Eutrophication is the Baltic Sea’s biggest problem

Eutrophication is the biggest threat to the Baltic Sea’s ecosystem. The process was accelerated after the Second World War, when the input of nitrogen and phosphorus into the seawater increased after the advent of industrialisation and intensive agriculture. Urbanisation also added to the environmental load.


The all-time peak in nutrient concentrations in the Baltic Sea was achieved in the 1980s and the 1990s. Today, the concentrations are either staying at a steady level or dropping. The state of eutrophication has been monitored since the 1960s, so we have ample data on its development.

The EU’s marine policy mandates monitoring the state of sea areas as part of marine management. According to the eutrophication indicators used by the EU, the Finnish areas of the Baltic Sea are not in a good state. The Baltic Marine Environment Protection Commission (HELCOM) was founded to monitor the state of the Baltic Sea and to organise environmental protection.

The sea’s condition is the worst in the coastal waters of the Gulf of Finland and the Archipelago Sea, and in the open sea areas of the northern parts of the Baltic Proper, Sea of Åland and Bothnian Sea.

Satellite image of large blue-green algae blooms in the northern parts of the Baltic Sea, western parts of the Gulf of Finland and the Archipelago Sea in Summer 2018. Picture: ESA, Copernicus Sentinel data/SYKE/ Jenni Attila.
Satellite image of large blue-green algae blooms in the northern parts of the Baltic Sea, western parts of the Gulf of Finland and the Archipelago Sea in Summer 2018. Picture: ESA, Copernicus Sentinel data/SYKE/Jenni Attila.

Nutrient runoff from land is the largest problem

The majority of the nutrient overload of the Baltic Sea originates from the drainage basin as a result of human activities. From the basin, the nutrients are eventually transported into coastal waters by rivers. A drainage basin represents an area of land where rainwater and snowmelt collect, for example. Water drains off from the basin into rivers and eventually ends up in the sea. Most of the harmful nutrients in seawater are a result of agriculture.

The nutrient input originating from settlements and industrial activities has decreased since the mid-1970s, when wastewater management became more efficient. According to HELCOM’s status assessment, the input of nutrients into the Baltic Sea has been decreasing significantly throughout the 21st century. Phosphorus input into the Gulf of Finland has begun to decrease mostly as a result of the development of wastewater management in St. Petersburg.

Image shows a graph of the compared data. Source: HELCOM.
Comparison of nitrogen and phosphorus inputs between 2012–2014 and 1997–2003. Source: HELCOM.

Nutrient input originating in Finland has been steadily decreasing when data from 1997–2003 are compared with data from 2008–2012. The load of nitrogen has decreased by an average of 5,000 tonnes and the load of phosphorus by an average of 300 tonnes each year.

Most of the Baltic Sea’s nutrient overload comes from rivers

The nutrient load brought into the sea by rivers is difficult to control. The majority of the nutrient overload of the Baltic Sea is brought by rivers. The efficiency of environmental protection actions targeted at drainage basins can be determined by studying the nutrient flux of rivers.

The phosphorus discharge from Finnish rivers decreased in the 1990s. The concentrations have remained at roughly the same level since then. On the other hand, nitrogen discharge has been steadily increasing, and nitrogen load has most notably increased in the Bothnian Bay in the 21st century.

Out of all Finnish sea areas, the Bothnian Bay receives the largest percentage of nutrient input, since it has the largest catchment area. The Archipelago Sea receives the largest percentage of nutrient input in proportion to the size of its catchment area.

The nutrient input originating in Finland varies greatly depending on hydrological conditions. In years with a high amount of rainfall, more nutrients are washed up from the soil, which means that the nutrient load of rivers becomes higher.

The southern parts of Finnish sea areas are in the worst condition

Nutrient concentrations regulate eutrophication, and the concentrations vary greatly across different parts of Finnish sea areas. The most notable nutrients are phosphorus and nitrogen.

The characteristics of each sea area and their catchment areas also play a part in eutrophication, as do adjacent sea areas. This is why the distinct sea areas of the Baltic Sea are treated as separate entities when designing plans to protect marine and freshwater ecosystems.

The nutrient concentration of the Bothnian Bay is largely regulated by the nutrient load brought in by the large rivers within the bay’s catchment area. Rivers flowing into the bay have low concentrations of phosphorus but carry a lot of nitrogen. The nitrogen concentration of the area bay is rather high, almost at the level of the Gulf of Finland. However, the low amount of phosphorus restricts algal growth.

The waters from the Bothnian Bay and the main basin of the Baltic are mixed in the Bothnian Sea. The nitrogen concentration of the Bothnian Sea is lower than that of the Bothnian Bay, because the nitrogen concentration of the main basin is significantly lower. On the other hand, the Bothnian Sea’s concentration of phosphorus is higher than the Bay’s, since the water coming in from the deep layer of the main basin is rich in phosphorus. The Bothnian Sea has been slowly becoming more eutrophicated since the 1980s, and the process has been particularly fast near the densely inhabited coastline since the 1990s.

The Gulf of Finland has a high concentration of phosphorus and a high level of eutrophication, since water from the deep layer of the main basin with high amounts of phosphorus flows freely into the gulf. Internal loading also brings the concentrations up. Internal loading refers to a state where phosphorus is released from the seafloor into to the water due to hypoxia. However, external phosphorus load into the Gulf of Finland has been decreasing throughout the 21st century. The concentration of nitrogen in the Gulf of Finland has especially increased by the Neva river, which is the largest of the rivers flowing into the Baltic Sea.

Graph on nutrient concentrations. Source: SYKE. Graph on nutrient concentrations. Source: SYKE.

The Status of Finland’s marine environment 2018 report states that all Finland’s sea areas are in a fairly poor state in terms of eutrophication. The HELCOM indicators used to assess the state of nutrients in the Baltic Sea, ‘Nitrogen/DIN’ and ‘Phosphorus/DIP’, show that none of Finland’s sea areas have achieved the status of ‘Good’. The low phosphorus concentration of the Bothnian Bay is an exception to this rule.

Concentrations of nutrients have fluctuated from one decade to the next

The concentration of phosphorus in the Bothnian Bay and the Bothnian Sea remained at a steady level from the late 1970s to the early 2010s. However, the phosphorus concentration of the Gulf of Finland has fluctuated strongly and currently seems to be on the rise again.

The concentrations of nitrogen in Finnish sea areas increased in the 1970s and the 1980s but have since then stayed at a steady level or even decreased. In the Gulf of Finland, the concentration of nitrogen started to increase again in the 21st century.

Direct and indirect effects of eutrophication

Eutrophication affects sea areas in a variety of ways. Some of the effects are direct, and we even see them when we are out and about at sea. Some of the effects are indirect, and we need different metrics and measurements to observe them.

The amount of algae is determined by the concentration levels of chlorophyll-a

Primary producers, such as phytoplankton, feed on sunlight via photosynthesis and contain chlorophyll-a. By measuring the concentration of chlorophyll-a in a body of water, we can estimate the amount of algae present. This is why chlorophyll-a concentration is one of the indicators used to assess water eutrophication.

The amount of chlorophyll-a has been increasing in all Finnish sea areas since the 1970s, although in many areas, the increase has stopped since the dawn of the 21st century. In the 2010s, the concentrations have even begun to decrease in some areas, for example in the Gulf of Finland.

Satellite image of summertime chlorophyll-a concentrations. Source: ESA / Sentinel / OLCI. Image processing: SYKE.
Satellite remote sensing provides the most comprehensive picture of how chlorophyll-a concentrations vary between the different areas of the Baltic Sea. The image shows average chlorophyll-a concentrations in the Baltic Sea in summer 2018. Source: ESA / Sentinel / OLCI. Image processing: SYKE.

The ‘Chlorophyll-a’ HELCOM indicator used to determine the amount of algae in the Baltic Sea shows that none of the Finnish sea areas or coastal waters have achieved the status of ‘Good’.

Blue-green algae blooms thrive in the south during warm summers

Blue-green algae, as all algae, turns the water green if there is a large amount of the algae present. This makes it possible to observe blue-green algae blooms with satellites.

Algal blooms mainly consisting of blue-green algae are mostly observed in late summer in the northern parts of the Baltic Proper and in the Gulf of Finland. Algal blooms also occur in the Archipelago Sea and the southern parts of the Bothnian Sea.

The frequency and scale of the blooms vary greatly from year to year. In addition to the amount of available nutrients, water temperature and weather conditions in general affect algal growth.

The largest algal blooms happen during warm summers with a lot of sunlight. Wind also has an effect on whether the algae are dispersed and mixed in the water or remain on the surface, easily observed by satellites.

Satellite image of blue-green algae blooms. Source: ESA / Sentinel / OLCI. Image processing: SYKE.
Blue-green algae on the surface of the sea in summer 2018. Source: ESA / Sentinel / OLCI. Image processing: SYKE.

Biomass of phytoplankton varies with the seasons

Phytoplankton blooms are one of the easiest signs of eutrophication to observe. The amount of algae varies very clearly between seasons, and the largest blooms only occur at certain points in the algal growth season.

During winter there is not enough sunlight for algae to grow, so there is very little of it. The biomass of phytoplankton is at its highest during spring, when the amount of sunlight increases and there is still plenty of nutrients available in the water after the winter months. The nutrients are mostly consumed by midsummer, and the amount of algae decreases accordingly. What follows next is the so-called stationary phase.

In July–August, blue-green algae start to bloom. Some species of blue-green algae can use the nitrogen dissolved in the water from the atmosphere as nutrition and store phosphorus. This is a significant advantage over other species of algae. The amount of algae starts to decrease as autumn arrives, when the stratification of the Baltic Sea decays and the amount of sunlight starts to decrease as well.

Secchi depth in Finnish sea areas has improved significantly

Secchi depth measures water transparency. It is measured with a so-called Secchi disc, which is a plain white disc. The disc is slowly lowered into the water and the Secchi depth is the depth in which the Secchi disc is no longer visible. When the Secchi disc is lifted back up, it becomes visible again when looked at from above the water’s surface. This measures the rate at which light entering the water is propagated. In addition to the characteristics of the water itself, the substances and organic and inorganic particles mixed in it, all have an effect on how light scatters in the water.

The Secchi depth of Finnish sea areas has significantly decreased in the last century. In the Bothnian Bay and Bothnian Sea, Secchi depth has remained at a steady level in the 21st century. In the Gulf of Finland and Northern Baltic, the decrease is still on-going.

The ‘Water clarity’ HELCOM indicator used to determine the Secchi depth in the Baltic Sea shows that none of the Finnish sea areas or coastal waters have achieved the status of ‘Good’.

Oxygen-deprived areas of the seafloor are among the indirect consequences of eutrophication

Saltwater is heavier than freshwater, which is why saltwater sinks to the bottom. In the deeps of the Baltic Proper and the western parts of the Gulf of Finland, is a layer of water that extends from the depth of around 60 metres to the seafloor, which has a significantly higher salinity.

The different layers do not mix due to differences in water density. This means that oxygen is not transported from the surface layer into the deep layer. In the stagnant deep water, biological decomposition quickly consumes all remaining oxygen. Once the oxygen is consumed, poisonous hydrogen sulphide starts forming in the deeps.

Eutrophication assists in the formation of these hypoxic deeps. Excessive algae growth in the surface layer, and the algae sinking to the deep layers as a result, increase the decomposition processes in the deep layer near the seafloor. This results in the remaining oxygen being consumed even faster.

Poor oxygen conditions are the result of stratification and eutrophication. In coastal waters, where the water is not layered, hypoxia has been observed in some smaller deeps. In these deeps however, eutrophication is the main cause of the hypoxia.

Low-oxygen seafloor conditions have become significantly more common in the 21st century. The main causes are the general eutrophication of the Baltic Sea and the pulses of saline water with plenty of oxygen coming in from the Atlantic Ocean becoming rarer.

Maps showing oxygen conditions on the Baltic Sea’s seafloor. Source: SYKE. Maps showing oxygen conditions on the Baltic Sea’s seafloor. Source: SYKE.

The oxygen conditions of the water in the deep layer in the Gulf of Finland varies greatly even from week to week. When weather conditions are favourable and there is strong wind from the east, oxygen-poor water is transported from the main basin into the seafloor of the Gulf of Finland. On the other hand, strong west winds push the oxygen-poor water near the seafloor out of the gulf, and the water leaving the gulf is replaced with surface water, which contains more oxygen.

The ‘Oxygen debt’ HELCOM indicator used to determine the oxygen conditions of the Baltic Sea shows that the seafloors of the Baltic Proper and the Gulf of Finland have not achieved the status of ‘Good’.

EU regulates water and marine environment protection

The EU aims to improve the status of the Baltic Sea and the coastal waters of Finland with legislation and EU-funded projects. The Marine Strategy Framework Directive (MSFD), approved in 2008, states that achieving or maintaining Good Environmental Status in the EU’s marine waters by 2020 as its main objective.

The state of the EU’s marine waters is assessed with 11 qualitative descriptors. HELCOM has established a set of indicators, in accordance with the definitions of the 11 descriptors, to help in concretely measuring whether Good Environmental Status is achieved.

The status of a sea area is always measured with these indicators. The indicators have quantitative threshold values to evaluate progress towards the goal of achieving Good Environmental Status as defined in the MSFD. All water and marine environment protection measures aim at achieving Good Environmental Status. If the holistic assessment falls short of the ‘Good’ status, actions that aim to correct the situation must be taken in that area.

An infographic showing the qualitative descriptors for a marine environment with good environmental status. Source: Meren pärskäys 2015.
The qualitative descriptors for a marine environment with Good Environmental Status. Source: Meren pärskäys 2015.

According to the EU’s definitions, Good Environmental Status is achieved when “human-induced eutrophication is minimized, especially adverse effects thereof, such as losses in biodiversity, ecosystem degradation, harmful algae blooms and oxygen deficiency in bottom waters.”

HELCOM indicators used to assess eutrophication are:

  • concentration of chlorophyll-a, which can be used to assess the biomass of phytoplankton
  • dissolved inorganic nitrogen (DIN)
  • dissolved inorganic phosphorus (DIP)
  • water clarity (Secchi depth)
  • total nitrogen (TN)
  • total phosphorus (TP)
  • cyanobacterial bloom index

According to HELCOM’s holistic assessment based on these indicators, none of Finland’s sea areas currently achieve Good Environmental Status in terms of eutrophication. With respect to individual indicators, the coastal waters of the Bothnian Bay achieved a ‘Good’ status when only the concentration of dissolved inorganic phosphorus was assessed.

Signs of improvement and reduction in the state of eutrophication are emerging, but the previous heavy load of nutrients is still maintaining a high level of eutrophication in the Baltic Sea.