Executive Summary

Phosphorus is considered to be the main limiting nutrient for primary production in most freshwater systems. Long-term increases in the concentration of phosphorus have occurred in many rivers and lakes in recent decades. External supplies of nutrients to surface waters can originate from point sources, which are localised and more readily monitored and controlled, and non-point sources, which are diffuse and much more difficult to monitor and regulate. Reductions in phosphorus inputs from point sources do not always reduce phosphorus concentrations in surface waters. While internal loading from sediments may be a factor in the maintenance of phosphorus availability in these systems, diffuse losses from agricultural sources are considered the major cause.

Export and utilisation of phosphorus show considerable temporal variation. Much of this variation is related directly and indirectly to climatic factors. Direct effects include the dominant role of rainfall in hydrology and phosphorus transport and the influence of temperature and light availability on chemical and biological cycles. Indirect effects include seasonal variations in land-use and agricultural management of particularly, in Ireland, grazing patterns and slurry disposal.

In pristine systems, the flux of phosphorus from the catchment to surface waters is mainly determined by the flow of water through the system and the underlying geology. Losses from these systems are generally low and occur by leaching over very long time periods. In general, lower phosphorus export is recorded in waters draining igneous catchments than from those with a sedimentary geology.

Atmospheric deposition may represent a significant source of phosphorus, particularly in oligotrophic catchments. In nutrient-poor systems, concentrations in rainwater may exceed those in runoff. Seasonal differences in phosphorus load from precipitation are relatively small in areas remote from intensive agricultural activity and urban centres. In heavily fertilised agricultural areas, however, the instantaneous phosphorus load from the atmosphere may be higher during the growing season than during the winter.

The major point source of phosphorus in urbanised areas is from municipal and industrial wastewater treatment plants. Improvements in treatment facilities required by EU regulations and the introduction of phosphorus free detergents are leading to a marked reduction in phosphorus from this source. While there maybe some temporal variation in the phosphorus load from these sources, there is generally no seasonal pattern.

The contribution of diffuse agricultural sources to the overall phosphorus load increases with percentage of agricultural land in the catchment. Although high phosphorus losses recorded from agricultural land may come from farmyards, most are attributed to excessive accumulation of phosphorus in soils because of long-term inputs of inorganic fertilisers and manures.

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