Water quality is a key pressure affecting habitats and species in coastal and marine areas. For marine species a build-up of contaminants in the water, either as toxic chemicals or biological waste reduces oxygen levels and can also result in immune suppression and reproductive problems. Coastal and marine habitats are particularly susceptible to the impacts of nutrient enrichment. This can lead to algal blooms and the build-up of algal mats which can smother both habitats and species and turn toxic on decomposition. The most visible impact is often marine litter which is covered in a separate topic on this CSSS hub.
Maintaining good water quality standards not only helps protect habitats and species but it helps preserve the intrinsic balance within our ecosystems, supporting local fisheries, carbon capture and coastal recreation.
The Solent is a complex site encompassing a major estuarine system on the south coast of England. The Solent and its inlets are unique in Britain and Europe for their hydrographic regime with double tides, as well as for the complexity of the marine and estuarine habitats. Sediment habitats within the estuaries include extensive areas of intertidal mudflats, often supporting eelgrass Zostera spp. and green algae, saltmarshes and natural shoreline transitions, such as drift line vegetation. All four species of cordgrass found within the UK are present within the Solent and it is one of only two UK sites with significant amounts of the native small cordgrass Spartina maritima. The rich intertidal mudflats, saltmarsh, shingle beaches and adjacent coastal habitats, including grazing marsh, reedbeds and damp woodland, support nationally and internationally important numbers of migratory and over-wintering waders and waterfowl as well as important breeding gull and tern populations (Source: Solent IPENS).
The most important sites around the coast are designated as Marine Protected Areas (MPA). Most of the Solent’s coast and marine water space is designated as an MPA. The Solent Maritime Special Area of Conservation was designated for its abundant sandflats, mudflats, Atlantic salt meadows, coastal lagoons and estuaries. The Solent’s three Special Protection Areas (SPAs) were designated due to the importance of the birdlife.
Natural England publishes condition assessments on its designated sites system where you can view site condition and pressures. Common Standards for Monitoring were developed in 2022 to provide an agreed approach to the condition assessment of designated Marine Protected Areas and subtidal and wider marine features.
The condition assessment for the Solent Maritime SAC states that the intertidal mud sub-feature is considered to be 'Unfavourable - no change' because its principal attribute (Structure: species composition of component communities) and secondary attributes (Supporting processes: water quality - nutrients and Supporting processes: water quality - contaminants) have been assessed as fail. The pressures (causes) listed are nutrient enrichment from agriculture and freshwater discharges (wastewater treatment works). It notes that there are some management measures in place to improve water quality, however, these would not currently be enough to deliver a full recovery.
Condition assessments for the SPAs have not yet taken place, although condition assessments for the underlying component SSSIs show poor water quality is the reason that many that are in sub-standard condition.
Natural England have published Site Improvement Plans for many Solent sites including The Solent, Solent and Isle of Wight Lagoons, and Solent Maritime. The Solent SIP lists water pollution as a priority issue detailing problems and actions with Nitrogen, macroalgae, storm discharges, wastewater from boats and herbicides/pesticides.
Further Information on Solent designated marine sites and condition assessments can be found on the SEMS webpages.
The Solent lies in the South East River Basin Management Plan (RBMP) and is currently classed as moderate ecological status. The target for the area is to improve the ecological status of the Transitional and Coastal Waters to good by 2027 or 2033, although there is low confidence in achieving this. In almost all cases Dissolved Inorganic Nitrogen is the reason for this moderate status, mainly attributed to pollution from agriculture and waste water treatment plants. All Solent catchments fail on chemical status due to the presence of priority substances mercury and polybrominated diphenyl ethers; the target for this improvement is not until 2063. Full detailed on the RBMP can be found on our governance page.
There are many species in the Solent that are not covered by a formal designation such as fish, shellfish and marine mammals that are impacted by water quality. Good water quality is vital for the production of high quality shellfish for human consumption and to support local shellfish-related economies. Shellfish waters can be impacted by pollution from various sources, such as run-off from agricultural land or discharges from sewage treatment works.
Oysters, clams, and other shellfish are efficient filter feeders that help remove excess nitrogen from waters by incorporating it into their shells and tissue as they grow. They provide an ecosystem service by helping to clean water so protecting and restoring them has many benefits.
Marine biologists often use the number of seals in an area as a barometer of water quality, the Solent has resident seals in Langstone and Chichester Harbours. A relatively stable numbers of seals indicates good water quality and reliable stocks of fish.
There are various initiatives underway across the Solent to help improve water quality using the Solent’s habitats and species as a tool.
Oyster beds contribute towards nutrient cycling and removal of organic matter through filter feeding and provide a nursery habitat for commercially viable fish stocks. The Blue Marine Foundation is working with partners to restore native oyster beds to the Solent and develop a sustainable fishery. Oyster reefs at Langstone Harbour are already fully established and it is expected will hatch one million oysters a year.
The Solent Nutrient Market Pilot is currently testing an online nutrient trading platform which aims to help landowners deliver nature-based solutions to reduce nitrate and phosphate pollution to support new developments to deliver nutrient neutrality.
Three projects are currently underway which aim to restore and identify sites for seagrass restoration. Seagrass can reduce nutrient loading and provides many ancillary benefits for other species.
RaNTrans is Interreg funded project that is trialling methods to physically remove algal mats and looks for a future use for them. It is being tested in the Solent and Poole Harbour and other areas around the UK and Europe.
ChaPron – Chichester Harbour Protection and Recovery of Nature
Based in Chichester Harbour this initiative is a long-term plan to help protect, enhance and drive nature recovery within the Harbour. The work includes coastal resilience and saltmarsh restoration, seagrass distribution, catchment sensitive farming, reducing bird disturbance, native oyster restoration and community engagement.