Executive Summary

Phosphorus is considered to be the main limitating nutrient for primaryproduction in most freshwater systems. Long-term increases in the concentrationof phosphorus have occurred in many rivers and lakes in recent decades. Externalsupplies of nutrients to surface waters can originate from point sources, which arelocalised and more readily monitored and controlled, and non-point sources,which are diffuse and much more difficult to monitor and regulate. Reductions inphosphorus inputs from point sources do not always reduce phosphorusconcentrations in surface waters. While internal loading from sediments may be afactor in the maintenance of phosphorus availability in these systems, diffuselosses from agricultural sources are considered the major cause.

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

In pristine systems, the flux of phosphorus from the catchment to surface waters ismainly determined by the flow of water through the system and the underlyinggeology. Losses from these systems are generally low and occur by leachingover very long time periods. In general, lower phosphorus export is recorded inwaters 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, concentrationsin rainwater may exceed those in runoff. Seasonal differences in phosphorus loadfrom precipitation are relatively small in areas remote from intensive agriculturalactivity and urban centres. In heavily fertilised agricultural areas, however, theinstantaneous phosphorus load from the atmosphere may be higher during thegrowing season than during the winter.

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

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

Full executive summary in report.