HELCOM Baltic Sea Environment Fact Sheet 2019; Published 5.9.2019
Authors: Lars M. SvendsenI and Bo GustafssonII
IDCE, Danish Center for Environment and Energy, Aarhus University Denmark
IIBNI; Baltic Nest Institute, Stockholm University, Sweden
 The authors want to thank colleagues contributing to data reporting, quality assuring and data management related to this BSEFS:
Peeter Ennet, Estonian Environment Agency), Dmitry Frank-Kamenetsky (HELCOM Secretariat), Juuso Haapaniemi (HELCOM Secretariat), Ilga Kokorite (Latvian Environment, Geology and Meteorology Center), Natalia Oblomkova (Institute for Engineering and Environmental Problems in Agricultural Production, Russia), Svajunas Plunge (Lithuanian Environmental Protection Agency), Antti Räike (Finnish Environment Agency (SYKE), Jan Pryzowicz (State Water Holding Polish Waters), Alexander Sokolov (Baltic Nest Institute, Stockholm University), Lars Sonesten (Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Science), Henrik Tornbjerg (Institute of Bioscience, Aarhus University) Antje Ullrich (German Environment Agency)
Annual water flow in 2017 to the Baltic Sea was with approx. 16,600 m3 s-1 about 5% higher than the average of 1995-2017. The annual waterborne inputs (inputs via rivers and via point sources discharging directly into the sea) of total nitrogen was in 2017 approx. 756,000 tonnes or 12% higher than the average of 1995-2017 while the corresponding annual total phosphorus inputs in 2017 amounted to approx. 27,900 tonnes, 14% lower than the average.
Inputs from point sources along the coasts discharging total nitrogen and phosphorus directly to the sea have decreased with approximately 50% and 75% since 1995, respectively. In 2017, the direct inputs of nitrogen and phosphorus constituted 4% and 5% of total waterborne inputs of these nutrients to the Baltic Sea. In 1995 the proportion of the direct inputs was 8% and 15%, respectively.
Annual flow weighted riverine TN concentration has decreased significantly (95% confidence) since 1995 to Bothnian Sea, Baltic Proper, Danish Straits and Kattegat, and for TP to Baltic Proper, Gulf of Finland and Danish Straits. Both TN and TP concentrations have decreased significantly for the total riverine inputs to the Baltic Sea.
Results and Assessment
Relevance of the BSEFS for describing developments in the environment
This fact sheet includes information on annual water flow and inputs of nitrogen and phosphorus via rivers (riverine inputs) and point sources discharging directly to the sea (direct inputs) together comprising the waterborne inputs to Baltic Sea sub-basins during 1995-2017. The inputs are the actual (not weather- normalized) annual inputs. A separate BSEFS on atmospheric nitrogen inputs is annually delivered by EMEP (e.g. Gauss et al., 2018).
The normalized waterborne inputs combined with the corresponding atmospheric nutrient inputs are annually evaluated in the HELCOM core pressure indicator: “Inputs of nutrients to the sub-basins of the Baltic Sea (the latest is HELCOM 2018).
Eutrophication in the Baltic Sea is largely driven by excessive inputs of the nutrients nitrogen and phosphorus due to accelerating anthropogenic activities during the 20th century. Nutrient over-enrichment (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. The main sources of waterborne inputs are diffuse sources (agriculture, managed forestry, scattered dwellings, storm overflows etc.), natural background sources, and point sources (as waste water treatment plants, industries and aquaculture). In addition, excess nutrients stored in bottom sediments can enter the water column and enhance primary production of plants.
Time series with information on annual nutrient inputs is needed to follow up the long-term changes in the nutrient inputs to the Baltic Sea. 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. Change in nutrient inputs combined with quantification of inputs from land-based sources and retention within the catchment, is 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. Reducing the effects of human-induced eutrophication is the stated goal of Descriptor 5 in the EU Marine Strategy Framework Directive (MSFD). Inputs of nutrients to the Baltic Sea marine environment have an effect on the nutrient levels under criterion D5C1 of the MSFD.
 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.
Policy relevance and policy references
Since the establishment of the Convention for the Protection of the Marine Environment of the Baltic Sea Area (Helsinki Convention) in 1974, the Commission for the Protection of the Marine Environment of the Baltic Sea Area (Helsinki Commission or HELCOM for short) has been working to reduce the inputs of nutrients to the sea.
In Article 3 and Article 16 of the Convention on the Protection of the Marine Environment of the Baltic Sea Area, 1992 (Helsinki Convention), the Contracting Parties agreed to undertake measures to prevent and eliminate pollution of the marine environment of the Baltic Sea and to provide pollution load data, as far as available. Through coordinated monitoring, since the mid-1980s HELCOM has been compiling information about the magnitude and sources of nutrient inputs into the Baltic Sea. By regularly compiling and reporting data on pollution inputs, HELCOM follows the progress towards reaching politically agreed nutrient reduction input targets.
HELCOM Baltic Sea Action Plan (BSAP) was adopted in 2007 by the Baltic Sea coastal countries and the European Union (HELCOM 2007). The BSAP sets the overall objective of reaching good environmental status in the Baltic Sea by 2021, by addressing eutrophication, hazardous substances, biodiversity and maritime activities. As an innovative feature the BSAP included a scientific based nutrient input reduction scheme identifying maximum allowable inputs (MAI) of nutrients to achieve good status in terms of eutrophication. The plan also adopted provisional country-wise allocation of reduction targets (CARTs) to fulfil, and the CART are converted to nutrient input ceilings country per Baltic Sea sub-basins.
The 2013 HELCOM Copenhagen Ministerial Declaration (HELCOM 2013a) revised maximum allowable inputs of nutrients and reduction targets using the best available scientific data and models. Also, national nutrient input ceilings were calculated for each country and each Baltic Sea sub-basin.
HELCOM Brussels Ministerial Declaration 2018 committed HELCOM member states to act further to achieve national reduction requirements based on Maximum Allowable Inputs of nutrients to the Baltic Sea sub-basins.
The information provided in this BSEFS also supports the follow-up of the implementation of the targets and measures under the following policies addressing reduction of nutrient inputs: EU Maritime Strategy Framework Directive (MSFD); EU Water Framework Directive (WFD);EU Nitrates Directive; EU Urban Waste-Water Treatment Directive; EU Industrial Emissions Directive (IED); Water Code of Russian Federation; Federal Act on the internal maritime waters, territorial sea and contiguous zone of the Russian Federation.
 Regarding atmospheric inputs the relevant policies are: The Gothenburg Protocol to Abate Acidification, Eutrophication and Ground-level Ozone under UNECE Convention on Long-range Transboundary Air pollution (CLRTAP); EU NEC Directive (2016/2284/EU); 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 of the Baltic Sea NOx emission control area (NECA).
Based on the reported annual riverine and direct water flows, total nitrogen (TN) and phosphorus (TP) inputs by Contracting Parties, Baltic Nest Institute (BNI), Stockholm University and Danish Centre for Environment and Energy (DCE), Aarhus University establish an assessment dataset with inputs country per sub-basin and per sub-basin to the Baltic Sea, -after checking for outliers, filling in data gaps and other validations procedures. The assessment data set covers all known waterborne inputs from the entire Baltic Sea catchment area.
This fact sheet provides information on the actual annual TN and TP amounts entering to the seven main Baltic Sea sub-basins (Figure 1) to allow for direct evaluation against environmental indicators in the sea. We focus mainly on riverine inputs as they in 2017 constituted 96% of TN and 95% of TP waterborne 2017 inputs to the Baltic Sea, respectively. In the evaluation of progress towards MAI and CART as publish in HELCOM (2018) (MAI) and Svendsen et al. (2018) (CART), we use (flow-)normalized nutrient inputs to allow for comprehensive statistical analysis for trends, break points, remaining or extra reduction as compared with reduction targets /inputs ceilings.
Table 1 provides key information on the annual water flow, total waterborne TN and TP inputs, flow- weighted annual TN and TP concentration of riverine inputs (mg l-1) to the sub-basins and total to the Baltic Sea in 2017 as compared with the average 1995-2017. Further, the catchment to the sub-basins and the area of the sub-basins are provided allowing for calculation of TN and TP inputs per catchment area and per sea area. Flow to the Baltic Sea in 2017 was about 5% higher than the 1995-2017 average. The flow was particularly higher to GUR (37%), BAP (13%) and DS (13%) compared with the average, while it was lower to KAT (17%) and BOS (10%). Higher flow usually will indicate higher waterborne TN and TP inputs, even though when comparing the 2017 waterborne input with the corresponding average 1995-2017 changes in waterborne inputs should be taken into account. Waterborne TN inputs in 2017 was 755,886 tons 12% higher, while corresponding TP inputs with 27,942 tons was 14% lower than the 1995-2017 average indicating overall reduction in TP inputs since 1995. For only GUR (39%) and BAP (26%) waterborne TN inputs was higher than the average of the time series, otherwise, it was equal (GUF) or lower. For waterborne TP the 2017 input was only higher to GUR (37%), equal to the 1995-2017 average for DS, but lower for the remaining sub-basins.
Annual flow-weighted riverine concentration (calculated by dividing annual riverine nutrient input with the corresponding water flow) in 2017 to BAS was 8% higher than the corresponding average of 1995-2017 for TN, but 13% lower for TP. Annual TN flow-weighted riverine concentration was particularly higher to BAP (14%), but markedly lower to BOS (13%), BOB (10%) and DS (9%) than the average of 1995-2017. Correspondingly, TP annual flow-weighted riverine concentration in 2017 was higher to KAT (11%), but markedly lower particularly to BAP (23%) and GUF (22%) compared with the 1995-2017 average.
DS has the highest specific waterborne catchment
inputs in 2017 (1,468 kg N km-2, 58 kg P km-2),
reflecting high population density and high agricultural land-use. The
corresponding lowest specific inputs are for BOB, BOS and GUR (approx. 185 N km-2
and 5.3-9.2 kg P km-2), catchments that have overall rather low
population densities and with high percentages of pristine or forested areas
and rather low pressure from agriculture. On the other hand, specific
waterborne inputs per sea area are highest to GUR ( 5,582 kg N km-2,
159 kg P km–2) and BAP (3,622 kg N km-2, 158 kg P km-2),
and lowest to BOS (529 kg N km-2, 25 kg P km-2).
 The preferred option would be the use of the measured nutrient concentrations per river. Back-calculating nutrient concentrations from nutrient inputs using the river flow leads to different results, but is currently, based on data availability in PLC, the only possible option. Calculated nutrient concentrations therefore can only provide an indication and should be treated with care.
Figure 1. The catchment of the Baltic Sea is divided by 9 HELCOM member countries Denmark (DK), Estonia (EE), Finland (FI), Germany (DE), Latvia (LV), Lithuania (LT), Poland (PL), Russia (RU) and Sweden (SE) and 5 transboundary countries (Belarus, Czech Republic, Slovakia, Norway and Ukraine). The Baltic Sea (BAS) is divided in 7 sub-basins: Bothnian Bay (BOB), Bothnian Sea (BOS), which includes Archipelago Sea, Gulf of Finland (GUF), Gulf of Riga (GUR), Baltic Proper (BAP), Danish Straits consisting of The Sound and of Western Baltic (DS) and Kattegat (KAT).
Table 1. Catchment area to and sea area of the seven sub-basins of the Baltic Sea (km2). Annual waterborne flow (m3 s-1), waterborne total nitrogen and phosphorus inputs (tonnes) in 2017 and in average for 1995-2017. Flow weighted TN and TP concentrations (mg l-1) of annual riverine inputs in 2017 and in average for 1995-2017. Further, waterborne inputs of TN and TP is given as specific inputs per km2 catchment area and per sea area (kg N, P km-2), respectively. For an explanation of abbreviations see the caption to figure 1.
The annual water flow, direct inputs of TN and TP, riverine TN and TP inputs and waterborne TN and TP inputs during 1995-2017 to the sub-basins and to the Baltic Sea are shown in figure 2 as well as in tables 2-7 in the “Data” section. There are significant reductions in inputs from direct inputs from 1995 to 2017 to all sub-basins except BOB, for TN e.g. to DS (74%), BAP (67%) and GUR (63%), and for TP e.g. to GUR (87%), BAP (84%), GUF (87%) and DS (72%), although data on direct inputs are more uncertain in the beginning of the times series. Even though 2017 direct inputs to BAS constitute only 4% of the waterborne TN and 5% of waterborne TP inputs, their importance is significantly higher to e.g. BOS (10%) for TN and DS (16%) for TP.
Figure 2: Annual riverine and direct inputs of total nitrogen (figures in the left column) and total phosphorus (figures in the right column) in tonnes and annual waterborne flow (m-3 s-1) to the seven Baltic Sea sub-basin and to the Baltic Sea. Data behind the figures are in tables 2-7. For an explanation of abbreviations see the caption to figure 1.
Annual riverine TN and TP inputs are plotted against the corresponding water flow together with the linear regressions between inputs and flow in Figure 3. Together with a statistical test (see caption to figure 3) it is used to allow for characterization and evaluation of the TN and TP riverine 1995-2017 inputs and these inputs specifically in 2017. For both TN and TP the relation between riverine inputs and flow is significant for all sub-basins and for BAS except GUF. Lack of significant correlation indicates some main challenges with the input data to GUF that is in large parts estimated both for unmonitored areas and for some rivers.
Even though riverine TN and TP input in 2017 was higher than corresponding average inputs during 1995-2017 to many basins, and very high in GUR they are within the expected range based on the relation shown in figure 3.
As a rule of thumb, a decrease in riverine TN and/or TP inputs during 1995 to 2017 is significant if most of the inputs in the latest 12-13 years falls below the dotted lines in figure 3. This can be seen for many sub-basin.
Figure 3. Linear regression plot of annual riverine flow (m3 s-1) against annual riverine total nitrogen inputs – TN – (left column) and total phosphorus inputs – TP – (right column) to the seven Baltic Sea sub-basins and to the Baltic Sea. 2017 is marked in red. The linear regression is indicated as y = a·X + b, Y = waterborne input (TN, TP), a = slope, b = intercept Y-axis, R2 indicates how much of the variation is explained by the regression, e.g. R2 =0.8667 say that nearly 87 % of variation explained (good correlation). From the statistical test, an F-value is calculated, and it is tested if the linear relation is significant (95 % confidence). All relations besides TN and TP for GUF are significant. For an explanation of abbreviations see the caption to figure 1.
Flow weighted annual concentrations is used as a rough evaluation of any trends in nutrient inputs combined with a simple linear regression analysis. In figure 4 the discharge weighted riverine TN and TP annual concentrations during 1995-2017 are shown. A statistical test on the linear regressions (test explained in the caption to figure 3) indicates that the discharged weighted TN riverine concentration has decreased significantly to BOS, BAP, DS, KAT and BAS. For TP there is a significant decrease to BOS, BAP, GUF, DS and BAS and a significant increase to GUR.
Figure 4. Annual average flow weighted riverine TN (upper figure) and TP (lower figure) concentration for the seven Baltic Sea sub-basins and the Baltic Sea (calculated as total annual riverine inputs divided with corresponding flow). For an explanation of abbreviations see the caption to figure 1.
Gauss, M. and Bartnicki, J. and Klein, H. 2018: Atmospheric nitrogen deposition to the Baltic Sea during 1995-2016. HELCOM Baltic Sea Environment Fact Sheet(s) 2015. Online, http://www.helcom.fi/baltic-sea-trends/environment-fact-sheets/.
Gauss, M. and Bartnicki, J. 2018. Nitrogen emissions to the air in the Baltic Sea area. HELCOM Baltic Sea Environment Fact Sheets. Online, http://www.helcom.fi/baltic-sea-trends/environment-fact-sheets/.
HELCOM 2007. HELCOM Baltic Sea Action Plan (BSAP). HELCOM Ministerial Meeting. Adopted in Krakow, Poland, 15 November 2007.
HELCOM 2012. Fifth Baltic Sea Pollution Load Compilation – An Executive Summary. Baltic Sea Environment Proceedings No. 128A.
HELCOM 2013a. HELCOM Copenhagen Declaration “Taking Further Action to Implement the Baltic Sea Action Plan – Reaching Good Environmental Status for a healthy Baltic Sea”. Adopted 3 October 2013.
HELCOM 2013b. Summary report on the development of revised Maximum Allowable Inputs (MAI) and updated Country Allocated Reduction Targets (CART) of the Baltic Sea Action Plan. Supporting document for the 2013 HELCOM Ministerial Meeting.
HELCOM 2013c. Approaches and methods for eutrophication target setting in the Baltic Sea region. Baltic Sea Environment Proceedings No. 133.
HELCOM 2013d. Review of the Fifth Baltic Sea Pollution Load Compilation for the 2013 HELCOM Ministerial Meeting. Baltic Sea Environment Proceedings No. 141.
HELCOM, 2015. Updated Fifth Baltic Sea pollution load compilation (PLC-5.5). Baltic Sea Environment Proceedings No. 145.
HELCOM 2016a. HELCOM Recommendation 37-38/1. Waterborne pollution input assessment (PLC-water). Adopted 16 June 2016. Supersedes HELCOM Recommendations 26/2.
HELCOM 2016b. Draft procedure for releasing the reported PLC water data. HELCOM HOD 50-2016 doc. 4-6.
HELCOM 2016c. HELCOM Recommendation 37-38-2 “Monitoring of airborne pollution input”. Adopted 16
June 2016. Supersedes HELCOM Recommendations 24/1.
HELCOM 2018. Inputs of nutrients to the sub-basins of the Baltic Sea. HELCOM core indicator report. Online. http://www.helcom.fi/baltic-sea-trends/indicators/inputs-of-nutrients-to-the-subbasins
HELCOM 2019. HELCOM Guidelines for the annual and periodical compilation and reporting of waterborne pollution inputs to the Baltic Sea (PLC-Water).
Larsen, S.E, & Svendsen, L.M. 2019. Statistical aspects in relation to Baltic Sea Pollution Load Compilation. Task under HELCOM PLC-7 project. Aarhus University, DCE – Danish Centre for Environment and Energy, 42 pp. Technical Report No. 137. http://dce2.au.dk/pub/TR137.pdf
Svendsen, L.M., Bartnicki J., Boutrup, S, Gustafsson, B., Jarosinski, W., Knuuttila, S., Kotilainen, P., Larsen, S.E., Pyhälä6, M., Ruoho-Airola, T., Sonesten, L and Staaf, H. 2015: . Updated Fifth Baltic Sea Pollution Load Compilation (PLC-5.5). Baltic Sea Environment Proceedings No. 145, HELCOM. 143 pp. Svendsen, L.M., Larsen S.E., Gustafsson, B., Sonesten L., Frank-Kamenetsky D. 2018. Progress towards national targets for input of nutrients. Online. http://www.helcom.fi/baltic-sea-action-plan/nutrient-reduction-scheme/progress-towards-country-wise-allocated-reduction-targets/
Numerical data for the period 1995-2017 are given in the following tables via this MS Excel file:
Table 1. Catchment area to and sea area of the seven sub-basins of the Baltic Sea (km2). Annual waterborne flow (m3 s-1), waterborne total nitrogen and phosphorus inputs (tonnes) in 2017 and in average for 1995-2017. Flow weighted TN and TP concentrations (mg l-1) of annual riverine inputs in 2017 and in average for 1995-2017. Further, waterborne inputs of TN and TP is given as specific inputs per km2 catchment area and per sea area (kg N, P km-2), respectively.
Table 2. Annual waterborne flow (sum of riverine flow and direct flow (flow for point sources discharging direct into the Baltic Sea)) to the seven Baltic Sea sub-basins and the Baltic Sea (in m3 s-1). For an explanation of abbreviations see the caption to figure 1.
Table 3. Annual total nitrogen (TN) direct inputs to the seven Baltic Sea sub-basins and the Baltic Sea (in tonnes). For an explanation of abbreviations seethe caption to figure 1.
Table 4. Annual total nitrogen (TN) riverine inputs to the seven Baltic Sea sub-basins and the Baltic Sea (in tonnes). For an explanation of abbreviations see the caption to figure 1.
Table 5. Annual total nitrogen (TN) waterborne (riverine + direct) inputs to the seven Baltic Sea sub-basins and the Baltic Sea (in tonnes). For an explanation of abbreviations see the caption to figure 1.
Table 6. Annual total phosphorus (TP) direct inputs to the seven Baltic Sea sub-basins and the Baltic Sea (in tonnes). For an explanation of abbreviations see the caption to figure 1.
Table 7. Annual total phosphorus (TP) direct inputs to the seven Baltic Sea sub-basins and the Baltic Sea (in tonnes). For an explanation of abbreviations see the caption to figure 1.
Table 8. Annual total phosphorus (TN) waterborne (riverine + direct) inputs to the seven Baltic Sea sub-basins and the Baltic Sea (in tonnes). For an explanation of abbreviations see the caption to figure 1.
The HELCOM Contracting Parties annually report annual water flow, inputs of total nitrogen and total phosphorus from rivers (riverine inputs) and annual inputs from direct point sources (direct inputs) to Baltic Sea sub-basins to the HELCOM PLC database (PLUS) according to HELCOM Recommendation 37-38-1 “Waterborne pollution input assessment (PLC-Water) (HELCOM 2016a). Further, 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) according to HELCOM Recommendation 37-38-2 “Monitoring of airborne pollution input” (HELCOM 2016c). EMEP subsequently compiles and reports this information to HELCOM including a BSEF on nutrient emissions and deposition (e.g. Gauss and Bartnicki, 2018 and Gauss et al., 2018).
Total nutrient inputs (air- + waterborne inputs) to the Baltic Sea and its sub-basins are assessed annually in a HELCOM core indicator report on water and airborne inputs (e.g. HELCOM 2018) and periodically in HELCOM PLC reports (e.g. HELCOM 2012, HELCOM 2013d and HELCOM 2015) and when assessing progress towards national nutrient ceilings (e.g. Svendsen et al., 2018).
2. Description of data:
Annual water flow together with load of nitrogen and phosphorus are reported from more than 300 monitoring stations in rivers covering the monitored part of the Baltic Sea catchment area. Direct inputs from point sources discharging directly into the Baltic Sea are reported from approximately 500 municipal waste water treatment plants, 220 industries and 170 marine fish farms. Further the nine HELCOM member countries model or estimate inputs for the unmonitored part of the catchment areas to the seven sub-basins shown in figure 1.
3. Geographical coverage:
Flow, nitrogen and phosphorus inputs from the entire catchment area to the Baltic Sea (approx. 1.73 mio. km2) are covered by monitoring (monitored part of the catchment which constitutes 83 % of the catchment area) or modelling/estimates (unmonitored part of the catchment). It includes catchments in the nine HELCOM member countries and catchments in five transboundary countries (see table 1 and figure 1). Further, annual flow and nutrient inputs from point sources discharging directly into the Baltic Sea are included in the compilation of total waterborne inputs to the Baltic Sea.
4. Temporal coverage:
Time series with annual water flow, total nitrogen and total phosphorus riverine and direct inputs together with the waterborne inputs to the seven sub-basins covering the Baltic Sea are available for the period 1995 – 2017.
5. Methodology and frequency of data collection:
Monitored part of the catchment and direct inputs
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. The flow should be monitored at least 12 times every year. 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-guidelines (HELCOM 2019).
For riverine inputs, as a minimum 12 water samples for measuring nutrients concentrations 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 major annual variations, the samples can be evenly distributed during the year (see PLC-guidelines HELCOM 2019). Overall, for substances transported in connection with suspended solids, lower bias and better precision is obtained with higher sampling frequency. National and EU regulation regulate the number of water samples from big point sources. For big point sources the sampling frequency is at least 24, and often much higher.
The load in rivers are typically calculated by multiplying daily flow with a daily concentration of TN and TP, respectively. Daily flow for most rivers is obtain from a stage-discharge relationship and daily concentration by linear interpolation between days with chemical sampling (HELCOM, 2019). For some rivers monthly average concentration are? multiplied with flow.
Unmonitored parts of the catchment
The nine HELCOM member countries estimate annual flow, load of total nitrogen and total phosphorus from the unmonitored catchment areas to the Baltic Sea by simple empirical or more advance physico-hydro-geochemical modelling, and/or extrapolation (see PLC-guideline HELCOM, 2019). In average 17% of the catchment is unmonitored, ranging from 4% unmonitored catchment (Gulf of Finland) to 52 % (Danish Straits).
Total waterborne inputs
Riverine and direct inputs and water flow data are quality assured by the Contracting Parties reporters before reporting to the PLC-PLUS database with the reporting WEB application. The data are further verified and quality assured using the PLC PLUS database verification tools and national expert quality assurance. After the national expert quality assurance in the PLC-PLUS database, BNI and DCE under the auspices of HELCOM RedCore DG make a quality assessment of the data in the PLC PLUS database. The experts amend the dataset filling in missing and correcting suspicious data to establish an assessment dataset, which is finally approved by the countries according to procedures described in HELCOM (2016b). The assessment dataset is used in the PLC assessments including thisBaltic Sea Environmental Fact Sheet. A description of the methods used to fill data gaps is given in PLC guidelines (HELCOM 2019) and HELCOM, 2013.
6. Strengths and weaknesses:
Strength: The data set is the most comprehensive and consistent time series of annual riverine and direct inputs 1995-2017 of total nitrogen and phosphorus to the Baltic Sea and its seven sub-basins covering the entire Baltic Sea catchment area. Data has been check with standardized quality assurance methods.
Weakness: Data from some part of the Baltic Sea catchment and some of the direct inputs in the beginning of the time series (1995-2017) are rather uncertain, and many estimates of missing data were required for the early years, particularly for direct inputs of nitrogen and phosphorus. Methods/models for estimating water flow and nutrient inputs from unmonitored areas are not completely comparable and consistent.
Further, the monitoring frequency and strategy are probably not adequate in some rivers with high variation in water flow and/or nitrogen and phosphorus concentrations, and where a substantial part of the annual load occurs within some days/few weeks.
The uncertainty of total nitrogen and total phosphorus inputs has not been estimated systematically by contracting parties. The PLC-group has roughly estimated an uncertainty (precision and bias) of 15-25% for annual total waterborne nitrogen and 20-30% for total inputs to Kattegat, Danish Straits, the main part of Baltic Proper, Bothnian Sea and Bothnian Bay. For the remaining part of BAP, and for Gulf of Finland and Gulf of Riga the uncertainty might be higher and up to 50 % for waterborne TP inputs (Svendsen et al, 2015).
8. Further work required:
Total nitrogen and phosphorus inputs from all unmonitored areas must be modelled/estimated with methods that provide consistent and comparable results. The sampling frequency and strategy in rivers should be adjusted to flow and concentrations regime and patterns in individual rivers, and at least 12 samples should be taken annually. Water flow, or at least the water level should be monitored continuously in rivers and in big direct point sources. Further, laboratories should use methods that actually provide the total nitrogen and phosphorus and with methods providing reproducible and comparable results between the involved laboratories.
For reference purposes, please cite this Baltic Sea environment fact sheet as follows:
[Author’s name(s)], [Year]. [Baltic Sea Environment Fact Sheets title]. HELCOM Baltic Sea Environment Fact Sheet 2019. Online. [Date Viewed], https://helcom.fi/baltic-sea-trends/environment-fact-sheets/eutrophication/waterborne-nitrogen-and-phosphorus-inputs/