There has been no increase in the concentration of dissolved inorganic nitrogen in the coastal and offshore waters of the Greater North Sea and Celtic Seas since 1990. Significant decreases have been observed in coastal waters in both regions. UK targets for nutrient concentrations in the sea have been met.

Background

UK Targets on nutrient levels

The UK target for eutrophication ‘non-problem areas’ is that there should be no increase in the assessed dissolved inorganic nitrogen and phosphorus concentrations resulting from anthropogenic nutrient input based on data from periodic surveys. The target for ‘problem areas’ is a downward trend in dissolved inorganic nitrogen and phosphorus concentrations, resulting from decreasing anthropogenic nutrient input over a 10-year period.

Key pressures and impacts

In the UK Initial Assessment (HM Government, 2012) the key pressures associated with this indicator were the riverine and direct inputs of nutrients (nitrogen and phosphorus) from various industrial and diffuse sources and from atmospheric deposition. These are still relevant but, considering that the small number of eutrophication problem areas are generally limited to coastal waters, small 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 the Water Framework Directive (European Commission, 2000) River Basin Management Plans, 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.

Monitoring and assessment

Risk based monitoring of nutrients is carried out under the Clean Seas Environment Monitoring Programme and assessments follow OSPAR guidance and procedures (OSPAR Commission, 2013). Mann-Kendall tests are used to analyse trends.

Regional cooperation

The UK regularly reports information on nutrient concentrations to OSPAR and has contributed to the work associated with the monitoring and assessment of the OSPAR nutrient common indicators. The UK results have been used in the OSPAR Intermediate Assessment 2017 (OSPAR Commission, 2017).

Further information

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 (European Commission, 1991b), 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 death of marine organisms. Identifying causal links between disturbances and nutrient enrichment is often difficult and complicated by other pressures. Cumulative effects of global change, 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 (see Figure 2). 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 (Painting and others, 2016).

Three stages in the identification of eutrophication. The criteria marked 1 are common indicators for the OSPAR Intermediate Assessment 2017.

Figure 1. Three stages in the identification of eutrophication. The criteria marked 1 are common indicators for the OSPAR Intermediate Assessment 2017. The criteria marked 2 are not relevant in all Contracting Parties’ waters.

Assessment method

Winter nutrient concentrations in marine waters (salinity > 30) were analysed for the Greater North Sea (OSPAR Region II) and Celtic Seas (OSPAR Region III), and for the sub-regions/regional seas within the Greater North Sea and Celtic Seas.  For the Greater North Sea, sub-regions are the northern North Sea, the southern North Sea and the eastern English Channel. For the Celtic Seas, sub-regions include the western Channel and Celtic Sea, the Irish Sea, Minches and western Scotland, the Scottish Continental Shelf and the Atlantic North-West Approaches.

Winter nutrient concentrations were analysed using data reported as dissolved inorganic nitrogen (nitrate + nitrite + ammonium), dissolved total oxidised nitrogen (nitrate + nitrite) and dissolved inorganic phosphorus. Data on both dissolved inorganic phosphorus and total oxidised nitrogen were used as total oxidised nitrogen is measured more frequently than dissolved inorganic nitrogen, for example, in the Scottish Continental Shelf region and by in situ nutrient analysers on SmartBuoys. Where data were available for both parameters, linear regressions indicate a close relationship between dissolved inorganic nitrogen and total oxidised nitrogen, suggesting that total oxidised nitrogen may be used as a proxy for dissolved inorganic nitrogen in most marine waters.

Data on dissolved nutrients and all supporting information (for example, latitude, longitude, water column depth, sample depth, temperature, salinity) were obtained from 1990 until 2014 (Figure 2). Data were obtained from  the International Council for the Exploration of the Seas database and from national (the UK Marine Environment Monitoring and Assessment National database (MERMAN), National Oceanographic Database) and institutional databases (Cefas Sapphire and SmartBuoy, Marine Scotland Science). Duplicates between databases were removed. Data were averaged over the whole water column for each cruise station and day, with the exception of the UK Marine Environment Monitoring and Assessment National Database data where date and time were used instead of day as there were multiple records in different locations (along transects) for the same station and day. Continuous data from SmartBuoys were averaged over 7-day intervals to be equivalent to survey-based data (see Capuzzo and others, 2015), these averages were included in the final data set. Data were filtered by salinity, to assign to coastal waters (salinity 30 - <34.5 in all regions, except in the Irish Sea, 30 - <34) or offshore waters (salinity >34.5 in all regions, except in the Irish Sea, salinity >34), and by season for nutrients (winter, November to February). Mean winter values were assigned to the year relevant to the phytoplankton growing season. For example, nutrient data for November and December of 2009 and January and February of 2010 were reported as 2010 winter nutrients.

Map showing where data on winter nutrient concentrations were available, 1990 to 2014. (i) dissolved inorganic nitrogen, (ii) total oxidised nitrogen.

Figure 2. Map showing where data on winter nutrient concentrations were available, 1990 to 2014. (i) dissolved inorganic nitrogen, (ii) total oxidised nitrogen. X = sampling locations at coastal (red) and offshore (blue) sites. Locations of Cefas SmartBuoy moorings and benthic landers, and Scottish coastal monitoring sites are shown. Active moorings in the southern North Sea (West Gabbard, the Warp, Dowsing) and Liverpool Bay are indicated by red triangles. Other moorings and landers are currently inactive. In the North-West Approaches, data were available from 1994-1997 only.

Mean winter nutrient concentrations were plotted per year to identify trends in the data. Where fewer than five data points were available in any given year in an assessment area, these data were excluded in order to improve the robustness of the analysis. 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, weighted least squares regression lines were calculated, using the 95% confidence intervals for the means.

The representativeness of the available data in space and time was calculated by following an approach similar to that described in the OSPAR guidance (OSPAR Commission, 2013). Overall confidence in representativeness is provided by the worst score in either space or time. For further details, see the UK National Report 2017 (Painting and others, 2016).

Within each regional sea, estuarine and coastal water bodies within 1 nautical mile of baseline (3 nautical miles in Scotland) were assessed under the Nitrates Directive (European Commission, 1991b), Urban Waste Water Treatment Directive (European Commission, 1991a) and Water Framework Directive (European Commission, 2000). These assessment methods are comparable with those used for OSPAR and Marine Strategy Framework Directive assessments. Assessment outcomes are used as the basis for reporting on eutrophication status.

Results

Findings from the 2012 UK Initial Assessment

The UK Initial Assessment (HM Government, 2012) did not provide a separate assessment of this indicator but described an overall assessment of eutrophication status and evidence that, over time, inputs of nutrients to the marine environment are generally decreasing.

Latest findings

Status assessment

In line with the UK targets for nutrient concentrations, we have only assessed trends for this indicator. Status assessments are carried out through the application of the OSPAR Common Procedure (OSPAR Commission, 2013) where nutrient concentration is one of several indicators used together to diagnose eutrophication status.

Trend assessment

Trend analysis (1990-2014) indicates significant decreases in winter dissolved inorganic nitrogen concentrations in coastal waters in the Greater North Sea and Celtic Seas. In offshore waters of both regions, no significant trends were observed. These findings indicate that the target for trends has been met.

Further information

Greater North Sea and Celtic Seas

Assessment of trends

Concentrations of mean winter dissolved inorganic nitrogen (Figure 3) were higher in coastal waters compared to offshore waters in the Greater North Sea and Celtic Seas. Mann-Kendall statistics (p <0.05, Table 1) showed significant decreases in winter dissolved inorganic nitrogen concentrations in coastal water in both the Greater North Sea and Celtic Seas, and no trends in offshore waters.

Trends in concentrations of mean winter dissolved inorganic nitrogen (µM) in the Greater North Sea and Celtic Seas from 1990 to 2014 in coastal (blue symbols) and offshore (orange symbols) water.

Figure 3. Trends in concentrations of mean winter dissolved inorganic nitrogen (µM) in the Greater North Sea and Celtic Seas from 1990 to 2014 in coastal (blue symbols) and offshore (orange symbols) water. Trendlines are shown where trends are significant (see Table a). Results are shown for years with five or more data points. Error bars indicate 95% confidence intervals. Obs = observations.

Table 1. Results of Mann-Kendall (MK) analyses for concentrations of mean winter dissolved inorganic nitrogen (µM) in coastal and offshore waters 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 less than 0.05, it is assumed that there is a significant trend. n = number of years with data.

Region

Coastal waters

Offshore waters

 

n

MK Statistic

p-value

n

MK Statistic

p-value

GNS

23

-3.0643

0.0022

25

-0.8175

0.4136

CS

20

-2.3035

0.0212

21

-1. 6613

0.0966

Sub-regions of the Greater North Sea and Celtic Seas

The UK sub-regions/regional seas of the Greater North Sea are the northern North Sea (Region 1), southern North Sea (Region 2), and eastern English Channel (Region 3). The sub-regions/regional seas of the Celtic Seas include the western Channel and Celtic Sea (Region 4), the Irish Sea (Region 5), Minches and western Scotland (Region 6), the Scottish Continental Shelf (Region 7), and the Atlantic North-West Approaches (Region 8).

Assessment of trends

Time series of mean winter dissolved inorganic nitrogen (Figure 4) show that concentrations were highest in coastal waters in the southern North Sea, the western channel and Celtic Sea and the Irish Sea. Time series plots also indicate considerable inter-annual variability in the data. Similar results were observed in the mean winter total oxidised nitrogen data (data not shown).

Trends in mean winter dissolved inorganic nitrogen (DIN) (µM) in Regional Seas 1 to 8, 1990 to 2014, in coastal (blue symbols) and offshore (orange symbols) water.

Figure 4. Trends in mean winter dissolved inorganic nitrogen (DIN) (µM) in Regional Seas 1 to 8, 1990 to 2014, in coastal (blue symbols) and offshore (orange symbols) water. Trendlines are shown where trends are significant (Tables c and d). Results are shown for years with five or more data points. Obs = observations. Error bars indicate 95% confidence intervals. See Table b for names of each regional sea/sub-region. Lines indicate assessment levels used as part of the overall OSPAR Common Procedure Assessment of eutrophication status.

Results of Mann-Kendall trend assessments for dissolved inorganic nitrogen in coastal and offshore waters in each sub-region are shown in Tables 2 to 4. These results show significant decreases in winter dissolved inorganic nitrogen concentrations in coastal water in the southern North Sea and Irish Sea, and in offshore waters in the western Channel and Celtic Sea. No other significant trends were observed.

Table 2. Summary of the trends obtained by applying Mann-Kendall analyses (1990-2014) to mean winter nutrient concentrations. ↘ = decreasing trend, ↗ = increasing trend, - = no trend. nan = no data. N/A = not applicable. Trends are statistically significant at the 95% level (α=0.05).

Region

Winter DIN

Winter TOxN

 

 

Coast

Offshore

Coast

Offshore

1

Northern North Sea

-

-

-

-

2

Southern North Sea

-

-

-

3

English Channel

-

-

-

-

4

Western Channel & Celtic Sea

-

-

-

5

Irish Sea

-

6

Minches & W Scotland

-

-

-

-

7

Scottish Continental shelf

-

-

-

8

Atlantic North-West Approaches

N/A

nan

N/A

nan

Table 3. Results of Mann-Kendall (MK) analyses for mean winter dissolved inorganic nitrogen (µM) in coastal areas. The sign of the Mann-Kendall statistic gives the direction of the trend. Where p-values are less than 0.05, it is assumed that there is a significant trend. n = number of years with data. See Table 2 for names of each regional sea/sub-region.

Regional Sea

n

MK Statistic

p-value

1

19

0.4898

0.6243

2

22

-2.3693

0.0178

3

4

-0.3536

0.7237

4

10

-1.61

0.1074

5

18

-2.3484

0.0189

6

13

-0.4276

0.6689

7

10

0.8944

0.3711

 

Table 4. Results of Mann-Kendall (MK) analyses for mean winter dissolved inorganic nitrogen (µM) in offshore areas. The sign of the Mann-Kendall statistic gives the direction of the trend. Where p-values are less than 0.05, it is assumed that there is a significant trend. n = number of years with data. See Table 2 for names of each regional sea/sub-region.

Regional Sea

n

MK Statistic

p-value

1

25

1.0044

0.3152

2

24

-0.0248

0.9802

3

15

-0.3961

0.6921

4

13

-2.2601

0.0238

5

15

-1.6833

0.0923

6

10

0.8944

0.3711

7

16

 

0.1627

8

 

 

 

Mann-Kendall trend assessments for total oxidised nitrogen (Table 5 and Table 6) show different results to dissolved inorganic nitrogen in that trends in coastal waters in the southern North Sea were not significant, possibly due to higher inter-annual variability in the data, particularly at the start of the time series. Total oxidised nitrogen trends indicate a significant decrease in offshore waters in more regions than does dissolved inorganic nitrogen - in addition to the western Channel and Celtic Sea, total oxidised nitrogen shows significant downward trends in the Irish Sea and on the Scottish Continental shelf.

Table 5. Results of Mann-Kendall (MK) analyses for mean winter total oxidised nitrogen (µM) in coastal areas. The sign of the Mann-Kendall statistic gives the direction of the trend. Where p-values are less than 0.05, it is assumed that there is a significant trend. n = number of years with data. See Table 2 for names of each regional sea/sub-region.

Regional Sea

n

MK Statistic

p-value

1

21

1.4801

0.1388

2

25

-1.4715

0.1412

3

4

-0.3536

0.7237

4

10

-1.61

0.1074

5

22

-3.0462

0.0023

6

15

0.198

0.8430

7

14

0.6576

0.5108

Table 6. Results of Mann-Kendall (MK) analyses for mean winter total oxidised nitrogen (µM) in offshore areas. The sign of the Mann-Kendall statistic gives the direction of the trend. Where p-values are less than 0.05, it is assumed that there is a significant trend. n = number of years with data. See Table 2 for names of each regional sea/sub-region.

Regional Sea

n

MK Statistic

p-value

1

25

0.6774

0.4982

2

25

1.4248

0.1542

3

17

1.0301

0.3030

4

17

-1.8542

0.0637

5

22

-2.2

0.0278

6

11

1.4013

0.1611

7

21

-2.0238

0.0430

8

 

 

 

The representativeness of dissolved inorganic nitrogen and total oxidised nitrogen data in time and space is given in Table 7. In the English Channel and western Channel and Celtic Seas, the representativeness of the dissolved inorganic nitrogen data was lower (<40%) than elsewhere. Representativeness of total oxidised nitrogen data was generally the same as or better than for dissolved inorganic nitrogen. For dissolved inorganic nitrogen, representativeness was lowest in time in Regions 2, 3, 4 and 6 (data not shown).

Table 7. Spatial and temporal representativeness of available data (%) for dissolved inorganic nitrogen and total oxidised nitrogen in each of the sub-regions. Note: these analyses use all data in an assessment region and do not consider coastal and offshore waters separately.

Assessment region

Dissolved Inorganic Nitrogen

Total Oxidised Nitrogen

1

80.30

80.30

2

72.54

83.30

3

39.75

35.82

4

38.78

61.35

5

88.56

88.83

6

91.47

94.68

7

62.96

62.96

8

x

x

Conclusions

Trend assessments indicate significant decreases in winter dissolved inorganic nitrogen concentrations in the coastal waters of the Greater North Sea and Celtic Seas. No trends are observed in offshore waters. These assessments indicate that UK targets for nutrient concentrations in the sea have been met.

Further information

In sub-regions of the Greater North Sea and Celtic Seas, winter nutrient concentrations show inter-annual variability and are highest in coastal waters in the southern North Sea, the western channel and Celtic Sea, and the Irish Sea. Winter dissolved inorganic nitrogen concentrations showed significant trends (downward) in coastal water (salinity ≥ 30) in the southern North Sea and the Irish Sea and in offshore waters in the western Channel and Celtic Sea, but not in any other region. Winter total oxidised nitrogen concentrations did not show significant trends in coastal waters in the southern North Sea. In offshore waters, however, significant downward trends were observed in the Irish Sea and on the Scottish Continental Shelf. The overall representativeness of nutrient data in time and space was good (> 60 %) in all sub-regions, except for the English Channel and western Channel and Celtic Sea (< 40 %).

Knowledge gaps

This indicator is well developed, and there are no major knowledge gaps.

References

Barry J and Maxwell D (2015) ‘Tools for environmental and ecological survey design and analysis’ (viewed on 31 October 2018)

Capuzzo E, Stephens D, Silva T, Barry J, Forster RM (2015) ‘Decrease in water clarity of the southern and central North Sea during the 20th century’ Global Change Biology 21: 2206–2214 (viewed 10 October 2018)

European Commission (1991a) ‘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 (1991b) ‘Council Directive 91/676/EEC of 12 December 1991 concerning the protection of waters against pollution caused by nitrates from agricultural sources’ Official Journal of the European Union L 375, 31.12.1991, pages 1–8 (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 21 September 2018)

HM Government (2012) ‘Marine Strategy Part One: UK initial assessment and Good Environmental Status’ (viewed 10 October 2018)

HM Government (2015) ‘Marine Strategy Part Three: UK Programme of Measures (viewed 5 July 2018)

Kendall M (1975) Rank correlation methods, 4th ed. Charles Griffith: London.

Mann, HB (1945) ‘Non-parametric tests against trend’ Econometrica 13: 245-259 (viewed 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 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, ISBN: 978-1-911458-34-0, Publication Number: 694/2017 (viewed on 8 October 2018)

Painting S and others (2016) ‘Common Procedure for the Identification of the Eutrophication Status of the UK Maritime Area’ (viewed 12 October 2018)

Acknowledgements

Assessment metadata
Assessment TypeUK Marine Strategy Framework Directive Indicator Assessment
 

D5

 

Eutrophication

 

In addition to links provided in ‘References’ section above:

European Court of Justice (2009) ‘Judgment of the Court (Third Chamber) of 10 December 2009, European Commission v United Kingdom of Great Britain and Northern Ireland’ Failure of a Member State to fulfil obligations - Environment - Directive 91/271/EEC - Urban waste water treatment - Article 3(1) and (2), Article 5(1) to (3) and (5) and Annexes I and II - Initial failure to identify sensitive areas - Concept of ‘eutrophication’ - Criteria - Burden of proof - Relevant date when considering the evidence - Implementation of collection obligations - Implementation of more stringent treatment of discharges into sensitive areas. Case C-390/07, European Court Reports 2009 I-00214*, ECLI identifier: ECLI:EU:C:2009:765 (viewed on 12 January 2019)

OSPAR Commission (2003) ‘OSPAR integrated report 2003 on the eutrophication status of the OSPAR maritime area based upon the first application of the Comprehensive Procedure’ OSPAR Eutrophication Series, publication 189/2003. OSPAR Commission, London (viewed on 12 January 2019)

OSPAR Commission (2008) ‘Second OSPAR integrated report on the eutrophication status of the OSPAR maritime area’ OSPAR Eutrophication Series, publication 372/2008. OSPAR Commission, London (viewed on 12 January 2019)

OSPAR Commission (2010) ‘The North-East Atlantic Environment Strategy: Strategy of the OSPAR Commission for the Protection of the Marine Environment of the North-East Atlantic 2010-2020’ 27 pages (viewed on 12 January 2019)

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)

Point of contact emailmarinestrategy@defra.gov.uk
Metadata dateMonday, October 1, 2018
TitleNutrient levels; chlorophyll concentrations; concentrations of dissolved oxygen near the seafloor
Resource abstract

Data on primary indicators for eutrophication status (nutrients, chlorophyll and dissolved oxygen) were assessed for the UK sector of the Greater North Sea and the Celtic Seas. Targets have been met for nutrients, chlorophyll and dissolved oxygen concentrations. Problem areas are limited to 21 areas in estuarine and inshore coastal waters.

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 2018 copyright Defra, licenced under the Open Government Licence (OGL).

and

OSPAR Data terms and conditions.

Assessment Lineage

Data were obtained for both OSPAR and Marine Strategy Framework Directive assessments. Data on chlorophyll concentrations, and all supporting information (e.g. latitude, longitude, water column depth, sample depth, temperature, salinity) were obtained from 1990 until 2014. Data were obtained from the ICES database and from national (MERMAN, NODB) and institutional databases (Cefas Sapphire and SmartBuoy databases, Marine Scotland Science). All available data were used, collected using all sampling platforms (e.g. ships and submersible sensors), and analysed by all analytical methods (e.g. fluorometry, spectrophotometry and pigment analysis). Duplicates between databases were removed. Data were averaged over the whole water column for each cruise station and day, with the exception of MERMAN data where datetime was used instead of day as there were multiple records in different locations (along transects) for the same station and day. Continuous data from SmartBuoys were averaged over 7-day intervals to be equivalent to survey based data; these averages were included in the final data set. Data were filtered by salinity, to assign to coastal waters (salinity 30 - <34.5) or offshore waters (salinity >34.5). For the OSPAR Common Procedure, salinity filters for the Irish Sea were 30 - <34 for coastal waters and >34 for offshore waters), and by season for chlorophyll (growing season, March to October inclusive). Chlorophyll 90th percentiles were calculated over the growing season.

The 90th percentiles were plotted per year to identify trends in the data. Where fewer than five data points were available in any given year in an assessment area, these data points were excluded, in order to improve the robustness of the analysis. 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, weighted least squares (WLS) regression lines (see http://statsmodels.sourceforge.net/) were calculated, using the 95% confidence intervals for the means.

For assessments of status (2006-2014), growing season chlorophyll 90th percentiles were compared against assessment thresholds for coastal waters (15 µg l-1) and offshore waters (10 µg l-1, see Foden et al 2011). Determination of the reference values and thresholds used for chlorophyll in coastal and offshore waters is described in Foden et al (2011). For chlorophyll, 90th percentiles during the growing season should be below the assessment thresholds.

Indicator assessment results
Dataset metadata
Dataset DOIContact marinestrategy@defra.gov.uk

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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. Winter nutrient concentration. UK Marine Online Assessment Tool, available at: https://moat.cefas.co.uk/pressures-from-human-activities/eutrophication/nutrient-concentrations/

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