Protecting Poole Harbour – Managing Nutrient Run-Off

Lee Hardy OBE JP, Chair Dorset Catchment Partnerships

Poole Harbour is recognised internationally for its huge ecological importance and renowned and protected for its outstanding landscape, birdlife and fisheries.  It is one of the largest natural harbours in the world and the largest microtidal estuary in Britain, exhibiting an enclosed lagoonal character with over 100km of enclosed coastline. Maintained, deep water channels allow shipping to access the Port of Poole but the Harbour is mostly shallow, containing a high proportion of intertidal saltmarshes and mudflats. These give way to freshwater marshes, reed beds and wet grasslands on low, poorly drained land above the tidal level, and transition to heathland on higher, sandy ground and heathland mires in small tributary valleys.

Four main rivers drain their catchment areas into the Harbour. The Frome is the largest flowing in from the west through Dorchester and Wareham with the smaller Piddle, Corfe and Sherford Rivers completing the picture. This connectivity with and influence from the freshwater environment is particularly important in terms the migratory passage of fish through the Harbour in both directions.

In recent years a number of management measures and interventions have been implemented to slow the decline of the environmental characteristics of the Harbour.  Nevertheless, 21st century lifestyles continue to place pressures on both short and long-term sustainability. Principal amongst the vulnerabilities is the threat to water quality brought about by nutrient pollution which is magnified by the Harbour’s constrained geography which limits the water’s ability to refresh itself.

The increase in nutrients entering the harbour waters over the last 50 years is a notable cause of deterioration of Poole Harbour’s marine ecology with nitrogen and phosphorus having the biggest impact. The amount of nitrogen entering the harbour has increased from around 1,000 tonnes per year in the 1960s to more than double that now. The effect has been significant and very visible, with examples of mudflats covered in green algae and the loss of some seagrass and saltmarsh habitats affecting wetland birds and other wildlife and the declining condition of the Poole Harbour Special Protected Area. Although there are other significant inputs, he largest source of nutrients to the catchment is the agriculture sector; 2011 modelling [1] indicated 75% of the N loading from the catchment comes from livestock and arable sources.  

The situation was becoming critical, and something needed to be done to arrest the decline in riparian and estuarine ecology by bringing nutrients to a level where these habitats are restored.  Doing so would also offer broader economic, welfare and amenity benefits. For example, saltmarsh and seagrass capture carbon, act as nursery grounds for commercial fish and provide improved coastal protection against sea-level rise in addition to enabling improved water quality.

A 2015 Judicial Review led the Environment Agency and Natural England to evaluate whether their existing mechanisms would deliver the improvements necessary and presented the threat of imposing a Water Protection Zone, a legislative last resort which would mean Defra enforcing restrictions on ‘polluting activities’ such as farming. A Water Protection Zone would likely have a severe impact on the ability of those farms in the catchment area to sustain their existing business models.

This context was the inspiration for the ground-breaking Poole Harbour Nutrient Management Scheme, or PHNMS, which was conceived and set up by farmers, for farmers, who formed the Poole Harbour Agriculture Group Community Interest Company to build and run the scheme. The Group has been supported by key partners to ensure that the scheme is fully fit for purpose. It takes a whole catchment approach, uniting all the farmers across its width and breadth.  In doing so, it provides the tools and support for delivering environmental benefits by reducing nutrient run-off; enabling farmers to make sustainable incremental changes to their agricultural practice and driving toward the vision that “The farmers in the Poole Harbour Catchment are the most nitrogen efficient in the world.”

The initial phase of the scheme was to develop a business case and secure funding.  That is now done, and effort is being focused on supporting the Environment Agency to develop a farm-level nutrient accounting tool which will enable farmers to calculate their nitrogen losses.  In 2022-23, all farmers in the catchment will be required to use this tool to calculate their farm baseline nitrogen losses, thereby equipping them to choose methods of maximising their nitrogen efficiency in 2023-24 and beyond through; improved targeting of fertiliser applications, crop choice, livestock management practice etc; in turn minimising any run-off into water courses.  This also offers a financial benefit to farmers by offering more cost-effective means of managing fertilisers and manures. Alongside this   a ‘trading platform’ is being developed.  This will allow overperforming farms to offer nitrogen ‘credits’ to other members needing assistance. As the scheme progresses year on year, the targets tighten, encouraging members to make real-world changes to meet the target rather than continuously ‘trade their way out of trouble’.

A small-scale trial of the scheme with 15-20 farmers commenced in Spring 2021. These volunteer farmers road-tested the scheme allowing it to be fine-tuned ensuring it is fit for purpose and demonstrating that the scheme works in principle. A larger, catchment-scale pilot will be rolled out over winter 2022-23.  A full time Development Manager has been appointed who will  build and run the pilot, with membership open to all farmers in the catchment. In 2024, the Environment Agency will review progress and efficiency of the scheme. If farms are shown to still be leaching excessive nitrate, Defra may impose a Water Protection Zone in order to deliver the required reduction in nitrate leaching by 2030.

The real success of PHNMS will not only see environmental and biological improvement in water quality within the Harbour, but in the future would also see the scheme replicated across other catchment areas around the UK where water quality is under threat. This would represent solid recognition of the foresight, initiative and impact of the local agricultural community and be a tangible demonstration of what can be achieved through working in partnership to safeguard the environment.

In conclusion, interest is surging among the private sector, local authorities, farmers and other landowners to collaborate on solutions that respond to regulatory and policy drivers to combat biodiversity decline and climate change. Farmers are at the heart of these efforts.

PHNMS is a leading example of potentially transformative nature-based solutions. It brings together farmers and a host of stakeholders to model a new approach to curbing farm nutrient losses and achieve cleaner waters on protected coastlines. The scheme will be closely watched as a potential blueprint for the water and other sectors beyond Dorset.


Environment Agency, “Poole Harbour Consent Order Technical Document,” 02 2021. [Online]. Available: [Accessed 13 09 2022].