Water quality

The water quality in the Baltic Sea is an important factor that influences the environment and the living conditions for associated fauna and flora. On this basis, and demonstrated by the require-ments outlined in the MSFD and WFD (see sections 4.2.5 and 4.2.6, respectively), water quality is considered an important receptor. This section describes the current water quality in the Baltic Sea, particularly with respect to contaminant and nutrient concentrations, turbidity and oxygen content.

Metals

The main sources of heavy metals to the Baltic Sea are diffuse sources (e.g. leakage from forest and agricultural soils) and industrial and municipal point sources /107/. Heavy metals are dis-charged directly, transported via rivers or supplied from the air. Significant airborne heavy metal pollution originates from sources outside the Baltic Sea catchment area.

Three metals, mercury (Hg), lead (Pb) and cadmium (Cd), are included in the HELCOM list of environmental core indicators, and their status was recently reported. The primary matrix for cad-mium and lead is water, as the primary threshold values for these two core indicators are agreed to be the respective EQS values for water (0.2 μg/l and 1.3 μg/l, respectively). The preferred matrix for monitoring of metals in the HELCOM COMBINE monitoring programme is biota and sediment, and as a result, very little direct data are available for cadmium and lead in Baltic Sea water.

Cadmium concentrations in the water phase have been measured by Russia (1995–1998), Ger-many (1998–2015), Lithuania (2007–2015) and Poland (2011-2015), and only a small percentage of these measurements were above the annual average EQS (AA-EQS) of 0.2 μg/l. Lead concen-trations in seawater have been measured by Russia (1995–1998), Germany (1998–2015), Lithua-nia (2007–2015) and Poland (2011-2015), and 11% of German and LithuaLithua-nian measurements were above the AA-EQS of 1.3 μg/l. When including monitoring results from sediments and biomass in the assessment, the environmental status of core indicators cadmium and lead in the waters around Bornholm (and most of the Baltic Sea in general) is considered poor. Mercury is measured in fish tissue as a primary matrix in the HELCOM monitoring programme, and no recent water measurements have been reported. However, a substantial dataset for fish tissue exists, indicating that the environmental status of the core indicator mercury is poor throughout the Baltic Sea, including in the waters around Bornholm.

Administrative Order 1625 of 19/12/2017 issued by DEPA /115/ lists a number of threshold con-centrations for metals that describe GES. These include values both for the AA-EQS and for maxi-mum allowable concentrations, as summarised in Table 7-24.

Table 7-24 Thresholds for GES of water in regard to metals /115/.

Metal Annual average concentration in

seawater, µg/l Maximum concentration in sea-water, µg/l

There have been substantial inputs of organic pollutants in the Baltic Sea from numerous sources over the past 50 years. These sources include industrial discharges, such as the organochlorines in effluent from pulp and paper mills, run-off from farmland, special paints used on ships and boats and dumped wastes. Several organic pollutants, such as DDT and technical-grade hexachlorocy-clohexanes (HCH isomers) have been completely banned since the 1980s.

Organic pollutants can reach the Baltic Sea via river run-off, atmospheric deposition and direct discharge of effluents or via inflowing water from the North Sea. Organic pollutants are usually adsorbed onto fine-grained particles in the water mass and carried to the seabed by sedimentation.

The concentrations of organic contaminants in the sediment are therefore generally several orders of magnitude higher than in the overlying water mass /136/.

Recent data regarding organic pollutants in the water are scarce, because the HELCOM COMBINE monitoring programme is based on measurements of biomass and sediment samples. The general status of the HELCOM core indicators PAH, PCB, organochlorine pesticides and organotin were discussed in section 7.3.3.

Administrative Order 1625 of 19/12/2017 issued by DEPA /115/ lists a number of threshold con-centrations for organic contaminants describing GES. These include values both for annual aver-ages and for maximum allowable concentrations, as summarised in Table 7-25.

Table 7-25 Thresholds for GES of water in regard to organic pollutants /115/.

Group Chemical Annual average

concentra-tion in seawater, µg/l Maximum concentration in seawater, µg/l

As discussed in section 7.3.3.7, increased concentrations of nutrients, mainly relating to nitrogen (N) and phosphorus (P) compounds, can cause eutrophication, considered one of the major pres-sures on the Baltic Sea ecosystem /111/. N and P concentrations in the water are among the core indicators of this pressure /110/.

7.5.3.1 Nutrient sources and input

Land-based nutrient inputs to the Baltic Sea are both air- and waterborne, as illustrated in Figure 7-23. Typical pathways of nutrient inputs to the offshore environment are discussed in /137/ and are summarised below:

• Direct atmospheric deposition on the water surface. Atmospheric emissions of airborne nitrogen compounds emitted from traffic or combustion of fossil fuels (heat and power generation) and from animal manure and husbandry, etc. A significant part of this load originates in areas outside the Baltic Sea catchment area.

• Riverine inputs of nutrients to the sea. Rivers transport nutrients that have been discharged or lost to inland surface waters within the Baltic Sea catchment area.

• Exchange with the North Sea via transport through the Danish straits.

• Point sources discharging directly to the sea. Point sources include inputs from municipalities, industries and fish farms discharging into inland surface waters and discharging directly into the Baltic Sea.

• Diffuse sources. These mainly originate from agriculture but also include nutrient losses from e.g. managed forestry and urban areas.

• Natural background sources. These mainly refer to natural erosion and leakage from unman-aged areas and the corresponding nutrient losses from e.g. agricultural and manunman-aged forested land that would occur irrespective of human activities.

Figure 7-23 Typical sources of nutrients to the sea /138/.

As illustrated in Figure 7-24, the total inputs of N and P to the Baltic Sea from waterborne sources have been reduced since the 1980s due to measures implemented by the Baltic countries.

Figure 7-24 Temporal development of waterborne inputs of total nitrogen and total phosphorus to the Baltic Sea /111/.

In spite of the reduced inputs of N and P, the concentrations of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) in seawater from the different sub-basins in the Baltic Sea have not decreased much since the 1980s, and, with a few exceptions, the core indicators DIN, total nitrogen (TN), DIP and total phosphorus (TP) all remain above the targets for GES in all sub-basins of the Baltic Sea /111/.

7.5.3.2 Eutrophication in Danish waters

Eutrophication is a condition in an aquatic ecosystem in which high nutrient concentrations stimu-late growth of algae, leading to imbalanced functioning of the system. Nitrogen and phosphorus are the main growth-limiting nutrients in the Baltic Sea, and therefore increased inflows of N and P can result in an increase in the growth of algae in the water. When the algae die and the biomass sinks to the bottom, a process of decomposition occurs and the nutrients contained within the organic matter are converted into inorganic salts and gases. This decomposition consumes oxygen and can result in oxygen deficiency. Hypoxic conditions at the seabed may in turn result in loss of important ecosystem functions carried out by benthic fauna, e.g. biogeochemical feedback loops and biomass production /139/.

Concentrations of DIN and DIP in seawater from the Arkona and Bornholm Basins during the years 2011 to 2015 are summarised in Table 7-26. Also shown in the table are the target concentrations corresponding to GES, as agreed by HELCOM /140//141/.

Table 7-26 Concentrations (average 2011-2015) and threshold values of DIN and DIP.

Basin DIN

Average 2011-2015 µmol/l

DIN Target value

µmol/l

DIP

Average 2011-2015 µmol/l

DIP Target value

µmol/l

Arkona 4.05 2.90 0.61 0.36

Bornholm 9.06 2.50 0.62 0.30

The general eutrophication state of the waters around Bornholm has recently been evaluated based on core indicators for nutrient levels (DIN, TN, DIP, TP), direct effects (chlorophyll a, Secchi depth, cyanobacterial bloom index), and indirect effects (oxygen debt and zoobenthos) /111/. It was con-cluded that the state of the water west of Bornholm was “poor”, and the state of the waters north, east and south of Bornholm was “bad”, see Figure 7-25.

Figure 7-25 HELCOM integrated eutrophication status assessment /111/.

Water turbidity

Water turbidity depends on the amount of particulate matter and dissolved substances in the water column. This may include suspended solids, plankton, humic acids and other dissolved coloured substances. Water turbidity varies naturally due to mobilisation and resuspension of seabed sedi-ments by waves and currents in shallow areas. Fine-grained sedisedi-ments (with a diameter <0.063 mm), e.g. silt and clay, are often cohesive and tend to flocculate and form aggregates in seawater.

When sediments are resuspended, the grains are transferred away from the seabed into the water column by turbulent mixing, with the lowest concentration in the upper part of the water column and the highest concentration near the seabed. In general, fine-grained sediments remain in sus-pension for a longer period and have the potential to travel relatively long distances before depos-iting, due to their low settling velocity.

Suspended solids usually settle to the seabed in accumulation areas, possibly after having been temporarily deposited and subsequently resuspended in shallow-water areas. As particles with a high organic content settle onto the seabed, they may form a very loose surface sediment layer with a low dry-weight content (a so-called “fluff-layer”). These surface sediments are easily resus-pended due to erosion caused by the shear stress imposed by wave and current action /142//143/.

Resuspension of the loosest surface sediments may occur even at relatively large depths due to storm wave action. Large waves have been found to be able to move sand, gravel and even cobbles up 20 cm in diameter at water depths greater than 20 m /144/.

Furthermore, turbidity increases during the summer throughout the Baltic Sea due to the increased growth of phytoplankton, see section 7.7.

The water turbidity in the Bornholm Basin and the Arkona Basin has improved during the last two decades, and compared with most other sub-regions of the Baltic Sea, Danish waters have a rela-tively low turbidity level /145/. As noted above, turbidity is strongly linked to the suspended sedi-ment concentration in the water column. The suspended sedisedi-ment concentration in the saline bot-tom water in the Baltic Sea is typically 1-2 mg/l, although during stormy periods, the concentration of suspended sediments has been shown to increase locally to 30-40 mg/l /146/.

Secchi depth (a measure of the clarity of the water) is employed by HELCOM as a core indicator for eutrophication and is monitored routinely. The target Secchi depth for the Arkona Basin is 7.2 m and for the Bornholm Basin the target is 7.1 m /147/. The results of Secchi depth measurements in the Bornholm and Arkona Basins during the years 2011 to 2015 are shown in Figure 7-26 /148/.

Secchi depths were also measured at several stations around Bornholm as part of the monitoring performed during NSP, and the results were in the range indicated in shown in Figure 7-26 /149/.

Figure 7-26 Summer (June-September) Secchi depth yearly averages in surface water from the Bornholm and Arkona Basins (blue columns). Also shown are the yearly averages for 2011-2015 (black line) and target levels as agreed by HELCOM HOD 39/2012 (red broken line).

Oxygen content

In the Baltic Proper, the low oxygen concentrations are a result of eutrophication and a weak renewal of water. Oxygen consumption increases in the period from late summer to early autumn, when relatively high bottom-water temperatures and the presence of degradable organic matter accelerate mineralisation of organic matter. Eutrophication provides a surplus of organic matter to the benthic environment, which further increases oxygen demand. The bottom water concentration of oxygen is therefore influenced by the balance between oxygen consumption at the seabed (which is affected by eutrophication) and the supply of oxygen from the surface layer due to vertical mixing and/or lateral transport of oxygen-rich water. Vertical exchange decreases with depth and is re-pressed by stratification caused by the salinity and temperature gradients, and oxygen replenish-ment below the prevailing halocline is largely limited to infrequent inflows of oxygen-rich marine water from the Kattegat. Seawater inflow and stratification of the water column is further described in section 7.4.2.

In the Kattegat, the Danish straits, the western Baltic Sea and coastal areas, oxygen depletion is a seasonal phenomenon, while hypoxic/anoxic conditions in the deep waters (i.e. Baltic Proper) seem to be persistent and independent of seasonality /150/.

The increasing distribution of areas with poor oxygen conditions in the bottom water has been noted as a particular concern for the Baltic Sea, and “oxygen debt” is included in HELCOM’s list of core indicators for the environmental status of the inner basins of the Baltic Sea (including the Bornholm Basin), but not for the area west of Bornholm or the Arkona Basin. In the Bornholm Basin, the average oxygen debt during the years 2007 to 2011 was -7.10 mg/l, which is only slightly more negative than the target of -6.37 mg/l. The Bornholm Basin was assessed to be sub-GES in regard to the oxygen debt indicator of eutrophication /151/.

Measurements performed by Nord Stream 2 AG east and south of Bornholm are discussed in 7.4.2, and confirmed the pattern of decreasing oxygen concentration with depth below the halo-cline.