The Bay of Bothnia has low phosphorus levels compared to further south

The amount of nutrients in the Bay of Bothnia is affected by loading from the large rivers in the area. The nutrient load here is also regulated by the water exchange with the Bothnian Sea. The nutrient load is also affected by the sea’s internal activities that either bind or remove nutrients.

A significant part of the nutrients introduced by rivers occurs as natural leaching, which is independent of human activities. Besides, agriculture, forestry, and peat production also burden the Bothnian Bay. The impact of coastal settlements and industrial loading is limited to coastal waters in the vicinity of the discharge sources.

Phosphorus levels have been very low in the open sea areas of the Bay of Bothnia since the 1990s. Indeed, the Gulf of Bothnia is spared from high phosphorus concentrations due to the relatively low flow of water into it from the deep layers of the Baltic Sea's main basin. By comparison, the deep waters of the Baltic Proper are high in phosphorus. The low phosphorus content of the Bay of Bothnia is also explained by the fact that its oxygen-rich bottom sediments are efficient at retaining phosphorus.

Compared to the peak years, nitrogen concentrations in the Bay of Bothnia have since decreased

Nitrogen concentrations in the Bothnian Bay peaked at the turn of the 1980s and 1990s. Since then, they have fallen slightly to the present day. This decrease could be explained by a reduction in the atmospheric deposition of nitrogen from the Baltic Sea air. In coastal areas, nitrogen levels have remained constant throughout the monitoring period.

The Bothnian Sea is gradually becoming eutrophic

Since the 1980s, the Bothnian Sea has been slowly becoming eutrophic. The nutrient status of the Bothnian Sea is influenced by phosphorus from more southern sea areas and nitrogen carried in currents from the Bay of Bothnia.

Water flowing from the main Baltic Sea basin into the Bothnian Sea is mainly composed of oxygenated and low salinity surface water, as well as small amounts of deep water which are hypoxic (low in oxygen), high in nutrients and saline.

Due to the reduced deep water currents, the Bothnian Sea is only slightly stratified by salinity. Because of this weak stratification, the shallow oxygenated layers can mix with the deep water layers and no oxygen depletion occurs. 

Coastal waters are burdened by agriculture but only in a narrow zone

The state of coastal waters is particularly influenced by loading from rivers. The worst situation is in the coastal area off the Kokemäenjoki River, which is burdened by agriculture from the catchment area.

The open coastal waters of the Bothnian Sea mix very effectively. As a result, the nutrient concentrations are only affected by municipal and industrial loading within a narrow coastal zone.

However, since the mid-1990s, the increased levels of cyanobacteria indicate gradual eutrophication of the area. This development is likely due to a decrease in the ratio of nitrogen to phosphorus, as a result of the increase in phosphorus levels.

Current nutrient levels correspond to the situation at the turn of the 80s and 90s

In the 1990s, the phosphorus levels in the open sea areas of the Bothnian Sea decreased. However, in the 2000s, this trend changed to a mild but steady growth. This increase was probably due to an increase in the phosphorus concentration in the surface layers of the Baltic Proper, from which water flows freely into the Bothnian Sea. The current phosphorous level in the open sea areas of the Bothnian Sea is similar to the situation at the turn of the 1980s and 1990s.

In the Kvarken Archipelago, the trend in phosphorus concentrations differs slightly from that occurring in the open sea. Although it rose in the 1980s, it remained unchanged in the following decade. In the 2000s, the trend again began to rise, reflecting the situation in the high seas.

The highest levels of nitrogen were observed in the open areas of the Bothnian Sea in the 1980s. Since then, concentrations have slowly declined until the mid-2000s. This change is likely to be mainly due to similar developments in atmospheric nitrogen deposition in the Baltic Sea.

In the Kvarken Archipelago, the nitrogen trend was initially declining, as in the open sea. However, nitrogen levels increased in the early 2000s. The concentration levels of nitrogen in the 2010s have been similar to those of the 1980s.

The Archipelago Sea is sensitive to eutrophication

Unlike the other sea areas in Finland, the Archipelago Sea has no actual offshore area. It is shallow, fragmented, and has only a limited water exchange. Therefore, it is sensitive to eutrophication.

The nutrient levels of the inner archipelago waters are most strongly influenced by agricultural loading. Conversely, the condition of the south-eastern and southern parts of the Archipelago Sea is particularly affected by nutrient-rich waters flowing in from the Gulf of Finland and the northern Baltic Proper. Moreover, nutrients released from the bottom increase the nutrient levels of some locations in both the middle and inner archipelago areas.

Phosphorus concentrations in the Archipelago Sea still continue to rise

In the Archipelago Sea, the level of phosphorus in surface waters has clearly risen almost continuously since the 1980s. This increase reflects a similar growth of phosphorus levels in the main basin of the Baltic Proper. It is also possible that the release of phosphorus from the bottom has increased. In particular, the phosphorus content of the off-bottom water has clearly increased.

On the other hand, although nitrogen levels have fluctuated strongly, its overall level has not changed significantly during the measurement period. The fluctuations in nitrogen loading introduced to the sea by rivers, as well as the changes in the relative proportion of river water in seawater, are reflected in the nitrogen content of the inner and middle archipelago zones.

The rise in nutrient levels in the northern Baltic Proper ceased in the 1980s

The closest part of the Baltic Proper to Finland lies at the intersection of currents from the Gulf of Finland, the Southern Baltic Sea, and the Gulf of Bothnia. As such, the nutrient status of the Baltic Sea basin is determined by the nutrient loading from its entire catchment area. Changes in nutrient levels in the area are thus linked to similar changes in the nutrient levels of the Baltic Proper itself.

The nutrient levels in the Baltic Proper have increased throughout the post-war industrialisation period up until the 1980s. Since then, the situation has remained unchanged or the level has fallen only slightly.

The Gulf of Finland suffers from high nutrient levels and oxygen depletion

In the Gulf of Finland, nutrient concentrations are regulated by the nutrient load coming from the catchment area, the deep water flowing in from the Baltic Proper, and the varying oxygen conditions on the seabed.

The condition of the eastern Gulf of Finland is strongly influenced by the nutrient load from the city of St. Petersburg and its neighbouring areas. Also, the Neva River introduces a lot of nutrients to the sea, especially nitrogen. Phosphorus loading in the Eastern Gulf of Finland has fallen sharply since the mid-2000s. This has been influenced by the improved treatment of municipal wastewater in St. Petersburg, as well as the reduction of the phosphorus load in the Laukaanjoki River since 2012.

Many factors increase the nutrient levels in the Gulf of Finland

Along the fragmented coastline of the Gulf of Finland, nutrient levels are influenced by the status of the waters in the open seas, as well as the nutrient load coming from the catchment area. In certain sea areas, particularly in the eastern part of the Greater Helsinki area, nutrient loading from rivers, as well as urban and industrial areas, cause eutrophication in coastal areas which have poor water exchange.

The oxygen-depleted state of the seafloor in the Gulf of Finland weakens the status of coastal waters in many places, including areas where there is no local nutrient loading. The nutrient levels in the deep coastal basins are significantly increased due to the phosphorus released from the seabed as a result of hypoxia.

The fluctuation of the off-bottom oxygen levels in the open sea areas of the Gulf of Finland regulates the internal loading and concentration of phosphorus in the water. The increase in phosphorus levels in the 1970s and 1980s was due to an increase in both external and internal loading. However, in the 1990s and early 2000s, the observed increase was mainly due to internal loading processes.

Phosphorus concentrations have decreased in the Gulf of Finland in the 2000s

In coastal areas, phosphorus levels increased markedly in the 1990s. Even in coastal waters, the large differences in phosphorus concentrations between years can be largely explained by variations in internal loading. The increase in phosphorous levels halted in the 2000s and began to decline, particularly in the easternmost coastal waters of the Gulf of Finland.

The decrease in the phosphorus content was due in particular to a reduction in internal loading. In the 2010s, the decrease in phosphorus loading from Russia has also improved the state of the eastern Gulf of Finland.

Nitrogen concentrations fluctuated for decades but have now stabilised

The increase in nitrogen levels in the open sea areas of the Gulf of Finland, which began in the 1970s, had ceased by the mid-1980s. This change was related to the simultaneous reduction of both atmospheric nitrogen deposition and nitrogen loading entering the sea from the catchment area.

Nitrogen concentrations began to rise again in the mid-2000s. However, no systematic increase in nitrogen loading in the Gulf of Finland has been observed in the 2000s.

In the 2000s, nitrogen levels in the outer archipelago of the Central Gulf of Finland have been in the same range as in the open sea areas in the Gulf. There have been no major changes in nitrogen levels since the early 1990s. In the easternmost part of the Gulf’s Finnish territorial waters, the nitrogen content is significantly higher than the central and western parts, due to the influence of inflowing river waters.