Salmon and Trout Association: Excess Nutrients

Aquatic plants need two essential nutrients to grow: nitrogen (N) and phosphorus (P). In a healthy freshwater system, nutrients occur in small amounts, and limit growth. In freshwater, plant and algae growths tend to be limited by phosphate levels, and in marine and estuarine water, nitrogen is the main limiting factor. However, if these nutrients occur in large quantities, they can cause major water quality problems.

The mains sources of excess human-derived nutrients are from;

• Urban; domestic sewage, industrial wastes and storm drainage
• agricultural runoff, polluted by fertilizers and animal effluent.
• phosphates are also found in detergents, and from mining activity.

Phosphorus binds strongly to soil particles; it can therefore easily be transported into watercourses when exposed soils are eroded in high rainfall events. (See: Fine Sediment) The amount from each source varies according to the soil types and human activities occurring in each watershed.

Higher than natural nutrient levels can lead to a process called eutrophication. This is when high nutrient levels result in rapid plant and algae growth, which removes oxygen from the water. High concentrations of algae can prevent light from penetrating the water column, block waterways and water treatment plants, and affect the taste and smell of the water.

When plants and algae die and decompose, they also use up large amounts of oxygen, which reduces the amount available for fish and other aquatic species. This stresses all animal life, and can lead to a decline in species diversity and a shift in dominant species. While mobile organisms, like fish, can often swim away from regions of low-oxygen, less-mobile and more sensitive species cannot escape. In extreme cases, excess nutrients can lead to a completely do-oxygenated (anoxic) environment, supporting nothing except a few species of bacteria able to survive without oxygen (anaerobic bacteria).

Eutrophication does occur naturally. However, human inputs of nutrients are accelerating the natural eutrophication of lakes (called cultural eutrophication). In waters that are naturally low in nutrients (called oligotrophic), small increases in nutrient levels can have large impacts on their plants and animals.

Nutrient Management

In England and Wales, 119 water bodies, including rivers, lakes and estuaries, have been designated as areas sensitive to eutrophication. This means that any sewage going into these areas must be treated to high standards, to make sure that all phosphates and nitrates have been removed. The designations are reviewed every four years.

Nitrate levels in many ground and surface waters still remain high or are even increasing. It is estimated that over 60% of nitrates enter water from agricultural land. The Nitrates Directive is designed to protect the water environment against nitrate pollution from agricultural land.

The Directive has recently been consulted on and revised (for S&TA views…), and from 1st January 2009, approximately 70% of England will be covered by Nitrate Vulnerable Zones (NVZs). All existing NVZs (those designated in 2002 or before) will be required to implement these measures from 1 January 2009, and new NVZ farmers will be required to start complying with the rules from 1st January 2010. The key measures set out in the revised Action Programme, which include:

• Whole farm manure N loading limit of 170 kg/ha of total N from livestock manures per calendar year, averaged across the farmed area.
• Closed period (organic manures): Prohibits the spreading of organic manures with high available nitrogen content during specified periods.
• Manure storage of all slurry produced by livestock (6 months for pigs and poultry and 5 months for cattle)
• Closed period from manufactured nitrogen fertilisers
• Spreading locations: Requires farmers to undertake a written assessment to identify areas of land at risk of runoff and causing water pollution.
• Spreading techniques: Prohibits the use of high trajectory application techniques for spreading slurry.
• Record-keeping: requirement to keep a record of all N applications to land

The revised Action Programme is also a Statutory Management Requirement (SMR) for cross compliance under the Single Payment Scheme, meaning farmers will have to comply with the measures to be entitled to their full subsidy payment; or failure to comply could lead to deductions.

Defra's water strategy, Future Water, outlines Government's long term vision for improving water policy by 2030, including measures to tackle non-agricultural diffuse water pollution (NADWP). A consultation was recently conducted on ‘options for controls on phosphates in domestic laundry cleaning products’. Click here for the S&TA response to this consultation.

Action

The S&A calls for further action to tackle issues of excess nutrients, and other sources of diffuse pollution. To remove nitrates and phosphates from sewage treatment works is expensive, energy intensive and on-going. Action must focus on catchment initiatives to reduce the sources of pollution, rather than relying on expensive ‘end of pipe’ solutions (See Blueprint for Water).

Other Information:

Defra. (2008). Nitrate Vulnerable Zones in England Map
Defra. (2007). Summary of methodology for identifying Nitrate Vulnerable Zones
EA. (2007). The Unseen Threat to Water Quality