Tonnes of throw-away plastic and massive runoff from chemical fertilizer are choking the world's oceans, the UN's environmental watchdog warned Thursday.

Taken together, the two sources of pollution threaten biodiversity, harm water quality, poison fish stocks and undermine coastal tourism, the United Nations Environment Programme (UNEP) said in its annual Year Book report.

Released ahead of a key meeting next week of environment ministers in Nairobi, the report highlights the need to protect marine environments already rendered fragile by over-exploitation and acidification caused by climate change.

Only better waste management and a coordinated shift towards cleaner engines of economic growth can insure the future health of the planet's aquatic commons, it said.

"The phosphorus fertilizer and marine plastic stories bring into sharp focus the urgent need ... to catalyze a global transition to a resource-efficient Green Economy," UNEP executive director Achim Steiner said.

Recent research suggests that both problems are more widespread -- and deleterious -- than once thought.

In the United States alone, for example, the costs associated with phosphorus pollution are estimated at more than two billion dollars a year, with the global tally in the tens of billions.

Some three dozen countries mine the phosphate rock found in growth-enhancing fertilizers. While a finite resource, supplies are not about to run out -- at current production rates, supplies will last an estimated 300 to 400 years.

Use of chemical-based fertilizers increased worldwide by 600 percent during the second half of the 20th century, but precisely how much flows into the environment is not known.

One study estimates that 22 tonnes of phosphorus wind up in marine environments each year, while concentrations in freshwater and land have grown by at least 75 percent since 1960.

Recycling waste water in the developing world's mega-cities could help stem that flow, the report said.

Marine plastics have also emerged as a growing threat, the Year Book warns.

Scientists have long observed that birds and aquatic animals can become entagled in plastic filaments, causing them to drown, or mistake them for food such as squid or jellyfish.

But a new concern is microplastics, tiny particles smaller that five millimetres in length discharged as pellets by industry or broken down by waves and sunlight.

Recent research suggests that microplastics could be moving through the food chain, becoming more toxic along the way.

Just how much plastic has been discarded into the sea in unknown, but consumption of plastic products continues to rise worldwide.

In North America and western Europe, per capita annual use stands at about 100 kilos (220 pounds), an amount like to increase by 40 percent within five years. The developing world only consumes at a fifth that level, but is catching up.

The report calls for stepped up recycling efforts.

"If plastic is treated as a valuable resource rather than just a waste product," it would create stronger incentives for collection and reprocessing, it argues.