Cleaning up our waters: actions to remove nitrogen pollution from estuaries and coasts

Fish farm feed and waste also produces higher levels of nitrogen.

For estuaries and coastal waters, enriched levels of nitrogen cause phytoplankton and algal blooms to grow. This reduces the water quality with toxic levels of ammonia, killing communities of macrobenthic organisms, and starving our waters of oxygen.

Nitrogen enrichment can cause macroalgae to grow excessively in intertidal waters as mats, spreading over the area, affecting invertebrates and reducing food for waders and waterfowl. 

Phytoplankton in coastal waters are affected by increased nitrogen levels. A study showed a link between nutrient levels in the estuaries, and the growth of phytoplankton in the adjacent coastal waters.

Lay of the land

Our waters are measured and assessed under the Water Framework Directive Regulations. A high ecological status is where the biological community matches what it would be in conditions where humans have no or minimal impact. A good ecological status means there’s been a slight variation from this condition.

Many of our estuaries and coastal waters are designated Natura 2000 sites, and increased nitrogen pollution threatens their status. The Natura 2000 network protects core breeding and resting sites for rare and threatened species and stretches across European land and marine areas. If pollution levels continue to rise, they may no longer be able to achieve statutory compliance under the Habitats and Water Framework Directive.

Estuaries with the highest levels of nitrogen include: the Severn estuary, the Mersey, the Clyde, the Humber, the Thames and the estuaries around the Solent. They drain catchments with generally high nitrate soils, often in Nitrate Vulnerable Zones (NVZs). Regulatory measures seek to protect NVZs, such as banning fertilisers during winter months due to more rainfall, reducing the number of fertilisers used, and changing times when farmers can spread manure onto fields. About 58% of England’s land is designated as NVZs, with 6% due to eutrophication in estuaries and lakes or reservoirs.

Sixteen shallow tidal harbours or estuaries in England and Wales are labelled as eutrophic under the Urban Waste Water Treatment and Nitrates Directives. 

Estuaries with the lowest nitrogen levels are in west Wales and north Scotland as they drain relatively infertile catchments, have lower populations and less human activity.

In comparison, 38% of Ireland’s transitional waters are in good or better ecological status. Nitrogen inputs to the Irish marine environment have increased by 16% since 2014. Eight percent of Irish coastal waters are in good or high ecological status because they are more open, have an exposed nature, and have greater capacity to absorb nutrients. 

Diverse plans for a complex issue

Nitrogen pollution sources are very diverse. We often have to take an ‘integrated catchment management’ approach, to sustainably manage all land, water, and human activity in a catchment.

River Basin Management Plans, produced by national environmental agencies, and under the Water Framework Directive Regulations, mean we must identify how we bring our water bodies up to ‘good’ status by measuring:

• the state of the water environment
• pressures affecting the quality of the water environment where it is in less than good condition
• actions to protect and improve the water environment
• a summary of outcomes after these actions have been implemented. 

With ‘catchment partnerships’, various stakeholders work together to reduce nitrogen levels, including farmers, water & sewerage companies and landowners. In 2018, a water catchment declaration between Thames Water, Yorkshire Water, and Anglian Water worked with farmers to improve water quality by using fertiliser more efficiently and improving soil quality.  

When Natura 2000 sites are threatened, we apply ‘nutrient management plans’ to identify sources, and management as a combined approach. These have been enacted for Special Areas of Conservation associated with the rivers Avon, Wye and Clun. In some cases, they resulted in restrictions on development, including house building.  

In some instances, it’s best to go directly to the source with measures including nitrogen removal at sewage treatment works and placing controls on fish farms.

Natural England (NE) has started to take firmer action, particularly at Natura 2000 sites. NE told Solent planning authorities that permission should not be granted for new developments unless they can demonstrate Nutrient Neutrality. In the Solent, dense mats of green algae were damaging protected habitats.

Mitigation measures by developers include nitrogen removal options from wastewater treatment plants, creating wetlands and reedbeds, and changing the purpose of the land on the site and surrounding area. The new NE restrictions seek to address the environmental problem but could have an impact on our national housing ambitions and growth unless cost-effective measures to enable nutrient neutrality can be developed.

Cleaning up our act

Cleaning up and removing nitrogen pollution is complex. It involves freshwater, estuarine and coastal processes, a wide range of sources and pathways, and diverse stakeholders, from farmers and fishermen to regulators and water companies. Other nutrients, including phosphorus, have to be considered and many of the impacts are indirect, such as eutrophication leading to low oxygen conditions.

Where Marine Protected Areas are affected, we must act urgently to reduce nutrient levels, by putting appropriate controls on development, upgrading sewage treatment facilities and shifting farming and land-use practices. The impact of this will have potential implications for society and the economy.

There are innovative measures, such as nitrogen offsetting through the ENTrade platform. These are likely to gain impetus with the proposed introduction in 2024 of DEFRA’s Environmental Land Management scheme in England and Wales. Farmers and land managers will be paid to carry out environmental work on their land, potentially including nitrogen reduction measures.

Despite work carried out so far, we have a long way to go. With 93% and 47% of our estuaries and coastal waters exceeding nitrogen standards, we can’t afford to stop, as our natural environment continues to suffer.