US researchers say they have developed an effective way to kill unwanted plants and animals that hitch a ride in the ballast waters of cargo vessels.

Tests showed that a continuous microwave system was able to remove all marine life within the water tanks.

The UN lists "invasive species" dispersed by ballast water discharges as one of the four main threats to the world's marine ecosystems.

The findings will appear in the journal Environmental Science and Technology.

Shipping moves more than 80% of the world's commodities and transfers up to five billion tonnes of ballast water internationally each year, data from the UN shows.

Vessels, especially large container ships, need ballast tanks to provide stability in the water and correct any shift in the ships’ mass.

When a ship's cargo is unloaded, it fills with ballast water; when it is later reloaded, often on the other side of the world, the water is discharged.

Co-author Dorin Boldor, from Louisiana State University’s Agricultural Center, said the team envisaged the microwave device being fitted to the exit valve of a ballast tank.

"The basic idea is that you take the ballast water and pump it through a microwave cavity."

He added that the system would follow the same principle as a household microwave oven.

"The power level is much higher and a different frequency, but it creates a very high intensity electric field in the centre of the cavity that oscillates rapidly.

"The water molecules are going to start spinning around very fast and they are going to create a lot of friction that generates heat," Dr Boldor explained.

"But it generates heat in the whole volume at the same time, unlike if you try to use another heating mechanism where you have to take the heat from somewhere else and conduct it through the liquid."

This means that the researchers have a high degree of confidence that the system is treating all of the water to remove the unwanted organisms.

"It is extremely fast and very efficient at transferring the energy from the microwaves into heat," he told BBC News.

Biological stowaways

For thousands of years, marine species have been dispersed throughout the oceans by natural means, such as currents and drifting on debris.

But natural barriers, such as temperature differences and land masses, have limited the range of some species' dispersal and allowed different marine ecosystems to evolve.

Since the emergence of the modern shipping fleet and growing trade between nations, these natural barriers have been broken down, allowing the introduction of alien species that upset the equilibrium of ecosystems.

The UN-led Global Ballast Water Management Programme (GloBallast) estimates that at least 7,000 species are able to be carried across the globe in ships' ballast tanks.

While many of these plants and creatures do not survive the journey, some find the new environment favourable enough to establish a reproductive population and go on to undermine native species.

For example, GloBallast says, European zebra mussels (Dreissena polymorpha) have infested more than 40% of the US's inland waterways.

Between 1989 and 2000, up to $1bn (£500m) is estimated to have been spent on controlling the spread of the alien invader.

The arrival of an invasive jellyfish-like organism, Mnemiopsis leidyi, led to a major ecological "regime change" in the Black Sea, which contributed to the collapse of commercial fisheries in the region.

At one stage, the species accounted for about 90% of the sea's entire biomass. Its appetite for native plankton stocks meant that other fish species were unable to compete and re-establish viable populations.

In February 2004, the international shipping community agreed to establish tougher measures to prevent discharges of ballast water releasing potentially invasive species.

The International Convention for the Control and Management of Ships' Ballast Water and Sediment requires all vessels over 400 tonnes to eventually fit systems to treat ballast water.

The team’s development is ideally suited to help commercial operators meet their obligation under this legislation, Dr Boldor explained.

"It will probably work very well for it to be installed on very large ships themselves, but when you are talking about smaller vessels it may be more cost effective to have some sort of barge system based in the ports.

"It can just pull up to the ship, take and treat the ballast water while the ships are waiting to berth at the dock."

• BBC article