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 sea floor (as excessive primary producers are degraded), and a cascade of other ecosystem changes. 97% of the region was assessed as eutrophied in 2011–2015 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 are not yet 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 has been evident in the Baltic Sea since the mid-1900s, accompanied by increasing severity of symptoms in the ecosystem (Larsson et al. 1985, Bonsdorff et al. 1997). Early symptoms of eutrophication are increased primary production (expressed through increased chlorophyll-a concentrations in the water column or growth of opportunistic benthic algae) and changes in the metabolism of organisms. The increased primary production leads to increased deposition of organic material which in turn leads to increased oxygen consumption. 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).
Concentrations of the main triggers of eutrophication (nitrogen and phosphorus) increased in many areas of the Baltic Sea up until the late 1980s, attributed to increased nutrient loading from land since the 1950s onwards (Figure 4.1.1, Gustafsson et al. 2012). As a result of locally improved waste water treatment, decreases in nutrient loading occurred in some local areas during the 1980s and 1990s, and in the 1990s the first effects of reducing loss of nutrients from agriculture were also seen. Since the late 1990s, the role of nutrient runoff from cultivated land has been recognised as a highly significant nutrient source in the Baltic Sea (HELCOM 1996). Nutrient inputs to the Baltic Sea have significantly decreased since the late 1990s, and in some sub-basins strong reductions have taken place recently (Figure 4.1.1-2, Box 4.1.2).
The goal of the Baltic Sea Action Plan is to a reach a Baltic Sea unaffected by eutrophication. Several eutrophication assessments have been carried out since its agreement (HELCOM 2009, 2010a, 2014a). Compared to previous HELCOM eutrophication assessments, this assessment was conducted with some new indicators and refined threshold values for evaluating status, leading to an approach which increasingly enables evaluation of progress towards improved status.
Indicators used in the assessment
Eutrophication status was evaluated in open-sea areas by assessing core indicator within three criteria: nutrient levels, direct effects and indirect effects of eutrophication (Core indicator reports: HELCOM 2017c-k).
To asses nutrient levels, core indicators on the concentrations of nitrogen and phosphorous, which primary producers need for growth, were used. Dissolved inorganic nitrogen and phosphorous are directly utilizable for phytoplankton, and are measured in the winter season when primary productivity is low. Measurements of total nitrogen and total phosphorous also include nutrients that are bound in phytoplankton, or in particles in the water. Thus, they describe the total level of nutrient enrichment in the sea. Including estimates of total nutrients makes it possible to take climate change into account in the assessment, since increased winter temperatures are expected to lead to the production of phytoplankton all year round, and thus to higher shares of nutrients being bound in phytoplankton biomass compared to dissolved forms.
To assess the direct effects of eutrophication, indicators on chlorophyll-a concentrations and water clarity (measured by the indictor ‘Secchi depth during summer’) were used. In addition, the ‘Cyanobacterial bloom index’ was included as a test indicator.
To assess indirect effects of eutrophication, the core indicator ‘Oxygen debt’ was used. This core indicator measures the volume-specific oxygen debt, which is the oxygen debt below the halocline divided by the volume. Hence, the indicator estimates how much oxygen is ’missing’ from the Baltic Sea deep water. In addition, the indicator ‘State of the soft-bottom macrofauna community’ was used to assess indirect effects of eutrophication in the open sea Gulf of Bothnia.
The coastal areas in eight countries were assessed by national indicators used in the Water Framework Directive, used to evaluate biological quality elements such as phytoplankton (chlorophyll-a), benthic invertebrate fauna and macrophytes (macroalgae and angiosperms), and supporting physical and chemical elements such as concentrations of nitrogen, phosphorus, and water clarity. Different indicators were used in different countries.
The integrated assessment of eutrophication was done using the HEAT tool which aggregates the indicator results into a quantitative estimate of overall eutrophication status (Supplementary report: HELCOM 2017B).
Integrated status assessment
The updated integrated eutrophication status assessment for 2011–2015 shows that the Baltic Sea is still affected by eutrophication (Figure 4.1.3). Out of the 247 assessment units included in the HELCOM assessment covering both coastal and open water bodies, only 17 achieved good status, showing that 97 % of the surface area in the Baltic Sea, from the Kattegat to the inner bays, is eutrophied (Figure 4.1.3). About 15 % of the surface area had eutrophication ratios in the category furthest away from good status. Only a few coastal areas are unaffected by eutrophication.
In most of the open-sea areas, good status was not achieved for the nutrient levels or the direct and indirect effects of eutrophication (Figures 4.1.4–4.1.5). Nutrient levels were in good status only in the Great Belt, and direct effects in the Kattegat (Figure 4.1.4). Indirect effects were in good status in the Bothnian Sea and the Quark, which cover 18 % of the open-sea area (Figures 4.1.4–4.1.5). The nutrient levels were generally furthest away from good status, and thus had highest overall influence on the integrated assessment results. Integrated eutrophication status had improved in only one but deteriorated in seven of the 17 open-sea assessment units since the last five year period (2007–2011).
Most coastal areas in the Baltic Sea failed to achieve good status based on nutrient levels and direct eutrophication effects, with exceptions mainly in the coastal areas of the Gulf of Bothnia and the Kattegat (Figure 4.1.4). Indirect effects achieved good status in many of the coastal areas, including the Swedish and Estonian coasts and Finnish coast of the Bothnian Sea.
Core indicator results
Table 4.1.1 shows the core indicator results for eutrophication in the open sea, and the integrated status assessment result for each of the open sea sub-basins.
The concentrations of dissolved inorganic nitrogen and total nitrogen did generally not achieve the threshold value. The threshold values were only achieved in the Kattegat and the Great Belt for total nitrogen and in the Gdansk Basin for dissolved inorganic nitrogen. The eutrophication ratios for dissolved inorganic nitrogen were highest in the Gulf of Riga and the Gulf of Finland. In addition, average concentrations were high in the Bornholm Basin due to influence from shallow stations in the Pomeranian Bay, which is influenced by the Odra plume.
Winter concentrations of dissolved inorganic nitrogen showed an increasing trend until the mid-1990s. They started declining in the late 1990’s, especially in the southwestern Baltic Sea and Kattegat (Figure 4.1.6). Compared to the previous five year period (2007–2011), dissolved inorganic nitrogen concentrations have increased substantially in four and decreased in three out of 17 sub-basins (Table 4.1.1). Concentrations of total nitrogen have remained at the same level since the period 2007–2011 in all sub-basins (Table 4.1.1).
For phosphorous, the indicator on dissolved inorganic phosphorous only achieved the threshold value in the Bothnian Bay, and the indicator on total phosphorous achieved it only in the Great Belt.
Dissolved inorganic phosphorus concentrations increased notably in the 1960s and 70s, and have shown relatively large fluctuations over time. A decrease from the high values in the mid-1980s to the present has been seen in the Kattegat, Danish Straits, Gulf of Riga and Bothnian Bay, but not in the Gulf of Finland or the Bothnian Sea. In these two sub-basins, dissolved inorganic phosphorus concentrations have increased since the early 2000s, despite decreases in the waterborne inputs from land (Figure 4.1.7). In the Baltic Proper, the concentrations decreased in the late 1990s, but increased again since then.
These recent increases probably reflect the release of phosphorus from anoxic sediments (Conley et al. 2002, 2009). Since the period 2007–2011, dissolved inorganic phosphorus concentrations have increased substantially in five sub-basins and decreased only in Gdansk Basin (Table 4.1.1). Within the same period, total phosphorus concentrations have increased substantially in three sub-basins.
The core indicators for direct effects (‘Chlorophyll-a’ and ‘Secchi depth during summer’ and additionally ’Cyanobacterial bloom index’) did not achieve the threshold value in any open sea sub-basin east of the Sound. West of the Sound, the chlorophyll-a core indicator achieved the threshold value in the Kattegat, and water clarity in the Kattegat and the Sound.
The longer term trend shows that chlorophyll-a concentrations have increased from the 1970’s to the present in most of the inner Baltic Sea (Figure 4.1.8). In the Kattegat and the Danish Straits, the chlorophyll-a concentration has been decreasing since the late 1980s (Figure 4.1.8). Compared to the previous five year period (2007–2011), the chlorophyll-a concentrations have decreased in seven sub-basins, but increased in the Bornholm Basin, Northern Baltic Proper, Gulf of Finland and Gulf of Riga (Table 4.1.1).
The longest time series available for water clarity have been recorded since the early 1900s in the Baltic Proper. The results show a steadily deteriorating situation over several decades (Figure 4.1.9). In more recent years, however, the decrease in water clarity has levelled off across most of the Baltic Sea, and the water clarity has remained on the same level since the period 2007–2011 in most of the sub-basins (Table 4.1.1). The water clarity reflects changes in the eutrophication-related abundance of phytoplankton, but is also affected by the presence of coloured dissolved organic matter and suspended particles.
The cyanobacterial bloom index was used as a test indicator in ten sub-areas, showing the worst status in the Gulf of Riga, the Northern Baltic Proper and the Bothnian Sea. The index has remained at the same level since the previous five year period 2007–2011 in most of the sub-basins (Table 4.1.1).
The core indicator ‘Oxygen debt’ did not achieve the threshold values in any open sea sub-basin. Oxygen debt has increased over the past century (Figure 4.1.10). It plateaued from the early 1980’s to the early 1990’s, but has subsequently increased again. Since the last assessment period (2007–2011), the oxygen debt has remained at the same level (Table 4.1.1). North of the Baltic Proper, the indicator ‘State of the soft-bottom macrofauna community’ was also included, to estimate the condition of the animal community at the seafloor. The core indicators achieved the threshold value in these areas suggesting the bottom fauna to be in good condition.
Impacts and recovery
Primary production is a key process in the ecosystem as it provides energy for all organisms, but nutrient enhanced excessive primary production leads to eutrophication symptoms and reduces the function of the food web in many cases. 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 reduces 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.
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. It should be noted, however, that in areas with vertical stratification and low water exchange, eutrophication acts on top of naturally low oxygen levels. Life in these deep water habitats is also highly dependent on aeration provided by inflows of marine water from the North Sea (see Chapter 1, Figure 1.9).
Even though some positive development in the eutrophication status is seen in the current assessment, such as a decrease in nutrient concentrations, improved water clarity in parts of the Baltic Sea, and a decrease in chlorophyll concentrations in some areas, 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. The 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 (Figure 4.1.2). Large scale responses to reduced loading are slow, and recently achieved reductions are not visible in the assessments over the short time frame. In addition, future development is foreseen to be dependent on changes in climate (Box 4.1.3).