Eutrophication

Eutrophication occurs when the enrichment of waters by nutrients (nitrogen and phosphorus) increases the growth of algae and plants in the water to the extent that they smother and suffocate wildlife.

Nutrient enrichment of watercourses results from agricultural run-off and sewage. Algae and plants take up dissolved nutrients and require approximately sixteen times more nitrogen than phosphorus. Compared to seawater, rivers are rich in nutrients, particularly nitrogen, so algae and plant growth in rivers are usually limited by phosphorus. Algal growth in seawater is usually limited by available nitrogen. The limiting nutrient in estuaries changes as river water is diluted with seawater so that phosphorus may be limiting growth at the head of an estuary and nitrogen at the mouth.

Nutrient inputs to the sea from rivers, sewage and industrial discharges have been monitored since 1990 for the OSPAR riverine inputs and direct discharges programme. Most of the nutrients are discharged to the East coast where the freshwater input is high compared to the rest of Scotland.