Whilst nutrients are essential to life in the oceans, there is growing concern over the accelerated nutrient enrichment of coastal waters as a result of human activities. Nitrate and phosphate concentrations around the UK coast have varied little since 1985. These nutrients, discharged directly and indirectly into rivers and the sea, can trigger an explosion in the growth of unicellular marine algae, often described as algal blooms. Dead algae sinking to the seabed are decomposed by bacteria using large amounts of oxygen. The surrounding water column becomes oxygen deficient, and this entire process is known as eutrophication. Water's suffering from eutrophication are detrimental to aquatic and marine life.

Broadly speaking, nutrients are delivered into the sea either from point sources and diffuse sources.

Typical point sources:

• domestic sewage outlets
• emergency storm overflow pipes and combined sewer overflows.
• industrial discharges.

Typical diffuse sources:

• Agriculture - organic and inorganic fertilisers, animal waste washing off farm land after rain.
• Urban - industrial chemicals, leaking sewers, organic waste and discarded refuse washing off city streets after rain.
• Atmospheric - air bourne nitrogen oxides from burnt fossil fuels can contribute significantly to the nitrate content of the oceans.

The Government estimates that almost two million tonnes of nutrients are discharged into UK coastal waters each year, and as point source discharges are cleaned up, so diffuse pollution becomes relatively more problematic. Diffuse pollution was described by former Environment Minister, Elliot Morley, in 2004 as "the biggest single challenge to enhancing the country's water quality".

Impacts

• Eutrophication can produce ‘dead zones' on the sea bed of up to 70,000 square kilometres in area. Non mobile marine animals and plants within this area risk death by smothering, light loss and suffocation.
• Red tides: excessive growth of the dinoflagellate algal class can produce ‘red tides', so named because the sea becomes discoloured by the concentration of algae. Some dinoflagellate species are toxic, and can be taken up and concentrated by shell fish such as mussels and oysters. If these contaminated shell fish are eaten by man and other animals it can cause illness ranging from minor stomach upsets to paralysis and heart attacks.
• Aesthetic impacts on coastal waters include increased turbidity, discolouration, unpleasant odours, slimes and foam formation which may adversely impact on recreational use and reduce tourism income.

what you can do

• Report sightings of algal blooms and sewage pollution accidents on the Environment Agency hotline: 0800 80 70
• Use eco-friendly detergent products (reduced phosphate content), and don't flush garden fertilisers down the drain.
• When abroad, avoid eating shell fish in areas known for sewage polluted sea.
• Farmers should contact the Environment Agency for advice on catchment sensitive farm practices to reduce diffuse pollution from agriculture. Click here for more information