1.2.1 Aquatic eutrophication

Aquatic eutrophication can be defined as nutrient enrichment of the aquatic environment. Excess input of nutrients increases the primary production of fast-growing algae such as phytoplankton, and as this algae biomass grows, the water becomes turbid. Slow-growing vascular plants (e.g. eelgrass) that are best adapted to low-nutrient environments decrease and the fish community shifts due to habitat changes (less light, changes in plant species). In tropical waters, nutrient enrichment stimulating production of macro-algae can lead to overgrowth and replacement of corals (Cloern, 2001). When driven to a far extent, nutrient enrichment of coastal stratified waters* can cause anaerobic conditions or low-oxygen conditions and result in significant bottom fauna mortality and losses of fish resources.

Generally, fresh waters (lakes, reservoirs, rivers) in temperate regions are phosphorus (P) limited, whereas nitrogen (N) is considered to be the primary limiting element in marine systems. This is however an oversimplification; it was early suggested that both N and P are important nutrients in estuaries and this has been confirmed with observations of P limitation during spring and N limitation during summer in coastal-near environments, such as the Gulf of Riga (Latvia), Roskilde Fjord (Denmark), Bay of Brest (France) and Delaware Bay (USA). Also, present understanding of the relative importance of N and P is strongly biased by the predominance of studies at temperate latitudes, since tropical marine systems seem to be more frequently P-limited (Cloern, 2001).

Increased human disturbance of the nitrogen and phosphorus cycle during the 20th century is the main cause for the eutrophication problem. Nitrogen fluxes in rivers in Europe and the US have increased significantly; movements of total dissolved N into most temperate-zone rivers in the North Atlantic Basin may have increased by as much as two to twenty-fold since pre-industrial times (Howarth et al., 1996). The highest N increases have been found in rivers in the North Sea region. Phosphorus loading to estuarine systems has increased two- to six-fold since 1900. When examined as a whole, existing nutrient records show a rapid change in the fertility of coastal ecosystems over the last half of the 20th century (Cloern, 2001).

Emissions of nitrogen to water from agriculture occur predominantly as nitrate leaching from the soils, but in severe cases it can also be in the form of discharged effluents from manure waste storages. The magnitude of soil leaching is determined by farming systems, type of soils and climate. High livestock density and crop rotations dominated by annual crops with high fertilising intensity and short growing season (e.g. potatoes) are examples of farming systems that are characterised by relatively high nitrate leaching, as opposed to crop rotations with a high degree of perennial crops, such as grasslands. Climate conditions with rainy and mild winters increase risks for leaching, and lighter sandy soils generally have higher nitrate leaching as compared to heavier clay soils.

During the transport of leached N in rivers, there are transformation processes leading to some of the emitted nitrate being removed by plant uptake and denitrification; these processes are referred to as ‘nitrogen retention’. In Sweden, current average retention of nitrogen lost from arable land has been estimated at approximately 40% (Arheimer et al., 1997). In other words, 40% of the leached N from arable land is disarmed in rivers and lakes mainly through the denitrification process transforming the nitrate into environmentally harmless nitrogen (N2), and 60% reaches surrounding seas and water via river mouths as reactive N and is thereby potentially environmentally harmful. During the second half of the 19th century and first half of 20th century, lakes and wetlands were extensively drained to gain more arable land in many European countries and this has affected the net losses of N to surrounding seas since the landscape has lost some of its capacity to neutralize the emitted reactive nitrate and transform it into inert N2. The rise in N concentration in rivers has often been connected to the increased input of fertiliser N. Studies show that there are also other mechanisms, notably draining of lakes and wetlands, which can be as important as the input of fertiliser N in affecting net losses to the surrounding seas from arable land (Hoffman et al., 2000).

Since the 1980s there has been an increasingly efficient removal of P by sewage wastewater systems in the developed world and this has resulted in agriculture’s contribution of diffuse P losses to aquatic environments becoming relatively more important to the eutrophication problem. Phosphorus is lost from arable land by soil erosion, surface runoff and leaching. One problem of today is that many agricultural soils have accumulated phosphorus in excess. In the first half of the 20th century it was seen as economically justifiable to add extra phosphorous when applying fertilisers in order to increase the soil P-content. But the adding of more phosphorus than crops remove and, moreover, application of farmyard manure that is often rich in P, has resulted in high accumulations of phosphorus in many soils. In Denmark, P-accumulation was 1400 kg ha− 1 during the 20th century (Damgaard Poulsen and Holten Rubaek, 2005); in Sweden, soil-P accumulation between from ~1950 to 2000 was around 700 kg ha− 1 as a nation average, and in regions with high animal density around 1000 kg ha− 1 (Andersson et al., 1998). The structural movement towards geographic concentration of livestock production has affected P accumulation in soils in regions with high animal density and this is bound to increase the risk of phosphorus losses to aquatic ecosystems.

1.2.2 Terrestrial eutrophication

Terrestrial eutrophication includes the effects of excess nutrients on plant functioning and species composition in natural or semi-natural terrestrial ecosystems. Under uninfluenced conditions, vegetation in natural ecosystems is mainly controlled by the limited availability of nitrogen. Atmospheric N deposition caused by human activities leads to increased loads of nitrogen and, from this, follows changes in structures and functions in N-limited ecosystems. For example, there is an increased competition from nitrogen adapted species at the expense of less adapted species and an altered tolerance towards diseases, drought, frost, etc. Based on measurements of precipitation in remote areas, annual wet deposition of inorganic N in unpolluted regions is estimated to be in the range 0.1 –...