Monitoring requirements

Monitoring methodology

Waterborne inputs

Contracting Parties measure water flow and concentrations of selected parameters in riverine water and point source discharges. Estimates of inputs from unmonitored areas are based on modelling including information of point sources discharges (monitored or estimated). These data are used to calculate total annual inputs to the sea.  These measurements and estimates are carried out by the Contracting Parties. The methods for monitoring and calculating waterborne pollution inputs are described in the HELCOM Pollution Load Compilation (PLC) guidelines. Updated guidelines were developed by PLC-6 project.

An overview of agreed monitoring of nutrient inputs is also described in the HELCOM Monitoring Manual. An overview of monitoring carried out by Contracting Parties in 2012 was compiled by the PLC-6 project, which summarizes the frequency of monitoring of different parameters in rivers and point sources.


Atmospheric inputs

Atmospheric emissions and measured atmospheric deposition are reported by countries to the Co-operative Programme for Monitoring and Evaluation of the Long Range Transboundary Air Pollutants in Europe (EMEP), which compiles and reports to HELCOM. EMEP models the deposition of nitrogen input based on emission measurements and estimates and information on meteorological parameters. The results of the EMEP Unified model are routinely compared to available measurements at EMEP and HELCOM stations. The deposition of phosphorus is not modelled but based on measurements from (rather few) monitoring stations and a fix deposition rate of 5 kg P per km2 have been used in the latest PLC assessment (HELCOM 2014b; HELCOM 2015). Details of the monitoring activities and the model are available in the HELCOM Monitoring Manual.


Current monitoring

Waterborne inputs

Inputs from large rivers are monitored and the measurements used for calculating inputs that are reported. Inputs from smaller unmonitored rivers are generally estimated by models. Inputs from point sources (municipal waste water treatment plants, industry and aquaculture) discharging directly to the Baltic Sea are reported separately.


Monitoring table 1a. Numbers of rivers, monitored area and percentages of waterborne nitrogen inputs that were monitored, unmonitored, and direct point source discharges of total waterborne nitrogen inputs to the [Baltic Sea sub-basin in 2014].

Sub-basinNumber of riversMonitored area (km2)Monitored area (% of total area)Total N monitored (%)

Total N unmonitored


Total N direct (%)
Bothnian Bay22237331898947
Bothnian Sea151910438780146
Baltic Proper455214519088102
Gulf of Finland22480315969118
Gulf of Riga71248738885141
Danish Straits74130994838566
Baltic Sea22416430188386104

Monitoring table 1b. Numbers of rivers, monitored area and percentages of waterborne phosphorus inputs that were monitored, unmonitored and direct point source discharges of total waterborne phosphorus inputs to the [Baltic Sea sub-basin in 2014].

Sub-basinNumber of riversMonitored area (km2)Monitored area (% of total area)Total P monitored (%)Total P unmonitored (%)Total P direct (%)
Bothnian Bay22237331898947
Bothnian Sea1519104387791110
Baltic Proper45521451909433
Gulf of Finland22480315969109
Gulf of Riga71248738886113
Danish Straits721309948295021
Baltic Sea2221643018838875


Monitoring tables 1a and 1b show that more than 80% of the total Baltic Sea catchment area is covered by monitoring based on 222/224 monitoring stations. For six of the seven sub-basins between 86% and 96% of the catchment areas are monitored, and these catchments are covered by monitoring in mainly large rivers. For Danish Straits only 47-49% of the catchment is monitored even though 74 monitoring stations or one third of all river monitoring stations in the Baltic Sea catchment area are situated in the catchment due to many small river catchments.

Monitoring tables 1a and 1b also show that estimated/calculated inputs from unmonitored areas constitute 10% of total nitrogen and 7% of total phosphorus waterborne inputs to the Baltic Sea.

Details of the monitoring activities are available in the HELCOM Monitoring Manual.


Atmospheric inputs

Details of the monitoring activities and the model are available in the HELCOM Monitoring Manual and Monitoring table 2 gives an overview of the number of nitrogen monitoring stations located at the Baltic Sea used to compare model and monitored nitrogen deposition.


Monitoring table 2. Number of monitoring stations situated close to the Baltic Sea used for measuring wet and dry deposition of nitrogen compounds in 2010.

 Sub-basinWet  deposition of NDry deposition of N
Bothnian Bay20
Bothnian Sea13
Baltic Proper66
Gulf of Finland22
Gulf of Riga00
Danish Straits23
Baltic Sea1517


Description of optimal monitoring

Waterborne inputs

Guidelines for sampling discharges from point sources and inputs via rivers are given in the PLC-6 guidelines. For riverine inputs, as a minimum 12 samples should be taken each year at a frequency that appropriately reflects the expected river flow pat­tern. If more samples are taken (e.g. 18, 26 or more) and/or the flow pattern does not show a major annual variation the samples can be more evenly distributed during the year. Overall, for substances transported in connection with suspended solids, lower bias and better precision is obtained with higher sampling frequency.

For rivers with hydrological stations the location of these stations, measurement equipment, frequency of water level and flow (velocity) measurement should at least follow the World Meteorological Organization (WMO) Guide to Hydrological Practices (WMO-No. 168, 2008) and national quality assurance (QA) standards. 

Preferably the discharge (or at least the water level) should be monitored continuously and close to where water samples for chemical analyses are taken. If the discharges are not monitored continuously the measurements must cover low, mean and high river flow rates, i.e. they should as a minimum reflect the main an­nual river flow pat­tern. Further details are provided in the PLC-6 guidelines.


Atmospheric inputs

Collection of air emission data and modelling atmospheric deposition are coordinated by EMEP. There are rather few stations located at the coast or on small islands in the Baltic Sea, and not all stations are measuring all components. Further, only some stations have long time series. Not all national monitoring stations are included in the list of "HELCOM stations" but could be used by EMEP.  There are also some problems with the representativeness of the stations, i.e. rather many in the south-western part of the Baltic Sea but few in the eastern and northern parts that cause challenges when verifying the EMEP model results.  For phosphorus it is especially important to establish a more extensive and representative monitoring station network, as there are no models developed to estimate the atmospheric phosphorus deposition. Thorough analysis of the monitoring data would improve the understanding of the development in the atmospheric deposition and also offer recommendations on how to improve and possibly expand monitoring.