Regime Shifts DataBase

Large persistent changes in ecosystem services

The oligotrophic Baltic Sea ( - 1950s)

Before excessive nutrient input, the Baltic Sea was predominantly oligotrophic, clear-water sea with submerged vegetation and commercially preferred fish species, e.g. Baltic cod. The deep waters were oxygenated with generally large volumes of inflow water.

The complex postglacial history of the Baltic Sea caused variations in the scale of the hypoxia. Seafloor sediment studies have shown that the stagnation, anoxia and hydrogen sulphide production are natural phenomena to a certain extent: they took place during the periods when there was a long duration between pulses of oxygen rich saline water from the North Sea, but the areal extent was smaller than today.

The Baltic Sea food web models suggest that the fourth trophic level top predators (mammals, large fish, marine birds) controlled the abundance of small fish, and that herbivorous invertebrates controlled the abundance of primary producers.

The eutrophic Baltic Sea (circa 1950s – present)

The eutrophic Baltic Sea is characterized by increased sedimentation, increased turbidity, reduced transparency in water and increased algal mats. Anoxic areas have spread and a food web regime shift from a cod-dominated to sprat-dominated regime has occurred. It is uncertain how much eutrophication affects the food web other than primary production and local changes caused by the dead zones.

Increased nutrient levels have led to altered nitrogen and phosphorus ratios, increased sedimentation rates and increased input of organic matter to the benthic system. This has in turn led to increased pelagic and benthic primary production, increased turbidity and reduced transparency in the water. Algal blooms can become very large and dense. Reduced oxygen reserves caused by the decomposition of organic matter together with increased occurrences of benthic sulphur bacteria cause dead zones. 

Increased anthropogenic nutrient load has been a major driving force behind the oxygen deficiency as it has increased algal production and hence sedimentation of organic matter. Extensive draining of wetlands and lakes has increased the proportion of nutrients that are transported to the Baltic Sea, and the speed with which water (and nutrients) run off the landscape into the Baltic.

The limited water exchange with the North Sea and the long residence time of water are the main reasons for the sensitivity of the Baltic Sea to eutrophication. Inflowing dense saline water replaces the old stagnant water in the deeps below the halocline. The displaced stagnant nutrient rich water is brought into the productive surface layer via upwelling (natural internal loading). Phosphorus loading accelerates eutrophication. Vertical stratification of water masses increases the vulnerability of the Baltic Sea to the eutrophication as it hinders or prevents ventilation and oxygenation of the bottom waters and sediments. Only a few Major Baltic Inflows (MBI), large volume inflows of high-salinity water, have been recognized since the mid-1970s. The lack of major inflows is believed to have resulted in a long-term stagnation. North Sea saltwater inflows are irregular and unpredictable. They are governed by large-scale and local meteorological variations and by sea level and salinity distributions. 

Increased nutrient loading have changed the Baltic Sea from an oligotrophic low nutrient clear-water sea into a eutrophic higher nutrient, murky sea. The shallow depth, low salinity, and slow rate of water turnover due to limited water exchange with the North Sea make the Baltic Sea particularly susceptible to increases in nutrient concentrations leading to eutrophication, because nutrients discharged to the sea will remain in the basin for a long time. The Baltic Sea has large variances and gradients in topography, geology, hydrography and climate. Hence, the consequences of the eutrophication are different in different parts of the sea.

Excessive nutrients promote the growth of photosynthetic plants and algae. Abundant phytoplankton species generate excess photosynthetic production, which the Baltic Sea ecosystem is unable to process. Algal mats sink down to the seafloor and decompose, depleting the oxygen in the near bottom water layer. When the organic biomass decomposing conditions become anoxic, the decomposing bacteria acting on these conditions start to produce hydrogen sulphide, which is extremely toxic. Gradually the living conditions become intolerable for fish and the benthic fauna. Fish move away and benthic fauna dies in masses, resulting in large areas of dead zones in the sea floor. The increase in phytoplankton leads to greater turbidity and thus decreased light penetration, which limits the habitat available for macrophytes.

Salinity in the Baltic Sea is low with extreme vertical and horizontal gradients and a permanent halocline at about 50-70m in the Baltic proper. The permanent halocline decreases vertical mixing of the water column, causing the deep areas of the Baltic Proper below the halocline to often run out of oxygen. Inflows of saline water replenish the deep-water layers of the Baltic Sea with saline and well-oxygenated North Sea water. The occurrence of major inflows is driven by major storms in the North Sea pushing water into the Baltic Sea and is therefore irregular and unpredictable. Salinity levels determine species distribution in the several large, ecologically distinct sub-basins of The Baltic Sea. Eutrophication has affected the structure of the food web as top-down control has decreased. Deepwater anoxia in the spawning areas has reduced the cod stock. 

Decrease in provisioning fisheries services can be seen in the declines of cod stock and in the cases of contaminated sea food. Cod has high commercial value for humans. Decline of cod has also in part caused a food web change from a cod-dominated to sprat-dominated regime (see case study: Pelagic food web, the Central Baltic Sea). Bladderwrack, which supports diverse faunal communities and is an important nursery environment for fish and many invertebrates, has suffered from the lack of light and declined. Regulative ecosystem services have been lost in water purification as the primary production has become too abundant for the Baltic Sea ecosystem to be able to process. Increase in phytoplankton leads to greater turbidity and thus decreased light penetration, which limits the habitat available for macrophytes. Anoxic areas and hydrogen sulphide production have increased due to decomposing biomass, resulting in large areas of dead zones in the sea floor.

Cultural services for water use and creation have been lost. Algal blooms have led to closed beaches, frequent public concerns, large cleanup costs, human health problems, lowered values of coastal properties, and loss of revenue from tourism and recreation. 

In 1974 some Baltic Sea countries began to acknowledge that eutrophication was an anthropogenic problem. Since the 1970s, the Helsinki Commission (HELCOM) has provided several recommendations for nutrient reductions. In the 1980s further attention was drawn to the load of nutrients entering the Baltic Sea and HELCOM started to work towards a 50% reduction target. In 1988 the ministers of the environment in the Baltic Sea countries signed a declaration stating that contracting parties were to reduce their emissions.

During the last few decades some of the Baltic Sea countries have managed to slow the increase of their nitrogen inputs and even reduce the inputs of phosphorus. During 1990-2000, the direct point-source inputs of phosphorus and nitrogen decreased by 68% and 60%. From 1990-2006 the total inputs to the Baltic Sea were reduced by 45% for phosphorus and 30% for nitrogen. Atmospheric nitrogen decreased since the mid-1990s and increased during 2003-2007. In 2005, the HELCOM launched the Baltic Sea Action Plan Plan. It is a cross-sectional, international program aiming to restore good ecological status of the Baltic marine environment by 2021. All major stakeholders are included and the measures can be taken at regional, national, European and global level.