) (Figure p).

Kattegat, Skagerrak and the Sound

Winter DIN and DIP concentrations

Long-term time series (1990–2014) of DIN concentrations show a statistically significant decrease in offshore areas (salinity >27) of the Skagerrak, in the Kattegat and in the Sound (Figure q). For DIN concentrations in coastal areas (salinity 0–27) of the Skagerrak no trend was observed. There are no statistically significant trends in DIP concentrations in the Skagerrak, the Kattegat, and the Sound for the period since 1990 (Figure r).

Short-term time series of DIN (Figure s) and DIP (Figure t) concentrations showed no statistically significant trends in the Skagerrak for the period 2006–2014 and in the Kattegat and the Sound for the period 2006–2013.

N/P ratios

NOx/DIP ratios are mostly around 6 to 12 in the Skagerrak, the Kattegat and the Sound except for high ratios near the Danish coast and the northern part of the Skagerrak (>50) and on the Danish-Kattegat coast (>30) and the north Danish coast and the northern part of the Skagerrak (>50) (Figure j). Long-term time series (1990–2014) of NOx/DIP ratios show no statistically significant trends for the Skagerrak (Figure u). NOx/DIP ratios in the Kattegat and the Sound show a pronounced decline after 1994 and a statistically significant decrease for the period 1990–2013 (Figure u). The short-term time series (2006–2014) of NOx/DIP ratios show no significant trends (Figure v).

Winter silicate concentrations

Mean (2006–2014) winter silicate concentrations show similar patterns in the northern North Sea and southern North Sea. High concentrations (>10 µM) are observed in coastal areas (salinity 0–27) of the Skagerrak and in the Kattegat. Concentrations are lower in the offshore areas (salinity >27) of the Skagerrak (around 4 µM) (Figure n). Concentrations at the Danish Kattegat coast were much higher than for the Swedish coast; gradients were similar to those for DIN and DIP since silicate is also discharged by rivers. There were no statistically significant long-term trends (1990–2014) in silicate concentrations in the Skagerrak and the Sound (Figure w). In the Kattegat, a statistically significant increase in silicate was observed over the period 1990–2014. Short-term time series (2006–2014) did not show any significant trends (Figure x).

Confidence rating

There is generally high confidence in the data, with mostly adequate spatial and temporal coverage of the data in the areas assessed, although some inadequacies remain. There are differences between assessed areas in the Greater North Sea. Data availability is high along the coasts of the areas assessed except along the Norwegian coast. There were fewer data available for the offshore waters of the southern North Sea. Spatial data coverage in the Skagerrak and the Kattegat was more scattered but broadly similar to North Sea coastal waters with 10–50 observations per square, for the period 2006–2014. In the Sound mainly one station was sampled. In the continental coastal areas (salinity 0–30), where data coverage is best, the standard deviation reaches 50–80% for DIN and DIP (Figures e and f). The standard deviation in the Skagerrak, the Kattegat and in coastal waters of the southern United Kingdom coast is also at 50–80%.

Compared to other OSPAR data series, data coverage for nutrient concentrations justifies the classification of confidence as ‘high’ for the purposes of the Intermediate Assessment 2017.

There is high confidence in the assessment methodology. The determination of winter nutrient concentrations has been used for many years in the OSPAR Common Procedure as well as in other assessments (such as the EU Water Framework Directive 2000/60/EC).