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Nutrient mitigation

Nutrient mitigation

Excessive nutrients (principally nitrogen) in the Solent’s European sites are causing eutrophication, leading to an increase in algae growth. This has an adverse impact on the habitats and species within the European nature conservation designations sites in and around the Solent to which the Habitat Regulations apply.

Following recent case law, Natural England has advised that there is a likely significant effect on the Solent’s designated sites due to the increase in wastewater from new housing. It has also advised that any development providing overnight accommodation which would discharge into the Solent would also be likely to cause a significant effect.

The internationally-designated habitats of the River Avon to the west of the New Forest are also affected by elevated levels of nitrates – in this case phosphates. The scale of planned new development in the catchment of the River Avon that falls within the National Park is low, but any proposals for net new dwellings and other forms of overnight accommodation in this catchment must demonstrate it is phosphate neutral. New Forest District Council has prepared an ‘Interim phosphate calculator’ that can be used to prepare a phosphate budget for new development in the River Avon catchment. Further details are available at Nutrient neutral development – New Forest District Council under ‘Achieving nutrient neutrality’.

Natural England recommends that the impact of development on water quality on the catchments of the Solent and the River Avon is examined within the Appropriate Assessment process. For this reason, we are requesting the submission of a Nitrate Mitigation Checklist for any planning application for a net increase in dwellings or providing overnight accommodation.

Natural England has published updated guidance on the issue of nitrates from new development affecting the internationally-protected Solent habitats. This updated information is available to download from this website and comprises:

Solent nutrients guidance – Version 5, June 2020

Solent nutrients advice – non-technical summary, June 2020. This includes a Frequently Asked Questions section in Annex A.

Nitrogen budget calculator: This spreadsheet standardises nutrient budget calculations

Water catchment areas in the New Forest National Park

Applicants are advised to review the latest Natural England guidance in preparing their applications. We have also also prepared a high-level map illustrating the respective water catchment areas for the National Park to indicate the areas affected by the respective nitrates and phosphates issues. This is available to download on the right hand side.

The New Forest National Park is covered by three main water catchment areas:

  • The Test
  • The New Forest
  • The Hampshire Avon

Detailed maps of these catchment areas can be viewed at Environment Agency – Catchment Data Explorer by searching using the three catchments named above.

Identifying the relevant catchment for mitigation measures 

Natural England’s guidance on nutrient neutrality (paragraphs 5.23 – 5.42) confirms that mitigation land should be within the same catchment as where the development is located. The Environment Agency’s detailed maps for the respective catchments help identify the areas for mitigation. In addition, Natural England’s advice is that development within the Bartley Water catchment of the New Forest (which includes Ashurst, Netley Marsh and Minstead) can be included with the Test and Itchen catchment due to the tidal flows between these estuaries. The catchment of the Bartley Water can be viewed on the Environment Agency’s website.

It is the responsibility of the applicant/developer to find suitable and sufficient nutrient mitigation solutions. A list of nutrient mitigation suppliers is available on the Partnership for South Hampshire’s website at Mitigation schemes available to developers – Partnership for South Hampshire ( This information includes details of the catchments that the mitigation schemes serve.

National Park Member


'Read our guidance notes before applying to save yourself time in the long run.'

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