Improving the water quality in Chichester Harbour is of critical importance for people and for nature. Harbour waters are heavily impacted by human activity and the green swathes of macroalgae weed that smother the mudflats in the summer are a visual representation of the excess nutrients within the harbour. Macroalgae weed reduces light in the water, and smothers our coastal habitats. But there are numerous other pollutants negatively effecting the coastal ecosystems – plastics, heavy metals and chemicals including endocrine disruptors impacting the behaviour and reproductivity of marine creatures and damaging the fragile ecosystems. Partnership working is key to improving the water quality within Chichester Harbour, and all of us have a role to play.
Pollution Pathways
The release of untreated sewage is the most-publicised of the pollution sources impacting Chichester Harbour. The outdated wastewater infrastructure means that rainwater drains from buildings and roads into wastewater treatment works alongside wastewater. During heavy rainfall the system becomes overwhelmed and untreated sewage is released into the harbour via Combined Storm Overflows (CSOs).
The Pollutants:
- Bacteria, a concern for human health.
- Excess nutrients causing eutrophication (causing the bloom of macroalgae weed).
- Microplastics from laundering man-made fibres like polyester and nylon also enter the water through our waste water.
- Chemicals and pharmaceuticals including household cleaning products, prescription and illegal drugs. A 2025 study found more than 100 compounds in water samples collected by the Clean Harbours Partnership.
What is being done?
Pressure has mounted on water companies to make meaningful infrastructure improvements and Southern Water has a programme of work underway.
What can I do?
As individuals we can make small differences for example:
- Help slow the flow from our property – use a water butt, planting to slow rain run-off, choosing permeable surfaces in our gardens and driveways.
- Consider the chemicals we use in our daily lives that will end up in the water, from cleaning products to pet flea treatments, suncreams and the chemicals in waterproof clothing and non-stick frying pans.
- Make other small changes in your daily life, for example using catchers or filters to collect microplastics from laundry.
Food production is of vital importance, and farmers often use nitrates and phosphates as fertilisers as well as approved pesticides and herbicides to grow crops. But not all the fertiliser applied to the land is taken up by plants. When it rains the excess fertiliser leaches into the ground or is washed into the waterways. The catchment for Chichester Harbour is huge, stretching up to the South Downs. The chalk of the South Downs acts as an aquifer, storing water but also pollutants from agriculture for decades that gradually filter into the harbour.
The pollutants:
- Nitrates (bringing excess nutrients causing eutrophication).
- Phosphates, herbicides and pesticides.
What is being done?
Around the Chichester Harbour National Landscape many farmers work with our Farming in Protected Landscapes team to reduce their use of fertilisers and farm with a view to improving water quality. Measures include intelligent GPS tools to help minimise the use of fertilisers as well as improving natural barriers before waterways.
There is wider work with partners across the whole catchment to try to better understand the issue, and work with the farming community to reduce pollution from agriculture, and look at learning from other catchments across the UK and beyond.
From the release of sewage from onboard toilets, to the “antifoul” chemicals used on vessel hulls, oils and fuels and plastics in the hulls of many boats, boating and sailing can have a detrimental effect on our water quality.
The pollutants:
- Sewage release from onboard toilets.
- Antifoul – The heavy metals and biocides in most antifoul treatments are toxic to marine life and can leach into the water. Antifoul chemicals often enter the water when scraped or removed without due care.
- Plastics can enter the water through routine maintenance of plastic and GRP-hulled vessels. A study has found accumulation of boating plastics within shellfish in Chichester Harbour.
- Oils and fuels.
What can be done?
- Boaters can install holding tanks and use the free pump-out facilities at Itchenor to dispose of sewage.
- Choose eco-friendly silicone-based antifoul and/or follow RYA environmental best practice guidelines.
- Take precautions when maintaining or refuelling your boat.
Urban run-off from houses, buildings, streets, roads and highways enters the waters of Chichester Harbour through drainage, waste-water treatment plants and storm overflows during rainfall events.
The pollutants include:
- Oils and fuels.
- Microplastics – a single tyre from an average car sheds 4kg of plastic particles during the lifetime of the tyre.
- Chemicals and compounds which negatively effect marine life and the environment.
In addition to the plastic pollution from boating, waste-water and urban run-off, plastics enter the marine environment through litter, and discarded fishing gear at sea.