Indicator concept

Relevance of the indicator

Policy Relevance

As a follow-up to the Baltic Sea Action Plan (2007), a revised HELCOM nutrient reduction scheme was adopted in the 2013 HELCOM Ministerial Declaration (HELCOM 2013a) in which reduction requirements for nitrogen inputs to the Baltic Proper, Gulf of Finland and Kattegat and for phosphorus inputs to the Baltic Proper, Gulf of Finland and Gulf of Riga were set. The HELCOM nutrient reduction scheme defines maximum allowable inputs (MAI) of nutrients, which indicate the maximum level of inputs of water- and airborne nitrogen and phosphorus to Baltic Sea sub-basins that can be allowed in order to obtain good environmental status (GES) in terms of eutrophication. This core indicator presents progress in the different Baltic Sea sub-basins towards reaching these maximum annual nutrient inputs levels. 

The progress of countries in reaching their share of the country-wise allocation of nutrient reduction targets (CART) is assessed separately in a follow-up system. Figure 5 illustrates how the nutrient reduction scheme fits into an eutrophication management cycle.

Figure 6.jpg 

Figure 5. The management cycle of the HELCOM Baltic Sea Action Plan.


Reducing the effects of human-induced eutrophication is the stated goal of Descriptor 5 in the EU Marine Strategy Framework Directive (MSFD). The indicator is an important part in following up the effectiveness of the measures taken to achieve GES under this Descriptor. Inputs of nutrients to the Baltic Sea marine environment have an effect on the nutrient levels under criterion 5.1. It is important to note that this pressure indicator on inputs of nutrients relates to HELCOM eutrophication state core indicators. More information on this is provided in the section below on Environmental Target and progress towards GES.

The information provided in this pressure indicator also supports follow-up of the effectiveness of measures implemented under the following agreements, as each of them addresses reduction in nutrient inputs in some way or other:  EU Nitrates Directive; EU Urban Waste-Water Treatment Directive; EU Industrial Emissions Directive, IED; EU Water Framework Directive, WFD; the Gothenburg Protocol to Abate Acidification, Eutrophication and Ground-level Ozone under UNECE Convention on Long-range Transboundary Air pollution (CLRTAP); IMO designation of the Baltic Sea as a "special area" for passenger ships under MARPOL (International Convention for the Prevention of Pollution from Ships) Annex IV (on sewage from ships); EC Directive 2000/59/EC on port reception facilities; and the Application on the Baltic Sea NOx emission control area (NECA) has not yet been submitted to IMO.

Role of nutrient inputs to the ecosystem

Eutrophication in the Baltic Sea is to a large extent driven by excessive inputs of the nutrients nitrogen and phosphorus due to accelerating anthropogenic activities during the 20th Century. Nutrient over-enrichment (or eutrophication) and/or changes in nutrient ratios in the aquatic environment cause elevated levels of algal and plant biomass, increased turbidity, oxygen depletion in bottom waters, changes in species composition and nuisance blooms of algae.

The majority of nutrient inputs originate from anthropogenic activities on land and at sea. Waterborne inputs enter the sea via riverine inputs and direct discharges from coastal areas. The main sources of waterborne inputs are point sources (e.g. waste water treatment plants, industries and aquaculture), diffuse sources (agriculture, managed forestry, scattered dwellings, storm overflows etc.) and natural background sources. The main sectors contributing to atmospheric inputs are combustion in energy production and industry as well as transportation for oxidized nitrogen and agriculture for reduced nitrogen. A large proportion of atmospheric inputs originate from distant sources outside the Baltic Sea region. Emissions from shipping in the Baltic and North seas also contribute significantly to atmospheric inputs of nitrogen.  In addition, excess nutrients stored in bottom sediments can enter the water column and enhance primary production of plants (Figure 6). For more information see HELCOM 2012 and HELCOM 2015.

 Figure 7.jpg

Figure 6. Different sources of nutrients to the sea and examples of nitrogen and phosphorus cycles. The flow related to ammonia volatilization shown in the figure applies only to nitrogen. In this report, also combustion and atmospheric deposition deal only with nitrogen. Emissions of phosphorus to the atmosphere by dust from soils are not shown in the figure. (Source: Ærtebjerg et al. 2003.)


Information on the quantity of nutrient inputs is of key importance in order to follow up the long-term changes in the nutrient inputs to the Baltic Sea. This information, together with information from land-based sources and retention within the catchment, is also crucial for determining the importance of different sources of nutrients for the pollution of the Baltic Sea as well as for assessing the effectiveness of measures taken to reduce the pollution inputs. Quantified input data is a prerequisite to interpret, evaluate and predict the state of the marine environment and related changes in the open sea and coastal waters.​

Environmental Target and progress towards GES

The environmental targets for nutrient inputs are the maximum allowable inputs (MAI) of the HELCOM nutrient reduction scheme (Table 5). The MAI indicate the maximal level of annual inputs of water- and airborne nitrogen and phosphorus to Baltic Sea sub-basins that can be allowed while still achieving good environmental status (GES) in terms of eutrophication.

A provisional nutrient reduction scheme was adopted in the HELCOM Baltic Sea Action Plan (HELCOM 2007). The presented MAI were revised based on improved scientific basis and models, and were adopted by the 2013 HELCOM Copenhagen Ministerial Meeting (HELCOM 2013a).  


Table 5. Maximum allowable annual inputs (MAI) of nitrogen and phosphorus to the Baltic Sea sub-basins.

Sub-basinMaximum allowable annual nitrogen inputs (tonnes)Maximum allowable annual phosphorus inputs (tonnes)
Bothnian Bay57,6222,675
Bothnian Sea79,3722,773
Baltic Proper325,0007,360
Gulf of Finland101,8003,600
Gulf of Riga88,4172,020
Danish Straits65,9981,601
Baltic Sea 792,20921,716


MAI was calculated by the Baltic Nest institute (BNI) - Sweden using the coupled physical-biogeochemical model BALTSEM. Obtaining MAI is formally an optimization problem: finding the highest possible inputs that will still satisfy given eutrophication targets (e.g. GES boundaries for eutrophication indicators).

The basin-wise MAI, were obtained by satisfying all eutrophication targets in all basins, taking into account ecological relevance and model accuracy. More details are provided in Gustafsson et al, in prep.

For basins without additional reduction requirements, the 1997-2003 averaged normalized inputs obtained within the PLC 5.5 project are used as MAI. For more information, see HELCOM 2013b.

The uncertainty in the determination of MAI can be divided into three sources: uncertainty in the eutrophication targets, uncertainties associated with model short-comings and uncertainties in the input data to the calculation. The confidence in the eutrophication targets has been classified as moderate or high, depending on the variable (HELCOM 2013c). It is straightforward but laborious to explore how MAI varies with changes in target values from the pressure-response relationships (i.e., the model derived change in target values for a given change in nutrient inputs). The laborious aspect arises from the numerous combinations of uncertainty that can arise if many indicator values and basins are simultaneously taken into account. However, the impression is that the nitrogen target causes the largest uncertainty in determination of MAI for most basins. Reasons are that in most cases there are no, or only few, trustworthy measurements to indicate the pre-eutrophied situation and also because the relationship between nitrogen input and concentrations in sea waters is rather weak in basins featuring hypoxia and strong nitrogen limitations (i.e. the Baltic Proper and the Gulf of Finland) because of large internal feedback from nitrogen fixation and denitrification.

When calculating MAI, attempts have been made to take into account biases in BALTSEM by discarding indicators in basins were they are not adequately modelled, and by raising a concern of whether MAI is really trustworthy because of model deficiency/bias.

Note: both MAI and CART calculations are affected by the input data to the model. If input data are inconsistent, it may cause over- or underestimation of MAI and CART, and thus an unfair distribution of reduction requirement between countries.

State indicators linked to the pressure of nutrient inputs

Response in the eutrophication status from changes in nutrient inputs may be considerably slow. Model simulations indicate that it would take perhaps half a century or even more after nutrient inputs reach MAI to reach the environmental targets (Gustafsson & Mörth, in prep.). However, the simulations indicate that significant improvements could be expected after 1 -2 decades. It should be noted that determination of these time-scales are regarded as more uncertain than the ultimate long-term state because of unexpected non-linear responses of, e.g., phosphorus to improved oxygen concentrations. In coastal areas one can expect faster responses, especially when significant direct point sources are removed. This is probably also the case for the eastern part of the Gulf of Finland.

The effect of changes in nutrient inputs on the core HELCOM eutrophication status indicators DIN, DIP, chlorophyll-a, Secchi depth and oxygen debt are thoroughly evaluated in Gustafsson and Mörth, in prep.

Relevant core indicators on eutrophication status:​

Information on other relevant supporting parameters:

See also the latest assessment on eutrophication status in the Baltic Sea (HELCOM 2014).

Assessment protocol

Data sources

The HELCOM Contracting Parties annually report waterborne inputs of nitrogen and phosphorus from rivers and direct point sources to Baltic Sea sub-basins. Data on atmospheric emissions and monitored atmospheric deposition are submitted by countries to the Co-operative programme for monitoring and evaluation of the long-range transmission of air pollutants in Europe (EMEP), which subsequently compiles and reports this information to HELCOM. In accordance with HELCOM Recommendation 26/2, periodic pollution load compilation (PLC) assessments (in principle every six year) are made to assess at a finer level the sources of inputs to inland surface waters (HELCOM 2005).

Nutrient input data can be viewed in HELCOM PLC reports (e.g. HELCOM 2012, HELCOM 2013d and HELCOM 2015).

Trend analysis and statistical processing

As part of the HELCOM PLC-5.5 project a trend analysis was carried out by DCE, Aarhus University (Denmark), with Mann-Kendall methodology (Hirsch et al. 1982) on:

  • annual flow normalized riverine inputs (A)

  • point sources discharging directly to the sea (direct inputs) (B)

  • flow normalized waterborne inputs (C = A+B)

  • normalized airborne inputs (D)

  • total normalized inputs (E = C+D) of nitrogen and phosphorus

for all relevant combinations of Contracting Parties and main sub-basins of the Baltic Sea. Where there is a significant trend, the annual changes were determined with a Theil-Sen slope estimator (Hirsch et al., 1982) and the change from 1995 to 2012 was calculated. The methodology has been agreed on by HELCOM LOAD (more information on trend analysis and determining the changes in input can be found in Larsen & Svendsen 2013).

For information about normalization of airborne and flow normalization of waterborne input data, see Annexes 9.3 and 9.4 of the PLC-5.5 report (HELCOM 2015).

Assessment units

Nutrient input data have been compiled in accordance with PLC guidelines for the following nine sub-basins: Bothnian Bay, Bothnian Sea, Archipelago Sea, Gulf of Finland, Gulf of Riga, Baltic Proper, Western Baltic, The Sound and Kattegat. The boundaries of the sub-basins coincide with the main terrestrial river basin catchments.

The BALTSEM model has divided the Baltic Sea into seven sub-basins in accordance with natural marine boundaries and hence the MAIs have been calculated for the following seven sub-basins: Kattegat, Danish Straits, Baltic Proper, Bothnian Sea, Bothnian Bay, Gulf of Riga and Gulf of Finland. In the BALTSEM sub-division, the Bothnian Sea includes the Archipelago Sea area and the Danish Straits combine Western Baltic and The Sound.

The entire Baltic Sea is covered by the assessment. ​