Nutrients in freshwater in Europe

Nutrient conditions in European surface waters have improved in recent decades. Average concentrations of nitrate and phosphate in rivers and total phosphorus in lakes have decreased. The decrease in nutrient concentrations is mainly related to improvements in wastewater treatment and the reduction of phosphorus in detergents. However, there has been a clear tendency for surface water concentrations to level off in recent years, in particular phosphorus in rivers. There has been little change in groundwater nitrate concentration in the past few decades.

Figure 1. Nutrient trends in European water bodies

Nitrate in groundwater

The average nitrate concentration in European groundwater decreased in the first few years after 1992, with little change since then (Figure 1). The shorter, but more spatially representative time series starting in 2007 indicates a decrease, although variability is large. Agricultural activities, such as over-use of fertiliser, is the main driver for nitrate in groundwater.

Nitrate in rivers

The average nitrate concentration in European rivers decreased steadily from 1992 but has levelled off in recent years. The shorter time series is parallel to the longer series, yet the concentration level is slightly higher. Agriculture remains the main contributor to nitrogen pollution, however, measures taken nationally and under the EU Nitrates Directive have contributed to lower concentrations. The apparent stabilisation in recent years calls for further measures.

Phosphate in rivers

The average phosphate concentration in European rivers more than halved over the period 1992-2012. From 2013, there is a tendency of an increase, more pronounced for the shorter time series. Variability is large, but the clear lack of a further decrease indicates a need for measures. The overall decrease in river phosphate can be related to measures introduced by national and European legislations, e.g. the Urban Waste Water Treatment Directive. Further, the change to phosphate-free detergents has contributed to lower phosphate concentrations.

Total phosphorus in lakes

The average total phosphorus concentration in European lakes has decreased since 1992, with an apparent stabilisation in recent years. The shorter time series shows a higher concentration level, but with a larger variability. As urban wastewater treatment has improved, phosphorus from detergents has been reduced, and many wastewater outlets have been diverted away from lakes, phosphorus from point sources has become less significant. However, diffuse run-off from agricultural land continues to be a major phosphorus source in European lakes. Phosphorus stored in sediment can keep lake concentrations high despite a reduction in inputs.

Overall, the economic crisis in central and Eastern European countries during the 1990s also contributed to decreasing pollution from manufacturing industries.

Figure 2. Nitrate levels in rivers in European countries (2018-2023)

Rivers that drain land with intense agriculture or a high population density generally have the highest nitrate concentrations, although slow-flowing, lowland rivers can have naturally higher levels. For the period 2018-2023, average nitrate concentrations exceeded 2.93mgNO₃-N/l in more than 50% of water bodies in Belgium and Denmark, and 40-50% in Czechia, Germany and Lithuania.

Statistical trend analysis (see Annex table RW nitrate) shows a significant decrease in nitrate concentrations since 1992 in 59% of the river water bodies shown in Figure 1, and a significant increase in 22%. The results were similar for the time series starting in 2007 (53% increasing and 23% decreasing trends).

The largest proportions of significantly decreasing trends since 2007 (> 80%) were in Croatia, Cyprus, Denmark, Germany and Slovenia. Lithuania had the highest proportion of increasing trends (83%). Average country time series contributing to the observed stabilisation in recent years (Figure 1) are mainly those where concentrations decline at the beginning and then stabilise (Czechia, Italy, Poland), and those with an increase (Estonia, Latvia, Lithuania).

Supporting information

Definition

This indicator shows concentrations of phosphate and nitrate in rivers, total phosphorus in lakes and nitrate in groundwater bodies. The indicator can be used to illustrate geographical variations in current nutrient concentrations and temporal trends. Large inputs of nitrogen and phosphorus to water bodies from wastewater and agricultural areas can lead to eutrophication. This causes ecological damage resulting in loss of aquatic biodiversity (reduction in ecological status) and negative impacts on the use of water for human consumption and recreation.

Methodology

A detailed methodology description is given in a separate document.

The indicator analysis is based on annual mean concentrations per monitoring site. The annual mean concentrations are calculated from sample data, unless the country has reported annual data only. The data undergo automatic and manual quality checks to remove errors and outliers.

Monitoring site time series from the same water body are combined and gap filled before further analysis. Gap filling is necessary to ensure that the average European time series represents the same water bodies every year. Water body time series are only included in the analysis if they have data within the first and last five years of the time range and data from at least 40% of the years in the time range. Two different time ranges are analysed: From 1992 for long-term trends and from 2007 for better spatial coverage (more water bodies meet the selection criteria).

Data are aggregated from site to water body level and gap-filled using generalised additive models (GAM) in the free software R, using the mgcv package. For water bodies with more than one site, a GAM model with concentration as response variable, year as fixed effect and site as random effect is used to estimate annual concentrations per site. Annual values for the water body are subsequently estimated by averaging the annual site predictions. For water bodies with only one site, or where the random effect for site is non-significant, a GAM without a random site effect is used to estimate annual values and for gap filling. The gap-filled water body time series are averaged by country and a weighted average of the country time series (weighting by the total length of rivers or total area of lakes/groundwater in the country) is calculated to give the time series for Europe. A 95% confidence interval for the weighted average is calculated using of the formula for standard error of a weighted mean recommended in Gatz and Smith (1995).

Trends are analysed with the Mann-Kendall method in the free software R, using the wql package. This is a non-parametric test suggested by Mann (1945) and has been extensively used for environmental time series. Mann-Kendall is a test for a monotonic trend in a time series y(x), which in this analysis is nutrient concentration (y) as a function of year (x). The size of the change is estimated by calculating the Sen slope (Theil, 1992; Sen, 1968). Absolute and relative Sen slopes are summarised across Europe and countries by averaging. The same water body time series as for the aggregated time series plots are used, but without the gap-filled values. Note that the smoothed GAM output makes it more likely to get significant trends.

In the present state analysis, the concentration per river water body is calculated as the average of available annual mean concentrations for the last six years. Any water body with at least one data point within the last six years can be used, giving far more water bodies than in the time series analysis. The water bodies are assigned to quintile classes based on the distribution of the 6-year average concentrations, and the results are summarised per country as percentage of water bodies per quintile class. The purpose of the analysis is to compare the distribution of concentrations among countries. Note that natural concentration levels vary between regions, so lower proportion of water bodies in the lowest concentration classes does not necessarily imply higher anthropogenic pollution pressure.

Policy/environmental relevance

Freshwater quality with respect to eutrophication and nutrient concentration is an objective of several directives and other policies: the Nitrates Directive (91/676/EEC); the Urban Waste Water Treatment Directive (91/271/EEC); the Industrial Emissions Directive (2010/75/EU); the Convention on Long-range Transboundary Air Pollution and the National Emission Ceilings Directive (2016/2284/EU); and the Water Framework Directive (2000/60/EC). The Drinking Water Directive (2020/2184) and the Groundwater Directive (2006/118) sets the maximum allowable concentration for nitrate of 50mgNO₃/l.

Reducing nutrient pollution from agriculture is an aspect of the European Green Deal, the ‘Farm to Fork’ Strategy, the Biodiversity Strategy and the Common Agricultural Policy.

Accuracy and uncertainties

Methodology uncertainty

Nutrient conditions vary throughout the year depending on, for example, season and flow conditions. Hence, the annual average concentrations should ideally be based on samples collected throughout the year. Using annual averages representing only part of the year introduces some uncertainty, but it also makes it possible to include more sites, which reduces the uncertainty in spatial coverage. Moreover, the majority of the annual averages represent the whole year. The GAM modelling introduces uncertainty, but this too increases the spatial coverage.

Nitrate concentrations in groundwater originate mainly from anthropogenic activities as a result of agricultural land use. Concentrations in water are the effect of a multidimensional and time-related process, which varies from groundwater body to groundwater body and is less quantified. To properly evaluate the nitrate concentration in groundwater and its development, closely-related parameters such as ammonium and dissolved oxygen should be taken into account.

Data sets uncertainty

The indicator is meant to give a representative overview of nutrient conditions in European rivers, lakes and groundwater. This means it should reflect the variability in nutrient conditions over space and time. Countries are asked to provide long time series without gaps and as complete geographic coverage as possible.

The datasets for groundwater and rivers include almost all countries within the EEA, but the time coverage varies from country to country. The coverage of lakes is less good. It is assumed that the data from each country represents the variability in space in their country. Likewise, it is assumed that the sampling frequency is sufficiently high to reflect variability in time. In practice, the representativity will vary between countries, and this introduces uncertainty to the analysis.

Each annual update of the indicator is based on the updated dataset. This also means that due to changes in the database, the derived results of the assessment vary in comparison to previous assessments. Such changes include changes in the quality procedure that excludes or re-includes individual sites or samples and retroactive reporting of data for the past periods, which may re-introduce lost time series that were not used in the recent indicator assessments.

Waterbase contains a large amount of data collected over many years. Ensuring the quality of the data has always been a high priority. Still, suspicious values or time series are sometimes detected and the automatic QC routines exclude some of the data. Through the communication with the reporting countries, the quality of the database can be further improved.

There is uncertainty in the spatial data used for calculating the weighted European average (WISE Spatial) related to the delineation of water bodies, especially for rivers. E.g. countries including a finer network of rivers in their river water bodies (and thus larger total length) will get a higher weight in the European average.

Rationale uncertainty

Using annual average values provides an overview of general trends and geographical patterns in line with the aim of the indicator. However, the severity of shorter-term, high-nutrient periods are not reflected.

Data sources and providers

Metadata

DPSIRTopicsTagsTemporal coverageGeographic coverage

TypologyUN SDGsUnit of measure

The concentration of nitrate is expressed as milligrams of nitrate per litre (mgNO₃/l) for groundwater and milligrams of nitrate-nitrogen per litre (mgNO₃-N/l) for rivers.

The concentration of phosphate in rivers is expressed as milligrams of phosphate-phosphorus per litre (mgPO₄-P/l) and total phosphorus in lakes is expressed as milligrams of phosphorus per litre (mgP/l)

The river sites are assigned to different concentration classes to visualise the distribution (percentage) of data in the dataset.

Frequency of dissemination

Nutrients in freshwater in Europe

Figure 1. Nutrient trends in European water bodies

Figure 2. Nitrate levels in rivers in European countries (2018-2023)

Supporting information

Metadata

References and footnotes