The Solent Forum

Working in partnership for the future


Algal mats Solent July 2021 Project RanTrans
Algal mats in the Solent © Project RanTrans
Algal mats in the Solent © Project RanTrans


A variety of chemical elements and compounds are essential to the growth and survival of living organisms. In aquatic ecosystems, nitrogen and phosphorus are the most important, as they are most often in short supply relative to the needs of plants, algae, and microbes. These two nutrients are the primary ones that, in excessive amounts, pollute our lakes, streams, and wetlands. These chemicals in excess, in particular nitrogen with its particular effects on estuarine habitats and phosphorous with its effects on freshwater habitats, can cause harm to habitats and species by causing eutrophication. Faecal contamination can also be a danger to amenity and public health of water bodies through Combined Sewage Overflows (CSOs) and misconnections can be a danger to the health of those that swim in it; detail on this are covered in the public and amenity page of this hub.

Eutrophication is a leading cause of impairment of many freshwater and coastal marine ecosystems in the world. It is characterised by excessive plant and algal growth due to the increased availability of one or more limiting growth factors needed for photosynthesis such as sunlight, carbon dioxide, and nutrient fertilizers. Although eutrophication can occur naturally over centuries as lakes age and are filled in with sediments, human activities have accelerated its rate and extent through both point-source discharges and the diffuse loadings of nitrogen and phosphorus. During the 1960s and 1970s, scientists linked algal blooms to nutrient enrichment resulting from anthropogenic activities such as agriculture, industry, and sewage disposal. The known consequences include blooms of blue-green algae, tainted drinking water supplies, degradation of recreational waters, and hypoxia. Eutrophication in the Solent area is not just an environmental problem; excessive algal growth can make launching boats and sea-angling more difficult and can create bad odours particularly in the summer months.

The predominant source of nitrogen is from rural diffuse pollution (mostly agriculture) however a significant component comes from housing waste water treatment. Natural England and the Environment Agency work with farmers on Catchment Sensitive Farming (CSF) to tackle agricultural inputs.

Solent Context and Issues

Algal mats Solent July 2021 Project RanTrans
Algal mat over seagrass bed © Hants and Wight Wildlife Trust
Algal mat over seagrass bed © Hants and Wight Wildlife Trust

The Solent’s designated sites are in unfavourable condition, one reason for this is eutrophic waters caused by the effects of historical high nutrient levels, predominantly nitrogen.  Historical pollution from agricultural discharges, wastewater, urban runoff, atmospheric deposition, combined sewer overflows (CSO), unlicenced sewer discharges, and sewer misconnections led several Solent estuaries to become eutrophic and experience algal blooms. The resulting dense mats of green algae impact negatively on the area’s protected habitats and bird species. They cover mudflats which stops oxygen reaching the animals in the sediment below and cause mass mortality, especially in hot weather. Algae also forms a feeding barrier to the birds which feed by probing the mud or picking off tiny invertebrates from its surface and it can smother habitats such as seagrass beds and saltmarshes causing damage and erosion.

Environment Agency data shows recent large decreases in nutrient loads, the Solent is now recovering from eutrophication. Solent waterbodies have much reduced nutrient loads compared to 20 years ago (e.g. in Langstone Harbour nitrogen load has been halved and phosphorous load reduced by over two thirds). Both Langstone and Chichester Harbours now achieve Water Framework Directive GOOD status for macroalgae. All Solent water bodies affected by eutrophication have experienced decreases in nutrient input and macroalgae. In some places there will be further improvements due to the ecological timelag as groundwater nitrogen decreases, even if we do no more improvement actions.

The greatest diffuse pollution input is still from agricultural run-off. This is currently controlled by regulations and legislation; Regulatory Nitrate Vulnerable Zones designations in 2008 have greatly reduced nitrates. Catchment sensitive farming provides advice and financial incentives to improve land management practices. The nitrogen and phosphorus concentrations observed in transitional and coastal waters, reflect the influence of direct and diffuse inputs of nutrients in the upstream catchments. The Solent has five river catchments that feed into it and the respective Catchment Partnerships (Test and Itchen; Island Rivers; East Hampshire and New Forest and Arun/western Stream), deliver the Catchment Based Approach (CaBa) to improve water quality in the upstream river catchments.

Southern Water acknowledge that they need to reduce storm overflow releases, and they are looking at innovative solutions to deliver infrastructure that's more resilient and adaptable; they set up the Clean Rivers and Seas Task Force in November 2021, demonstrating their commitment to drive down the use of storm overflows. The Task Force is responsible for delivering six pathfinder projects over a number of years, as well as delivering a regional plan to reduce storm overflows between now and 2030. One of these pathfinder projects is in the Solent at Sandown Isle of Wight a catchment which has had a large number of storm overflow ; It is made up of eight sub-catchments and covers more than 90% of the population of the Isle of Wight. Different solutions are being trialled including enhanced wastewater pumping station control, surface water removal and storage solutions (uniquely using water butts installed in residential properties.


Regulatory bodies have been aware for several decades of eutrophication in some Solent estuaries. Since 1998 there have been a series of eutrophication reviews, particularly by the Environment Agency, and regulatory means to reduce nitrogen (N) inputs. These N reductions have included N removal from sewage discharges from both marine and riverine environments (via the Urban Waste Water Treatment Directive (UWWTD) and Habitats Directive) and reduced N inputs from agriculture (via the Nitrates Directive, which enabled designation of Nitrate Vulnerable Zones (NVZ) and subsequent actions required by farmers).

During the summer of 2018, Natural England undertook a review of the condition of the water environment in designated sites in the Solent harbours. The best available up-to-date evidence identified that some interest features at the designated sites, such as intertidal mudflat habitat and the wildlife they support, are widely in unfavourable condition due to existing levels of nutrients and are therefore at risk from additional nutrient inputs. It is Natural England’s view that there is a likely significant effect on several internationally designated sites (Special Protection Areas, Special Areas of Conservation and Ramsar sites) due to the increase in wastewater from new housing developments coming forward. An important European case set a precedent in law in 2019 (Cooperation Mobilisation for the Environment v Verenigin Leefmilieu) makes it difficult for regulatory authorities to approve plans or projects that might permit additional pollutants to sites which are already in unfavourable condition. As the approval of new housing would lead to additional pollutants from nitrates through wastewater, there is a need for plans/projects to demonstrate nutrient neutrality.

The Environment Agency published an overview of Solent Eutrophication and Recovery in January 2023. This review details nutrient reduction measures that have been put in place by the Environment Agency for several decades in some Solent estuaries, particularly the Harbours. They have included putting in place permits that require nitrogen reductions from both marine and riverine sewage discharges as well as reducing nitrogen inputs from agriculture. This review asserts that recovery from eutrophication is well underway.

Nutrient Neutrality

Although a small proportion of the overall nutrient loading, the additionality of nutrient pollution from new housing development has been the focus of recent attention to satisfy new Habitats Regulations requirements. Natural England (NE) published a position statement in 2020 that advised that there is uncertainty about the potential in-combination impacts of new housing upon the Solent’s designated sites. As sufficient certainty is required at the Appropriate Assessment stage, the onus was put upon new housing plans to show that the housing is nutrient neutral; this would show that the housing would not result in any additional nutrients entering into the Solent. NE have since developed a methodology which provides specific guidance including a nitrogen budget calculator, to enable Developers and Local Authorities to understand the issue and work out how best to respond.

In order to progress the development of new housing, local authorities in the Solent have been working individually and together (through the Partnership for South Hampshire) with Southern Water, Portsmouth Water, Natural England and the Environment Agency to identify potential nutrient mitigation solutions. A range of mitigation projects are starting to emerge, provided by individual Councils, the HIWWT and by private land owners. Developers will be able to purchase offsetting nitrogen credits from these projects equivalent to the additional total nitrogen load their new housing would generate. DEFRA is also piloting a nutrient trading platform in the Solent, which is expected to offer additional mitigation credits later in 2022 by harnassing the nitrogen reduction potential of smaller land holdings and localised environmental projects. However, the Department for Levelling Up, Housing and Communities has announced plans for an amendment to the Levelling Up and Regeneration Bill, which could have significant implications for nutrient mitigation work. It is expected that this amendment, if adopted, will exempt waste water from housing from consideration in a Habitats Regulation Assessment in areas currently affected.

The solutions to the housing neutrality issue include:


Bloomin’ Algae” is developed by the Centre for Ecology & Hydrology for the general public to record the presence of possible harmful algal blooms in freshwaters around the UK. You can download the app directly from Google Play or App Store.

Southern Water have prepared a guide on the work they do removing nutrients from wastewater at their waste water treatment plants in the Solent. 

Portsmouth Water has launched the Portsmouth Water Nitrate Intervention Payment for Ecosystem Services (PES) scheme which directly assists farmers (and other land managers) in high sensitive catchment areas to deliver measures that aim to reduce nitrate leaching and improve farming efficiency. 

A Defra led Solent Nitrate Pilot with funding of £3.9m over two years (announced September 2020) is seeking to deliver an online nitrates trading platform to unlock housing development in south Hampshire. 

The Rapid reduction of Nutrients in Transitional waters (RaNTrans) Interreg project, partnered by the University of Portsmouth, will develop and test innovative and cost-effective methods to reduce algal mat coverage and reduce nutrient levels in mudflats. 

Environment Agency 'Overview of Solent Eutrophication Briefing note' 2020. The Environment Agency encourages any enquiries, reports or concerns about nutrient pollution to be reported on their 24/7 hotline – 0800 807060.

Havant BC have identified evidence of high levels of nitrogen and phosphorus in the water environment with evidence of eutrophication at some designated sites. As such, there is considered to be a likely significant effect from any development resulting in an increase in overnight accommodation located anywhere in Havant Borough. As a result, an avoidance and mitigation package is needed in order for the Council to lawfully grant any planning permission. The Council has published a Nutrient Neutrality Position Statement and Mitigation Plan which sets out how it is possible to mitigate the impact of development.


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