Pollution from rivers in Wales is affecting the marine environment and “suffocating our sea”, campaigners have warned.

WWF Cymru said excess nutrients from farming and sewage were leading to the growth of algal blooms, smothering the seabed.

New assessments of the condition of protected areas along the Welsh coast, published by environment watchdog Natural Resources Wales (NRW), reveal several sites suffering from nutrient pollution with more than half of the species and habitats assessed across all areas in unfavourable condition.

The Welsh government said the reports would provide “critical information” to help improve the marine environment.

NRW had previously confirmed that seven out of the nine most protected rivers in Wales – known as special area of conservation or SAC rivers – were failing water quality targets due in part to high levels of nutrients.

Run-off and slurry spills from agricultural land, wastewater discharges and pollution from industry were all contributors to the problem.

The situation has already led to limits being placed on housebuilding along SAC rivers.

Launching a manifesto of asks to politicians ahead of next year’s Senedd election, WWF Cymru said a similar picture was now emerging in the sea.

Pollution fed in from rivers is damaging marine ecosystems and the ocean’s ability to store carbon, “making conditions for recovery or restoration of carbon-rich habitats, such as seagrass, challenging”, it said.

The charity called for a “strong recovery plan” for ocean habitats, backed by law and funding.

Like with rivers, estuaries and areas of the sea around Wales are protected for their important habitats and wildlife.

NRW described its report on the condition of these marine SACs and special protection areas (SPAs) as “the most extensive to date on the state of marine protected areas in Europe”.

A total of 85 important features were assessed – ranging from rare species such as grey seals and otters to protected habitats like mudflats and saltmarshes.

Across both SACs and SPAs 55% of the features assessed were in unfavourable condition, with 38% classed as favourable and 7% unknown.

NRW noted that nutrient pollution was “one of the main reasons” for the issues at some sites.

Individual reports were prepared for seven SACs, with five of those failing water quality targets due to excessive nutrients.

These included the Pembrokeshire Marine Special Area of Conservation, one of the country’s most famous and popular tourist hotspots.

In the marine environment the nutrient of concern is generally dissolved inorganic nitrogen, predominately from agricultural run-off and pollution incidents, the reports suggest.

The assessments did find some good news for both bottlenose dolphins and grey seals in Welsh waters.

Welsh SACs are considered vital in maintaining their populations, and both animals were found to be in a favourable condition across Cardigan Bay, Lleyn Peninsula and Sarnau.

In the Pembrokeshire Marine SAC where approximately 90% of Welsh seal pups are born, long term measures show the population growing.

Ric Cooper helps run The Cleddau Project in Pembrokeshire where a group of volunteers are working to restore the county’s main rivers – the Western and Eastern Cleddau – back to good health.

“I live on the estuary and like others had become increasingly distraught with the state of the river,” he said.

“It would often smell of sewage and sometimes slurry, and the life in the river was deteriorating before our eyes.

“We have green algal banks, we have what they can algal mats in the summer months which are due to excess nutrients in the water, sunlight and warm temperatures.”

The condition of the river and marine environment are “very linked”, Mr Cooper explained, but he felt the impact on the coast and sea were “almost like a forgotten story”.

“There hasn’t been the same pressure to improve things for the estuary as there has been for the freshwater Cleddau.”

He said the group, which has spent the last year coordinating a citizen science project monitoring water quality at 49 sites, involving 100 volunteers, was very much looking forward to reading NRW’s reports.

“We want it to be highlighted that this water body is failing, it’s in a bad way and then the powers that be will have to decide what can be done about it.

“We’re hoping this catalyses more action.”

Shea Buckland-Jones, head of policy and advocacy at WWF Cymru, said NRW’s assessments highlighted “the terrible state of our waterways and the urgent action needed from Welsh government”.

“Sadly, the public do not need to view a report to appreciate the dire state of our waterways as it is all too apparent.

“This is why there is so much support from across Wales to bring Welsh rivers back to full health,” he added, pointing to polling carried on the charity’s behalf.

It suggested 87% of the Welsh public supported action from the Welsh government to reduce and where possible eliminate river pollution caused by agriculture and sewage.

Results show 79% of those asked supported restoring carbon-rich marine habitats such as seagrass, saltmarsh and oyster reefs.

“As we have set out in our new manifesto this means sustained action to restore ecosystems and prevent all sources of river pollution, focusing on the two biggest sources; agriculture and sewage,” Mr Buckland-Jones said.

A spokesman for NFU Cymru said the farming union recognised “the role that farmers have to play in maintaining and enhancing water quality in Wales”.

“Farmers take their environmental responsibilities seriously and NFU Cymru continues to work with partners to take action to reduce incidences of agricultural point source and diffuse pollution, where the evidence shows this is needed,” they said.

Welsh Water said it planned to invest £4bn – including £2.5bn on environmental projects – between now and 2030.

“During the next two years we will be building on NRW’s work and carrying out investigations to understand the different sources of nutrients found in sea water whether from runoff water from the land or wastewater,” the company said.

“River pollution and marine water quality is not just an issue for water companies. We need to work together with other sectors who impact water quality to ensure improvement.”

Mary Lewis from NRW said a huge amount of knowledge and expertise had been drawn upon “to produce such comprehensive evidence about the condition of our protected marine areas”.

“This will now provide an important baseline for us to monitor progress,” she said.

The Welsh government said it was “committed to improving the health of our rivers and seas” and undertaking “a range of actions to improve water quality”.

This includes implementing “at pace all recommendations from the recent Control of Agricultural Pollution Regulations Review,” it added.

“We remain dedicated to developing and strengthening environmental law in Wales. The Environment (Principles, Governance and Biodiversity Targets) (Wales) Bill, recently laid in the Senedd, reflects the ongoing need to deliver transformative change, including tackling drivers of biodiversity loss such as nutrient pollution.”