Ballast water harbours billions of life forms

November 22, 2000 - Global trade has been responsible for much of the spread of invasive species around the world. But today one of the greatest threats to humans posed by such species may come from hitchhiking organisms we can't even see.

Species that are introduced to an ecosystem can become a problem if their new environment lacks a suitable predator for them or otherwise creates favourable conditions for their survival. These "invasive" species threaten biological diversity because, lacking normal controls, they can out-compete native species for food or other resources. In fact, they are considered the second greatest threat to biological diversity world-wide, after habitat loss.

One of the most common methods of transportation for foreign species today is in cargo ships, particularly in ballast water. Ships pick up ballast water at their points of origin to help stabilize the huge vessels on their long journeys overseas. But once the crew reaches their destination, they often dump this water into the local harbour. The United States alone receives 79 million tonnes of ballast water from overseas locations every year. Chesapeake Bay on the US east coast receives 10 billion litres of it annually.

The problem is that ballast water is a literal aquarium of sea life from the ship's starting point, harbouring everything from fish to viruses. According to Canadian fisheries scientist Colin Levings, an analysis of 67 samples of ballast water from foreign ocean-going ships entering Canada found as many as 12,392 marine creatures per cubic metre of water.

Some marine creatures do not survive their entire journeys, but many do. A study in the Journal of Plankton Research found that some organisms actually thrive in ballast tanks, which may function as an incubator during the cruise for some species.

And many of these species have made themselves at home in their new surroundings. The European green crab, for example, is thought to have been introduced to the west coast of North America from ballast water and has now established itself in California and is making its way up the coast. According to a paper published in the journal Ecology, following the invasion some local clam species declined in abundance five times faster than in previous years, likely due to predation by the green crab. Many other introduced aquatic species are causing problems in North America as well, including the European zebra mussel and most recently the invasive European algae,

Perhaps most disturbing is the incredible number of microorganisms found in ballast water. A recent study published in the journal Nature reported that each litre of ballast water tested from ships in Chesapeake Bay contained about a billion bacteria and up to seven billion virus-like particles. The bacteria Vibrio cholerae 01 and 0139, which cause the deadly human disease cholera, were found in 93 per cent of the samples tested.

The authors conclude that, "the transfer and release of V. cholerae by ships creates an opportunity for the colonization of coastal ecosystems." V. cholerae has been known to survive in seawater for more than 50 days and ballast water containing V. cholerae emptied into Peruvian ports is the probable cause of the outbreak of cholera in Peru in 1991 that killed 3,000 people.

Some ports, like Vancouver, BC, have regulations requiring foreign ships to discharge ballast water at sea before entering the harbour, but many others do not. Furthermore, discharge alone does not remove all the organisms from the water and if conditions are rough, ships don't have to obey the regulation for safety reasons.

Currently, billions of litres of foreign ballast water are still finding their way into ports across North America, but there are few studies on the fate of the vast number of foreign microorganisms hitchhiking with it. Faced with ever-increasing world trade and changing climactic conditions that could prove favourable to some pathogens, we need to know what we are up against and how best to deal with the problem.

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