Nutrient inputs to UK regional seas
There is no significant trend in the waterborne input of nitrogen to the Greater North Sea (1990 to 2014) but there is a significant reduction in phosphorus input. There are significant downward trends in both nitrogen and phosphorus inputs to the Celtic Seas.
Background
UK target relating to nutrient inputs
There is no formal Marine Strategy Framework Directive target for nutrient inputs, as such. There is, however, an expectation that nutrient inputs will not increase or continue to contribute to nutrient enrichment. The UK target for nutrient concentrations in ‘non-problem areas’ is that they should not increase as a result of nutrient inputs. The target for ‘problem areas’ is a downward trend in dissolved inorganic nitrogen and phosphorus concentrations, resulting from decreasing anthropogenic nutrient input over a ten-year period.
Key pressures and impacts
In the UK Initial Assessment (HM Government, 2012) the key pressures associated with this indicator were riverine and direct inputs of nutrients (nitrogen and phosphorus) from various sources and atmospheric deposition. These are still relevant but, considering that the small number of eutrophication problem areas are generally limited to coastal waters, estuaries and embayments, the key pressures are likely to be from local (catchment related) agriculture and wastewater treatment.
Measures taken to address the impacts
The UK Marine Strategy Part Three (HM Government, 2015) describes the robust UK legislative framework for controlling and reducing discharges, emissions and losses of nutrients from sources affecting specific eutrophication problem areas. Measures include Water Framework Directive River Basin Management Plans (European Commission, 2000) which are particularly relevant for UK problem areas in coastal waters. There are improvements in some of the areas where measures are in place but it can take many years for measures to reduce nutrient loads and to improve the issues related to eutrophication.
Areas that have been assessed
Trend assessments were conducted for UK inputs to the Greater North Sea and Celtic Seas Marine Strategy Framework Directive sub-regions. Assessments were also carried out at the smaller scale of the eight UK biogeographic marine regions set out in Charting Progress 2 (Defra, 2010) and by river catchment areas. Atmospheric loads were not assessed but, estimates for the whole of the Greater North Sea can be found in the OSPAR Intermediate Assessment 2017 (OSPAR Commission, 2017).
All UK marine waters (coastal and transitional waters are covered under the Water Framework Directive (European Commission, 2000) have been assessed for the purposes of the eutrophication related indicators (Nutrient Inputs, Nutrient Concentrations, Chlorophyll Concentration and Dissolved Oxygen Concentration) whether they have been assessed as ’problem areas’ or ‘non-problem areas’ using the OSPAR Common Procedure (OSPAR Commission, 2013).
Further information
Nutrient inputs can result in excessive enrichment of coastal waters and, under certain circumstances, may contribute to eutrophication. Enrichment can lead to accelerated growth of algae and/or higher forms of plant life (European Commission, 1991), and may result in an undesirable disturbance to the balance of organisms present and overall water quality. Undesirable disturbances include shifts in the composition and extent of flora and fauna and depletion of oxygen due to decomposition of accumulated biomass. Such disturbances can have other effects, such as changes in habitats and biodiversity, blooms of nuisance algae or macroalgae, and behavioural changes or death of fish and other species. Low oxygen levels, for example, may result in increased abundances of species which are able to tolerate low oxygen conditions. Very low oxygen levels may result in the death of marine organisms. Identifying causal links between disturbances and nutrient enrichment is often difficult and complicated by other pressures. Cumulative effects of prevailing conditions, including climate change, may have similar effects on biological communities and dissolved oxygen, complicating efforts to demonstrate causal links. Although oxygen depletion can be an indirect effect of nutrient enrichment, other factors which influence oxygen concentrations include changes in water temperature and salinity. Additionally, some degree of seasonal oxygen depletion is a natural process, particularly where the water column stratifies seasonally.
For the Marine Strategy Framework Directive Article 8 Assessment of Eutrophication (European Commission, 2008), four eutrophication criteria have been assessed: riverine inputs, nutrient concentrations and ratios, chlorophyll concentrations, and dissolved oxygen levels (Figure 1). The individual assessment results of any one of these indicators do not diagnose eutrophication by themselves. However, the assessments provide useful information about progress and are important for informing management measures. For further details, see the UK National Report to OSPAR (Painting and others, 2016).
Figure 1. Three stages in the identification of eutrophication. The criteria marked 1 are common indicators for the OSPAR Intermediate Assessment (OSPAR Commission, 2017). The criteria marked 2 are not relevant in all Contracting Parties’ waters.
Assessment method
The aim of this assessment is to show changes in nutrient inputs to the UK portion of the Greater North Sea and Celtic Seas since 1990, using data on waterborne sources of nutrient input via rivers and direct inputs. Data for the entire Greater North Sea are given in the OSPAR Intermediate Assessment (OSPAR Commission, 2017), which includes data from other member states as well as the UK data shown here.
Anthropogenic inputs of dissolved inorganic nutrients into coastal waters via rivers and direct point sources (notably sewage and industrial discharges) are monitored under the Riverine Inputs and Direct Discharges programme for reporting to OSPAR. In the UK, Riverine Inputs and Direct Discharges data are recorded by river and may be aggregated over different scales, for example by catchment (Paris Commission areas, Figure 2), regional sea or OSPAR region. UK riverine inputs and direct discharges data from 1990 to 2014 were obtained from the Marine Environment Monitoring and Assessment National database (MERMAN). For the purposes of this assessment, inputs of dissolved nitrogen and phosphorus to the UK-portion of the Greater North Sea and the Celtic Seas were calculated as total inputs (loads), as well as separately for riverine, industrial and sewage sources. Reported loads were not adjusted for water flow rates. Mann-Kendall non-parametric tests were used for trend analysis (Mann, 1945; Kendall, 1975, Barry and Maxwell, 2015). Where p-values are greater than 0.05, a trend cannot be detected statistically. Where p-values are less than 0.05, it is assumed that there is a significant trend. Where trends were significant, a linear regression was calculated.
Figure 2. Paris Commission river catchment areas (NI1 & 2, E1-E30, SC1-SC5), which broadly relate to UK regional seas (sub-regions) in the Greater North Sea (OSPAR Region II) and the Celtic Seas (OSPAR Region III).
For the third national application of the OSPAR Common Procedure (Painting and others, 2016), nutrient inputs were calculated by UK regional seas. In the Greater North Sea, these are the northern North Sea, the southern North Sea and the eastern English Channel. In the Celtic Seas, these are the western Channel and Celtic Sea, the Irish Sea, Minches and western Scotland, the Scottish Continental Shelf, and the Atlantic North-West Approaches. Trends in inputs of riverine nitrogen and phosphorus were assessed by fitting a smoother to the log loads, and assuming the errors were normally distributed and correlated with an AR1 structure. The trends were summarised by the estimated yearly percentage change in loadings between 1990 and 2014, obtained from the fitted slope (if a linear model was adequate) or the fitted values in 1990 and 2014 (if there was a nonlinear trend). Significance was determined by the significance of the fitted slope (linear trend) or a Wald test comparing the fitted values in 1990 and 2014 (nonlinear trend). Inputs to each Paris Commission area were also analysed, and trends were assessed using Mann-Kendall tests.
Results
Findings from the 2012 UK Initial Assessment
The initial assessment (HM Government, 2012) reported that over time, inputs of nutrients to the marine environment were generally decreasing and that eutrophication problems in UK seas were restricted to a small number of estuaries, embayments and coastal waters.
Latest findings
Status assessment
There is no status assessment for this nutrient input indicator as assessment thresholds have not been set.
Trend assessment
Trends have been assessed for this indicator. This is in line with the regional approach in the OSPAR Intermediate Assessment (OSPAR Commission, 2017).
Total waterborne nitrogen input (1990 to 2014) shows no trend in the Greater North Sea and a significant reduction in the Celtic Seas (Figure 3). Inputs to both regions are dominated by riverine inputs reflecting diffuse sources. Total waterborne phosphorus inputs show significant reductions in both the Greater North Sea and Celtic Seas (Figure 3). The main inputs are riverine, reflecting diffuse sources, and direct sewage sources.
Overall, waterborne nutrient inputs to the Celtic Seas are considerably lower (0.28 t nitrogen km-2; 0.02 t phosphorus km-2) than those to the Greater North Sea (0.8 t nitrogen km-2; 0.06 t phosphorus km-2) reflecting differences in agriculture and population density. Nitrogen inputs vary from about 150 to 280 kilotonnes per year (kt y-1) in the Greater North Sea and from about 90 to 180 kt y-1 in the Celtic Seas (Figure 3). Total waterborne phosphorus inputs vary from 9 to 25 kt y-1 to the Greater North Sea and from 6 to 19 kt y-1 to the Celtic Seas. In the Greater North Sea and Celtic Seas nitrogen and phosphorus inputs appear to be increasing in a number of catchments. The reason for these apparent increases is unclear and will be subject to further investigation.
Figure 3. Trends in total inputs of nitrogen (N, kt y-1) and phosphorus (P, kt y-1) to the Greater North Sea and Celtic Seas. Trendlines are shown only where trends are significant using Mann-Kendall analysis. Inputs were not adjusted for water flow rates and riverine contributions may be higher in years with more rainfall.
Overall confidence in the assessment methodology and in the data is moderate (see also OSPAR Commission, 2017). Confidence in results is reduced when analysis is performed at smaller spatial scales due to associated reduced amount of data available.
Further information
Total waterborne nitrogen inputs to both the Greater North Sea and the Celtic Seas are dominated (60 - 90 %) by riverine inputs which show no trends from 1990 to 2014 (Figure 4, Table 1). Nitrogen inputs from sewage and industry are considerably lower than from rivers and have reduced significantly since 1990 (Figure 4). In the Greater North Sea, these reductions are insufficient to reduce total loads (Figure 3, Table 2).
Figure 4: Trends in inputs of nitrogen (N, left hand column) and phosphorous (P, right hand column) from rivers (top), industry (centre) and sewage (bottom). Trendlines are shown only where trends are significant using Mann Kendall analysis (see Table 1). Inputs were not adjusted for water flow rates. Note changes of scale between plots.
Table 1. Mann-Kendall (MK) results for rivers, sewage and industry (kt y-1) in the Greater North Sea and the Celtic Seas. The sign of the Mann-Kendall statistic gives the direction of the trend. Where p-values are greater than 0.05, it is assumed that there is no significant trend. * indicates significant trends. n = number of years with data. Inputs were not adjusted for water flow rates.
|
|
Dissolved Inorganic Nitrogen |
Dissolved Inorganic Phosphorus |
||||
|
|
n |
MK Statistic |
p-value |
n |
MK Statistic |
p-value |
|
Greater North Sea |
25 |
|
|
25 |
|
|
|
River |
25 |
16 |
0.73242 |
25 |
-144* |
6 x 10-4 |
|
Sewage |
25 |
-164* |
8 x 10-5 |
25 |
-244* |
1 x 10-5 |
|
Industry |
25 |
-212* |
1 x 10-5 |
25 |
62 |
0.15645 |
|
Celtic Seas |
|
|
|
|
|
|
|
River |
25 |
-50 |
0.254 |
25 |
-150* |
3 x 10-4 |
|
Sewage |
25 |
-188* |
1 x 10-5 |
25 |
-210* |
1 x 10-5 |
|
Industry |
25 |
-244* |
1 x 10-5 |
25 |
-195* |
1 x 10-5 |
Table 2. Mann-Kendall (MK) results for total nutrient inputs (kt y-1) in the Greater North Sea and the Celtic Seas. The sign of the Mann-Kendall statistic gives the direction of the trend. Where p-values are greater than 0.05, it is assumed that there is no significant trend. * indicates significant trends. n = number of years with data. Inputs were not adjusted for water flow rates.
|
Region |
Dissolved Inorganic Nitrogen |
Dissolved Inorganic Phosphorus |
||||
|
|
n |
MK Statistic |
p-value |
n |
MK Statistic |
p-value |
|
GNS |
25 |
-44 |
0.31903 |
25 |
-182* |
0.00001 |
|
CS |
25 |
-128* |
0.00240 |
25 |
-236* |
0.00001 |
Total phosphorus inputs from 1990 to 2014 are generally dominated (50 – 60 %) by riverine inputs. In the Greater North Sea, reductions are observed in inputs via rivers and sewage. In the Celtic Seas, all inputs have reduced significantly since 1990 (Figure 3, Table 1). These reductions have resulted in significantly reduced total loads in the Greater North Sea and Celtic Seas (Figure 3, Table 2).
Total waterborne inputs of nitrogen and phosphorus are approximately three times higher in the Greater North Sea than in the Celtic Seas (Table 3). The higher inputs are likely to reflect differences in agriculture (such as riverine loads) and population densities (such as sewage loads).
Table 3. Annual mean area-specific nitrogen and phosphorus inputs for the UK portions of the Greater North Sea and the Celtic Seas (1990-2014).
|
|
Area (x 1000 km2) |
Annual Mean Area-Specific Inputs (tonnes/km2) |
|
|
Nitrogen |
Phosphorus |
||
|
Greater North Sea |
265 |
0.80 |
0.06 |
|
Celtic Seas |
471 |
0.28 |
0.02 |
The use of smaller assessment regions, such as regional seas and/or river catchment areas provides greater detail on spatial and temporal differences in waterborne nutrient inputs. For the third application of the OSPAR Common Procedure (OSPAR Commission, 2013), nutrient inputs were calculated by regions which are broadly equivalent to regional seas (Greater North Sea: northern North Sea, southern North Sea, English Channel. Celtic Seas: Celtic Sea, Irish Sea, Atlantic; see Figure 2, Table 4). Trends were summarised by the estimated yearly percentage change in loadings between 1990 and 2014.In regional seas, riverine data (1990-2014) show that nitrogen and phosphorus inputs were highest and most variable in the southern North Sea (Figures 5 and 6). Apart from nitrogen inputs to the northern North Sea, all trends are linear (Figure 5). Riverine results for nitrogen inputs show significant downward trends in the Atlantic and northern North Sea, but not in any of the other regional seas (Table 4). However, riverine results for phosphorus inputs show significant downward trends in all regional seas apart from in the northern North Sea (Table 4).
Table 4. Estimated percentage annual change in riverine inputs of dissolved inorganic nitrogen and phosphorus to UK regional seas over the 25-year period (1990 to 2014). Negative numbers indicate downward trends, positive numbers indicate upward trends. * indicates that the change is significant at the 5% level. Riverine inputs were not adjusted for water flow rates.
|
|
Nitrogen |
Phosphorus |
|
Atlantic |
-2.6* |
-1.8* |
|
Celtic Sea |
0.2 |
-3.9* |
|
Channel |
1.3 |
-2.5* |
|
Irish Sea |
-0.4 |
-2.1* |
|
North Sea North |
-1.1* |
-1.3 |
|
North Sea South |
0.7 |
-4.2* |
Figure 5. Trends in total riverine inputs of nitrogen (N, Kt) to regional seas. Note changes in scale between plots.
Figure 6. Trends in total riverine inputs of phosphorus (P, Kt) to regional seas. Note changes in scale between plots.
Results for riverine inputs to the regional seas agree broadly with results for the Greater North Sea and Celtic Seas, that is predominantly no trends or reductions in nitrogen inputs and decreasing trends in phosphorus inputs. In the Greater North Sea and Celtic Seas nitrogen and phosphorus inputs appear to be increasing in a number of individual catchments. The reason for these apparent increases is unclear and will be subject to further investigation.
Conclusions
Inputs of total nitrogen via waterborne sources show no trends in the Greater North Sea and significant reductions in the Celtic Seas. In both regions, the dominant inputs of nitrogen are via rivers, which show no trends in inputs since 1990. Nitrogen inputs from industry and sewage sources have reduced but, in the Greater North Sea, these reductions do not influence total inputs of nitrogen.
Inputs of total phosphorus via waterborne sources have been reduced in both the Greater North Sea and in the Celtic Seas. The dominant inputs are generally via rivers. In the Greater North Sea, reductions in riverine and sewage inputs have contributed to overall reductions in total phosphorus inputs. In the Celtic Seas, reductions are observed in riverine, sewage and industry sources. In the Greater North Sea and Celtic Seas, nitrogen and phosphorus inputs appear to be increasing in some individual catchments. The reason for these apparent increases is unclear and will be subject to further investigation.
Measures are in place to reduce nutrient inputs to areas identified as eutrophication problem areas. Reduced nutrient inputs to the UK-portion of the Greater North Sea and Celtic Seas contribute to the achievement of the UK targets for nutrient concentrations in the sea.
Knowledge gaps
This indicator is well developed and there are no major knowledge gaps.
In both the Greater North Sea and Celtic Seas, nitrogen and phosphorus inputs appear to be increasing in several individual catchments. The reason for these apparent increases is unclear and will be subject to further investigation with the national Environment Agencies responsible for measures.
References
Barry J and Maxwell D (2015) ‘Tools for environmental and ecological survey design and analysis’ (viewed on 31 October 2018)
Defra (2010) ‘Charting Progress 2. The State of UK seas’ (viewed on 27 July 2018)
European Commission (1991) ‘Council Directive 91/271/EEC of 21 May 1991 concerning urban waste-water treatment’ Official Journal of the European Union L 135, 30.5.1991, pages 40–52 (viewed on 8 October 2018)
European Commission (2000) ‘Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for Community action in the field of water policy’ Official Journal of the European Union L 327, 22.12.2000, pages 1–73 (viewed on 8 October 2018)
European Commission (2008) ‘Directive 2008/56/EC of the European Parliament and of the Council of 17 June 2008 establishing a framework for community action in the field of marine environmental policy (Marine Strategy Framework Directive)’ Official Journal of the European Union L 164, 25.6.2008, pages 19-40 (viewed on 21 September 2018)
Kendall MG (1975) ‘Rank correlation methods’, 4th ed., Charles Griffith: London.
HM Government (2012) ‘Marine Strategy Part One: UK initial assessment and Good Environmental Status’ (viewed on 10 October 2018)
HM Government (2015) ‘Marine Strategy Part Three: UK Programme of Measures’ (viewed on 5 July 2018)
Mann HB (1945) ‘Non-parametric tests against trend’. Econometrica: 13, pages 245-259 (viewed on 9 October 2018)
OSPAR Commission (2013) ‘Common Procedure for the Identification of the Eutrophication Status of the OSPAR Maritime Area’ OSPAR Agreement 2013-8, supersedes Agreements 1997-11, 2002-20 and 2005-3 (viewed on 12 October 2018)
OSPAR Commission (2017) ‘Eutrophication Status of the OSPAR Maritime Area: Third Integrated Report on the Eutrophication Status of the OSPAR Maritime Area’ Third Common Procedure Task Team of the Intersessional Correspondence Group on Eutrophication. Publication Number: 694/2017 (viewed on 8 October 2018)
Painting S, Garcia L, Collingridge K (2016) ‘Common Procedure for the Identification of the Eutrophication Status of the UK Maritime Area’ (viewed on 8 October 2018)
Acknowledgements
Assessment metadata
| Assessment Type | UK Marine Strategy Framework Directive Indicator Assessment |
|---|---|
D5 | |
Eutrophication | |
| Point of contact email | marinestrategy@defra.gov.uk |
| Metadata date | Monday, October 1, 2018 |
| Title | Nutrient Inputs |
| Resource abstract | UK riverine inputs and direct discharges data from 1990 to 2014 were obtained from the Marine Environment Monitoring and Assessment National database (MERMAN) and submitted to the Riverine Inputs and Direct Discharges OSPAR programme. For the purposes of this assessment, inputs of dissolved nitrogen and phosphorus to the UK-portion of the Greater North Sea and the Celtic Seas were calculated as total inputs (loads), as well as separately for riverine, industrial and sewage sources. |
| Linkage | Painting S and others (2016) UK National Report. Common Procedure for the Identification of the Eutrophication Status of the UK Maritime Area (viewed 12 October 2018) |
| Conditions applying to access and use | © Crown copyright, licenced under the Open Government Licence (OGL). and |
| Assessment Lineage | For the third national application of the OSPAR Common Procedure (Painting and others, 2016), nutrient inputs were calculated by UK regional seas. In the Greater North Sea, these are the northern North Sea, the southern North Sea and the eastern English Channel. In the Celtic Seas, these are the western Channel and Celtic Sea, the Irish Sea, Minches and western Scotland, the Scottish Continental Shelf, and the Atlantic North-West Approaches. Trends in inputs of riverine nitrogen and phosphorus were assessed by fitting a smoother to the log loads, and assuming the errors were normally distributed and correlated with an AR1 structure. The trends were summarised by the estimated yearly percentage change in loadings between 1990 and 2014, obtained from the fitted slope (if a linear model was adequate) or the fitted values in 1990 and 2014 (if there was a nonlinear trend). Significance was determined by the significance of the fitted slope (linear trend) or a Wald test comparing the fitted values in 1990 and 2014 (nonlinear trend). Inputs to each Paris Commission area were also analysed, and trends were assessed using Mann-Kendall tests. |
| Dataset metadata | please see nutrient inputs assessment metadata sheet reference HASEC17/D504 https://oap.ospar.org/en/ospar-assessments/intermediate-assessment-2017/pressures-human-activities/eutrophication/nutrient-inputs/ |
| Dataset DOI | Collingridge et al (2019). Data for UK OSPAR Common Procedure - Eutrophication Assessment Cefas, UK. V1. doi: https://doi.org/10.14466/CefasDataHub.79 |
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Recommended reference for this indicator assessment
Suzanne Painting1, Kate Collingridge1, Luz Garcia1, Jon Barry1, Simon Leaf2, Mike Best2, Alison Miles2, Michael McAliskey3, Mark Charlesworth4, Lucie Haines4, Rob Fryer5, Pamela Walsham5, Lynda Webster5, Eileen Bresnan5, Ashley Roberts6, Clare Scanlan6 and Clemens Engelke6 2018. Nutrient inputs in water and air. UK Marine Online Assessment Tool, available at: https://moat.cefas.co.uk/pressures-from-human-activities/eutrophication/nutrient-inputs/
1Centre for Environment, Fisheries and Aquaculture Science
2Environment Agency
3Department of Environment, Agriculture & Rural Affairs, Northern Ireland
4Natural Resources Wales
5Marine Scotland
6Scottish Environment Protection Agency