What is nutrient pollution?

Nutrient pollution refers to the excessive input of nutrients into the aquatic environment. Nitrogen and phosphorus are among the main growth limiting nutrients and as such do not pose any direct hazards to marine organisms. Eutrophication, however, is a condition in an aquatic ecosystem where high nutrient concentrations stimulate growth of algae which leads to imbalanced functioning of the system:

• intense algal growth: excess of filamentous perennial algae and phytoplankton blooms;

• production of excess organic matter;

• increase in oxygen consumption;

• oxygen depletion;

• death of benthic organism, including fish.

Where do the excess nutrients come from?

Nutrients enter the Baltic Sea via rivers, via atmospheric deposition, or via direct discharges from pollution sources located on the coastline. Nutrient discharges may originate from point sources, such as industrial or municipal wastewater outlets, or from diffuse sources, such as farmland, homes in rural areas and atmospheric  deposition within the Baltic Sea catchment area.

	For further information about nutrient loads to the Baltic Sea, see: The Fourth Baltic Sea Pollution Load Compilation (PLC-4) (2004) Atmospheric Supply of Nitrogen, Lead, Cadmium, Mercury and Lindane to the Baltic Sea in the period 1996 - 2000 and the Indicator Fact Sheets for atmospheric deposition of nitrogen to the sea in 2000 and 2001.

What happens to excess nutrients?

Nitrogen and phosphorus are the main nutrients at the bottom of all food chains. The smallest marine plants, known in ecological terms as primary producers, fix these nutrients into their biomass through primary production. When excessive amounts of nutrients enter the sea, this primary production increases rapidly, and the natural ecological balance of marine ecosystems is disturbed, resulting e.g. in algal blooms. Secondary production will also increase, as secondary producers such as zooplankton and fish feed on the increasing numbers of primary producers. This whole phenomenon of the excessive nutrient enrichment of an ecosystem is known as eutrophication.

Marine life can be suffocated by eutrophication

In the Baltic Sea, the excess biomass of primary producers accumulates on the sea-bed, and the decomposition of this organic material consumes oxygen from the water. This leads to oxygen depletion, especially in areas where mixing of water is restricted, such as in deeper waters below the halocline, or in shallower waters affected by thermal stratification during the summer.
 

	Oxygen depletion and internal nutrient loads – a vicious circle A vicious circle can develop where anoxic conditions affect the nutrient cycle in the bottom water and sediments, intensifying eutrophication and accelerating oxygen depletion. This is because in anoxic conditions phosphorus is released from sediments back into the water, while nitrogen accumulates as ammonium because of a lack of denitrification and nitrogen release from sediments. These nutrients are then further utilised by more primary producers, compounding eutrophication through a factor known as internal nutrient load.

Further information about algal blooms can be found in the following indicator reports:

Temporal trends and regional differences in chlorophyll concentrations from satellite remote sensing of ocean colour

Phytoplankton spring bloom biomass in the Gulf of Finland, Northern Baltic Proper and Arkona Basin in 2003

Phytoplankton biomass and species succession in the Gulf of Finland, Northern Baltic Proper and Arkona Basin in 2003

Cyanobacterial blooms in the Baltic Sea

Cyanobacteria bloom index