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Questions and answers

Why does our study focus on phosphorus?

Phosphorus is a key nutrient in the eutrophication of waters. In lakes it is usually the low level of phosphorus that limits the growth of algae, as the amount of phosphorus is the lowest compared with other nutrients required by algae. In the Baltic Sea nitrogen has a greater role, but even there phosphorus affects the mass occurrences of blue-green algae, for example.

Gaps in knowledge remain in the impact that phosphorus has on eutrophication. For example, we do not know the extent to which the fight against eutrophication should focus on dissolved and particulate phosphorus. Information is needed for the correct targeting of measures to reduce loading.

How much does agricultural phosphorus affect waters?

The total amount of phosphorus from agriculture that ends up in waters is fairly well known. Average annual phosphorus loss from fields is about one kilo of per hectare. However, there is considerable variation in different areas and under different weather conditions. The load has been studied both on parcels of land, in small catchment basins, and in large river catchments.

Finnish agriculture accounts for nearly 60 percent of the anthropogenic phosphorus load on waters caused by Finns. The share of Finnish agriculture on the load imposed on the Baltic Sea is about five percent.

There are uncertainties that affect the evaluation of diffuse loads from agriculture and other sources. For example, we do not know how much natural wash takes place - in other words, how much phosphorus would flow into the waters from areas dominated by clayey soils, for example, if they were not under cultivation. Also causing uncertainty is that water samples are not taken frequently enough, and that there are not enough research catchment areas representing different environmental conditions and agricultural practices.

What factors affect phosphorus loading from agriculture?

Weather is a factor that has a strong impact on the load. The warmer autumn and winter are, and the more precipitation there is, the more nutrients wash off from fields. Factors that have a great impact on the load include fertilisation, manure spreading, and the structure, tilling, and drainage of the soil.

The phosphorus from agriculture comprises two main types: dissolved phosphorus and particulate phosphorus (phosphorus bound by eroded soil particles). The higher the soil phosphorus content is, the more dissolved phosphorus can be leached into waterways. The greater the erosion of fields, the more particulate phosphorus ends up in the water. Erosion is smallest from land that is covered by vegetation year-round and the greatest in fields with a high clay and loam content, and which are ploughed in the autumn and left bare for the winter.

How has the load imposed by agriculture been reduced?

The environmental compensation system has great significance in targeting environmental measures. To qualify for environmental payments, a farm must commit to the conditions set for the system. For example, there are restrictions concerning the use of phosphorus, which apply to all farms. A farm may also choose from among elective measures. Very popular among these has been winter vegetation cover. The system also supports, for example, the establishment of wetlands and buffer zones. Also regulating the nutrient load from agriculture is the Nitrate Directive, the Fertiliser Regulation, and licensing practices pertaining to large cattle farms.

Some of the methods can be of minor importance on a national level, but effective locally. These include wetlands and buffer zones, for example. Some of the measures, such as restrictions on the use of nutrients, are effective only over a longer period of time. Some, on the other hand, can have a conflicting impact. For instance, reduced primary tillage of soil can reduce the total phosphorus loading to watercourses, but may increase the leaching of dissolved phosphorus.

Has the load from agriculture declined?

It is hard to evaluate changes in the phosphorus load over the long term, because weather conditions vary from year to year. In monitoring the predominantly agricultural catchment areas there have been no long-term changes in the phosphorus load over the long term. Observations of rivers, meanwhile, suggest that the load would have declined.

However, the decline has not been as great as might be expected based on reductions in the use of fertilisers. The reason is the storage phenomenon: many parcels of land continue to have large amounts of phosphorus that have accumulated in the previous decades. This slows down the decline in the load.

With climate change the snow cover that protects fields is coming later and melting earlier. This increases risk of phosphorus loading.

What kind of phosphorus causes eutrophication?

There is inadequate knowledge on the degree to which phosphorus of agricultural origin causes eutrophication. Especially the release of phosphorus bound in soil particles into a dissolved form that algae are able to utilise is poorly understood. The release probably depends on characteristics of both the soil as well as the receiving body of water. Some of the phosphorus in the soil particles is released immediately when it gets into the water, but most of it settles on the bottom with the particles. What happens to the phosphorus on the bottom remains a mystery. In the oxygen-free conditions of the bottom, biological material decays. There is much uncertainty concerning the decaying process, such as the reactions of iron that that are linked to phosphorus release.

What is the condition of waters in Finland and elsewhere?

Of Finland's lakes 85 percent are in good to excellent condition with respect to their ecological state. The lakes whose condition is less than good need to be restored to good condition in accordance with the European Water Framework Directive. Sixty five percent of river waters are in a good or excellent ecological state, but only 25 percent of coastal waters meet the standard. A key factor weakening the ecological state is eutrophication. The state of water can be examined with the help of the Water map and the Lake&Water Wiki. The Open Information pages of the Finnish Environment Institute contain detailed information on the chemistry, biology, and physics of water as well as on plans to improve the situation.

The global rise in population and the rise in their standard of living combined with climate change lead to an increased phosphorus load and water eutrophication in areas that are under intense cultivation. Especially the state of coastal waters of the seas is expected to weaken in the future in certain areas.

Published 2019-05-14 at 16:48, updated 2019-05-14 at 16:39