The Baltic Sea still suffers from eutrophication. Excessive input of nutrients to the marine environment enhances the growth of phytoplankton, leading to reduced light conditions in the water, oxygen depletion at the seafloor (as excessive primary producers are degraded), and a cascade of other ecosystem changes. At least 97 percent of the region was assessed as eutrophied in 2011–2016 according to the integrated status assessment. Nutrient inputs from land have decreased as a result of regionally reduced nutrient loading, but the effect of these measures has not yet been detected by the integrated status assessment. Although signs of improvement are seen in some areas, effects of past and current nutrient inputs still predominate the overall status.
Eutrophication, or increase in the supply of organic matter to an ecosystem through nutrient enrichment, is induced by excessive availability of nitrogen and phosphorus for primary producers (algae, cyanobacteria and benthic macrovegetation). Its early symptoms are enhanced primary production, which is expressed through increased chlorophyll-a concentrations in the water column and/or the growth of opportunistic benthic algae, as well as changes in the metabolism of organisms. The increased primary production may lead to reduced water clarity and increased deposition of organic material, which in turn increase oxygen consumption at the seafloor and may lead to oxygen depletion. These changes may in turn affect species composition and food web interactions (as species that benefit from the eutrophied conditions are favoured directly or via effects on habitat quality and feeding conditions; Cloern 2001).
Inputs of nitrogen and phosphorus have been increasing for a long time in the Baltic Sea, mainly between the 1950s and the late 1980s (Figure 4.1.1, Gustafsson et al. 2012), causing eutrophication symptoms of increasing severity to the ecosystem (Larsson et al. 1985, Bonsdorff et al. 1997, Andersen et al. 2017). As a response to the deteriorating development, actions to reduce nutrient loading were agreed on by the 1988 HELCOM Ministerial Declaration, and reaching a Baltic Sea unaffected by eutrophication is included as one of the main goals of the Baltic Sea Action Plan (BSAP; HELCOM 2007). Maximum allowable inputs (MAI) for the whole Baltic Sea and each sub-basin, and Country-Allocated Reduction Targets (CART) were set in 2007, and updated in the 2013 HELCOM Ministerial Declaration (HELCOM 2013a).
Several HELCOM eutrophication assessments have been carried out since the agreement of the Baltic Sea Action Plan, to follow-up on the status of eutrophication of the Baltic Sea (HELCOM 2009, 2010a, 2014a; see also Box 4.1.1). The current assessment covers the situation during years 2011-2016. In comparison to previous HELCOM eutrophication assessments, some new indicators are included, enhancing the coverage of assessment criteria. For other indicators, threshold values for evaluating status have been refined, leading to an approach which increasingly enables evaluation of progress towards improved status.
Nutrient inputs to the Baltic Sea
Eutrophication was first recognized as a large-scale pressure of the Baltic Sea in the early 1980s, and in part attributed to anthropogenic nutrient loading (HELCOM 1987, 2009). Actions to reduce nutrient loading in the order of 50 % were agreed on by the 1988 HELCOM Ministerial Declaration, and reaching a Baltic Sea unaffected by eutrophication was identified as one of the goals of the Baltic Sea Action Plan in 2007 (HELCOM 2007, 1988).
Trends in nutrient inputs
Since the 1980s, nutrient inputs to the Baltic Sea have decreased, and in some sub-basins strong reductions have taken place. For example, waterborne nitrogen inputs to the Baltic Sea are currently at the level that they were in the 1960s, and the phosphorus inputs at the level of 1950s (Figure 4.1.1). The total nitrogen input to the Baltic Sea was about 7 % larger than the maximum allowable input in 2015, whereas phosphorus input remained 44 % above this threshold value (HELCOM 2018i).
The current annual total input of nutrients to the Baltic Sea amounts to about 826,000 tonnes of nitrogen and 30,900 tonnes of phosphorus (HELCOM 2018h). Most of the input is riverine for both nitrogen and phosphorus (Figure 4.1.2). Atmospheric inputs account for about 30 % of the total nitrogen inputs, originating mainly from combustion processes related to shipping, road transportation, energy production, and agriculture. The largest relative decreases in inputs of nitrogen and phosphorus over recent decades have occurred in direct sources, which currently account for 4-5 % of the total loads (Figure 4.1.2). The atmospheric input of nitrogen has decreased by between 24 and 30 % during 1995-2015 for all sub-basins, while changes in waterborne nitrogen input are clearly more variable (HELCOM 2018i).
Natural sources constitute about one third of the riverine inputs of nitrogen and phosphorus to the Baltic Sea (Figure 4.1.2). A major part of the anthropogenic part originates from diffuse sources, mainly agriculture, while point sources, dominated by municipal waste water treatment plants, contribute with 12 % and 24 % of the riverine nitrogen and phosphorus loads, respectively.
Nutrient reduction targets for sub-basins
Based on the revised maximum allowable inputs (MAI) for the seven sub-basins of the Baltic Sea within the HELCOM nutrient reduction scheme, reductions of nitrogen input were needed in three sub-basins (HELCOM 2013a). Of these, the MAI has been fulfilled in the Kattegat, whereas reductions are still required for nitrogen input to the Gulf of Finland and Baltic Proper (HELCOM 2018i). In the remaining four sub-basins, the input of nitrogen has remained within or close to the maximum allowable input (Figure 4.1.3).
Reductions of phosphorus input were set for three sub-basins: the Baltic Proper, Gulf of Finland and Gulf of Riga (HELCOM 2013a). In all three cases, reductions are seen but notable further reductions are still needed in order to reach the allowable levels (Figure 4.1.3). So far, the most pronounced results are seen for the Gulf of Finland, where the phosphorus input has been cut with more than half compared to the reference period (Figure 4.1.4). This reduction has been attributed to improved waste water treatment in St. Petersburg and actions to prevent phosphorus release from a fertilizer factory in the catchment of river Luga (Raateoja and Setälä 2016).
Overall, the normalized input of nitrogen was reduced by 12 % and the normalized input of phosphorus by 25 % between the reference period (1997-2003) and 2015 (HELCOM 2018i). The strongest relative changes over the past decades are seen in the Kattegat and the Danish straits for nitrogen input and in the Gulf of Finland for phosphorus input (Figure 4.1.4).
Indicators included in the assessment
The integrated assessment of eutrophication was done using the HELCOM HEAT tool which aggregates the indicator results into a quantitative estimate of overall eutrophication status. Eutrophication status was evaluated by indicators within three criteria: nutrient levels, direct effects and indirect effects of eutrophication.
To assess nutrient levels in the surface water, eutrophication core indicators on the concentrations of nitrogen and phosphorus were used (Core indicator reports: HELCOM 2018j-m). Primary producers need both nitrogen and phosphorus for growth. Dissolved inorganic nitrogen and phosphorus, which are directly utilisable by primary producers, are assessed in the winter season when primary productivity is low and their concentrations are largely unaffected by uptake. Hence, these indicators represent the nutrient pool available for phytoplankton growth. Core indicators for total nitrogen and total phosphorus also include dissolved organic nutrients (such as proteins, urea and humic substances), as well as nutrients which are bound in particulate organic matter (such as phytoplankton and detritus). The inorganic nutrients which enter the sea are rapidly taken up by organisms and bound to their biomass. Via excretion and decay they are then transformed into dissolved organic nitrogen and phosphorus, which again re-mineralise (Markager et al. 2011, Knudsen-Leerbeck et al. 2017). Hence, the total nutrient indicators provide an estimate of the total level of nutrient enrichment in the sea , .
To assess the direct effects of eutrophication, core indicators on chlorophyll-a concentrations in the surface water and water clarity were used (Core indicator reports: HELCOM 2018n-o). In addition, the ‘Cyanobacterial bloom index’, which is not yet an agreed core indicator, was included as test (HELCOM 2018p).
To assess indirect effects of eutrophication, the core indicator ‘Oxygen debt’ was used (Core indicator report: HELCOM 2018q). This indicator measures the volume-specific oxygen debt, which is the oxygen debt below the halocline divided by the volume of the water mass below the halocline. Hence, it estimates how much oxygen is ’missing’ from the Baltic Sea deep water, primarily as a result of degradation of organic matter. In the open sea of the Bothnian Bay, Quark, Bothnian Sea, and Gulf of Riga, where the oxygen debt indicator was not applicable, the biodiversity core indicator ‘State of the soft-bottom macrofauna community’ was used in order to address indirect effects of eutrophication (Core indicator report: HELCOM 2018r). In these areas, this indicator was seen to be suitable for the eutrophication assessment, since it responds only or mainly to eutrophication-related pressures.
Coastal areas were assessed by national indicators mainly derived from the implementation of the Water Framework Directive (EC 2000). These indicators varied between different national coastal areas. They included indicators describing the level of phytoplankton (via biomass or chlorophyll-a -concentration), benthic invertebrate fauna, macrophytes (macroalgae and angiosperms), concentrations of nitrogen, concentrations of phosphorus, and water clarity (For more information, see Thematic assessment; HELCOM 2018B).
Integrated status assessment
The integrated eutrophication status assessment for 2011–2016 shows that the Baltic Sea is still affected by eutrophication (Figure 4.1.5). Out of the 247 assessment units included in the HELCOM assessment of coastal and open water bodies, only 17 achieved good status.
In terms of areas covered, 96 % of the surface area in the Baltic Sea, from the Kattegat to the inner bays, is below good status in regards to eutrophication. The assessment results were in the category furthest away from good status in about 12 % of the area. Only a few coastal areas were not affected by eutrophication.
In many open-sea areas, good status was not achieved with respect to any of the assessed criteria; nutrient levels, direct or indirect effects of eutrophication (Figure 4.1.6 a-c, 4.1.7). Generally, indicators for nutrient levels were furthest away from good status, and thus had highest influence on the integrated assessment results. This was especially evident for Bornholm Basin where shallow stations located in the Pomeranian Bay had significant impact on nutrient level results (Figure 4.1.8). Nutrient levels were in good status only in the Great Belt, being just below the limit for good status, and direct effects were in good status only in the Kattegat. For indirect effects of eutrophication, good status was seen north of and including the Åland Sea, covering 25 % of the total open-sea area.
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Confidence in the assessment
The final confidence of the integrated assessment was moderate in most of the open sea. It was low in the Gulf of Riga, the Åland Sea and the Quark, and high in the Arkona Basin and Bornholm Basin (For more information, see the thematic assessment: HELCOM 2018B).
Changes in comparison to the previous assessment
Compared to previous assessment results (2007-2011; HELCOM 2014a, 2015a) the integrated eutrophication status has improved in the Gdansk Basin, but deteriorated in four of the seventeen open-sea assessment units (Figure 4.1.8). However, a long-term analysis of integrated assessment results using HEAT 3.0 indicate an improving eutrophication status since the mid-1990s in the westernmost parts of the Baltic Sea: the Kattegat, Danish Straits and Arkona Basin (Andersen et al. 2017).
The limited improvement in comparison to the previous assessment could in part be attributed to natural variability acting on top of the human induced eutrophication effects. Past nutrient inputs have enhanced the occurrence of oxygen deficiency and led to an excess of nutrients in deep waters of the central Baltic Sea (Thematic assessment; HELCOM 2018B). Further, inflow events of marine water from the North Sea may have caused intrusions of nutrient-rich deep water from the Central Baltic Sea to adjacent areas leading to enhanced anoxia in the receiving areas and hence an enhanced release of phosphorus from the sediments.
Figure 4.1.8 shows the numerical integrated status assessment results for each of the open sea sub-basins, together with the corresponding core indicator results. More detailed results are presented in the thematic assessment: HELCOM (2018B).
Longer term changes in the core indicators
Assessments of longer term trends additionally show possible effects of nutrient reduction efforts over a larger time scale. When assessing a shorter time span, such as when comparing two assessment periods of six years each, as above, natural variability in climate and hydrography may result in temporarily worsened conditions even if the long term development shows a different pattern. A recent example is the major saline inflow which occurred in December 2014, which has caused intrusions of deep sea water with high phosphate concentration into surface waters (Finnish environment institute 2016). Further, the Baltic Sea has a long water residence time, lasting over decades. Hence, pools of nutrients and organic matter which have accumulated over decades with high nutrient inputs are very large and will delay the improvement in environmental conditions.
Analyses of developments since 1990 show an improving eutrophication status in the westernmost parts of the Baltic Sea (Thematic assessment; HELCOM 2018B). Levels of nitrogen are predominantly decreasing, with the exception of some sub-basins in the southern Baltic Sea. The results can be viewed as responses to substantial decreases in nitrogen loadings, proving that the nutrient reductions are effective. Phosphorus concentrations do not show the same improvement. For most areas the levels of phosphorus are constant or even increasing, with the exception of a decrease in total phosphorus concentrations in the Great Belt and Kiel Bay. This result reflects that phosphorus is stored in the sediment to a much higher degree than nitrogen, and the present conditions additionally encompass previous high inputs. In addition, the aforementioned major saline inflow has affected the situation in recent years. Ongoing reductions in phosphorus input are expected to lead to decreasing phosphorous concentrations over the coming years.
A summary of how selected indicators representing nutrient levels, direct and indirect effects have changed over the past decades is given below. More results are presented in HELCOM (2018B), and more details about each of the agreed HELCOM core indicators are given in the core indicator reports (HELCOM 2018j-r).
The concentrations of dissolved inorganic nitrogen and total nitrogen did generally not achieve the threshold value with the exception of Kattegat and Great Belt where the threshold values were achieved for total nitrogen (Figure 4.1.8). The highest eutrophication ratios occurred for dissolved inorganic nitrogen in the Gulf of Riga, the Gulf of Finland, and the Bornholm Basin. Average concentrations in the Bornholm Basin were high due to influence from shallow stations in the Pomeranian Bay under influence from the river Odra plume.
Winter concentrations of dissolved inorganic nitrogen have shown an increasing trend up until the early 1990s, but the increase has thereafter ceased throughout the Baltic Sea. They have decreased significantly in twelve of the seventeen sub-basins since the 1990s (Thematic assessment: HELCOM 2018B). Total nitrogen concentrations decreased significantly between 1990 and 2016 in ten of the sub-basins, but they increased in the Bornholm Basin, Gdansk Basin and the Eastern Gotland Basin (For examples, see Figure 4.1.9, see also HELCOM 2018B). Increasing variability is likely attributed to increased monitoring frequency in several sub-basins. In the Bornholm Basin, this also reflects influence from the river Odra.
In more recent times, comparing the last five year assessment period (2007–2011) to the current one (as presented in Figure 4.1.8 above), dissolved inorganic nitrogen concentrations have increased substantially in four out of fifteen addressed sub-basins. Concentrations of total nitrogen have decreased in the Sound and the Gulf of Riga and increased in the Gdansk Basin compared to the period 2007–2011.
The indicator for dissolved inorganic phosphorus achieved the threshold value only in the Bothnian Bay, and total phosphorus achieved it only in the Great Belt. A notable increase in total phosphorus was seen in the 1960s and 1970s. This increase ceased around 1990, and relatively large fluctuations have occurred over time (For examples, see Figure 4.1.10; see also Thematic assessment, HELCOM 2018B). During the assessed time period 1990-2016, an increase in concentrations of dissolved inorganic phosphorus occurred in one sub-basin, the Åland Sea. Concentrations of total phosphorus increased significantly in the Northern Baltic Proper, the Bornholm Basin and the Western Gotland Basin, but decreased in the Great Belt and Kiel Bay (HELCOM 2018B).
In comparison to the latest assessment period (2007–2011) the current levels of dissolved inorganic phosphorus are higher (>15 %) in eight of the seventeen sub-basins (Figure 4.1.8). Total phosphorus concentrations have increased substantially in the Gdansk Bay and the Gulf of Riga and decreased in the Northern Baltic Proper and the Quark. In areas with deep water oxygen deficiency, increases in phosphorus concentrations can at least partly be attributed to release of phosphorus from sediments during transition to anoxic conditions (Conley et al. 2002, 2009, Lehtoranta et al. 2016).
None of the core indicators for direct effects, namely ‘Chlorophyll-a’ and ‘Water clarity’, nor the pre-core indicator ’Cyanobacterial bloom index’ achieved the threshold value east of the Sound (Figure 4.1.5). ‘The indicator for Chlorophyll-a achieved the threshold value in the Kattegat, and that for water clarity in the Kattegat and the Sound.
The chlorophyll concentrations have remained essentially unchanged during the past few decades (1990-2016), with the exception of the most western parts of the Baltic Sea, where it shows decreasing trends (Figure 4.1.11; see also Thematic assessment: HELCOM 2018B). The result corresponds well with decreases in nitrogen inputs and concentrations in the western parts, where nitrogen is considered the most limiting nutrient for phytoplankton growth. In the central and eastern parts of the Baltic Sea, where summer chlorophyll-a concentration is mainly related to phosphorus concentrations the indicator shows no changes. A deteriorating trend was detected only in the Bornholm Basin, which is attributed to influence from measurements at shallow stations in the Pomeranian Bay and outflow from the river Odra.
Compared to the previous five year period (2007–2011), chlorophyll-a concentrations have decreased in the Kattegat, Great Belt and the Sound, but increased in the Northern Baltic Proper and the Gulf of Riga (Figure 4.1.8).
The long-term series for water clarity show a steadily deteriorating situation over several decades, most profoundly in the north-eastern sub-basins (Fleming-Lehtinen and Laamanen 2012). In more recent years, however, the decrease in water clarity has levelled off across most of the Baltic Sea (Figure 4.1.12; Thematic assessment: HELCOM 2018B). Looking over the time period 1990-2016, water clarity has decreased in four of the seventeen sub-basins, and has increased (improved) in the Kattegat and the Great Belt.
Water clarity is affected by the abundance of phytoplankton (which is related to eutrophication), but is also affected by the total amount of organic matter in the system. Particulate as well as dissolved organic matter affect the attenuation of light, and both of them have eutrophication and non-eutrophication related components. Eutrophication is attributed to the portion of organic matter produced within the sea, in the form of either phytoplankton or other organic matter.
As the total amount of organic matter in the system is still at a high level after many decades of elevated nutrient inputs, water clarity is not expected to decrease until the pools of organic matter are degraded or washed out of the Baltic Sea. Recovery is expected to take decades, although improvements in the most northern parts are promising.
In comparison to the period 2007–2011, water clarity has improved in three western sub-basins and decreased (deteriorated) in the Bothnian Bay and the Bothnian Sea under 2011-2016 (Figure 4.1.8).
The ‘Cyanobacterial bloom index’ did not achieve the threshold value in any of the ten sub-basins where it was tested. The worst status was indicated for the Gulf of Riga, the Northern Baltic Proper and the Bothnian Sea. Long-term data was available for the Eastern Gotland Basin, the Northern Baltic Proper and the Gulf of Finland, showing a deteriorating trend in the Baltic Proper during 1990-2016 (Figure 4.1.13).
Compared to the previous five year period 2007–2011, the ‘Cyanobacterial bloom index’ has further deteriorated in the Gulf of Riga and the Bay of Mecklenburg and improved in the Gdansk Basin during the current assessment period 2011-2016 (Figure 4.1.8).
The core indicator ‘Oxygen debt’ did not achieve the threshold values in any assessed open sea sub-basin (Figure 4.1.5). The indicator has increased over the past century (Figure 4.1.14). It levelled off between the early 1980s and the early 1990s, but has subsequently increased again. In comparison with the most recent previous assessment period (2007–2011), oxygen debt during 2011-2016 has remained at the same level (Figure 4.1.8).
North of the Baltic Proper, the indicator ‘State of the soft-bottom macrofauna community’was included to evaluate the condition of the animal community at the seafloor. The indicator achieved the threshold value in these areas.
Impacts and future perspective
Primary production is a key process in the ecosystem as it provides energy for all organisms. On the other hand, excessive primary production leads to eutrophication symptoms and reduces the function of the food web in many cases, as well as socioeconomic effects (Box 4.1.2). An increased intensity and frequency of phytoplankton blooms typically leads to decreased water clarity and increased sedimentation. These conditions further limit the distribution of submerged vegetation, such as macroalgae and macrophytes, and reduce the habitat quality of coastal areas. Increased sedimentation and microbial degradation of organic matter increases oxygen consumption and depletes oxygen conditions in areas with poor water exchange, including deep water areas. The extent of oxygen-deficient waters has increased more than ten- fold over the past one-hundred and fifteen years (Carstensen et al. 2014). After a stagnation period, the oxygen deficiency has expanded again over the last two decades (Carstensen et al. 2014). Also in the coastal areas, hypoxia has steadily increased since the 1950s (Conley et al. 2011).
By the 1960s the soft bottom fauna was already disturbed in some parts of the Baltic Sea, attributed to eutrophication. Human induced nutrient inputs have contributed to the enhanced distribution of areas with poor oxygen conditions seen today, including deep waters. In areas with vertical stratification and low water exchange, eutrophication acts on top of naturally low oxygen levels, further enhancing these conditions. Life in deep water habitats is also highly dependent on aeration provided by inflows of marine water from the North Sea.
Some positive development in the eutrophication status is seen in the current assessment, such as a decrease in nitrogen concentrations in most of the Baltic Sea and improved water clarity and decreased chlorophyll-a concentrations in some western parts of the Baltic Sea. Moreover, the intensity of the spring blooms is seen to have been reduced from 2000 to 2014 due to reductions in nutrient loading (Groetsch et al. 2016). However, the results show that the Baltic Sea is still highly affected by eutrophication and that the impacts on organisms and human well-being will continue. Large scale responses to reduced loading are slow, and recently achieved reductions are not visible in the short time-frame of the assessments.
The recovery of the Baltic Sea from eutrophication depends on the continuing efforts to reduce nutrient loading. Ongoing and agreed reductions of nutrient inputs according to the HELCOM Baltic Sea Action Plan are foreseen to be effective in decreasing the eutrophication symptoms in the long term. Based on modelling simulations of the Baltic Sea biogeochemistry under different nutrient reduction schemes, implementation of the BSAP nutrient reductions will lead to significantly improved eutrophication state of the Baltic Sea within this century, including reduced primary productivity, nitrogen fixation and hypoxia (Saraiva et al. 2018). Climate change is foreseen to amplify eutrophication symptoms, with biogeochemical responses depending on the implemented nutrient reductions (Box 4.1.3), hence enhancing the importance of nutrient reductions (Saraiva et al. 2018).