Questions Abound About Tiny, Tiny Stowaways in Ballast Water

By Fred Dobbs

Every commercial ship chugging into every port in the world inadvertently carries with it stowaways. No romantic adventure or piratical skullduggery at work here—these stowaways are microorganisms entrained with water deliberately brought within the ship’s hull and held in its ballast tanks. Such tanks are designed to hold enormous quantities of water—each has a capacity of hundreds, even thousands of metric tons—and are crucial to ships’ cargo operations and safety at sea. By pumping water in or out of these tanks and adjusting the ship’s waterline, the crew can compensate for cargo loading, increase propulsion efficiency, and maximize the ship’s stability in rough waves.

Huge amounts of ballast water are transported every day as the global fleet plies its trade. Any organisms that have survived since “coming on board” are liable to be released when ballast water is next discharged. Unfortunately, we know all too well that many non-native invertebrates and plants have been introduced to new environments in this manner. Some of these introduced species proliferate, their populations released from control by predators, parasites, or other factors. When they outcompete and deleteriously affect native organisms, we further classify them as “invasive species.” In some spectacular cases, such as the zebra mussel in North America, invertebrates introduced by ballast water have fundamentally altered the ecosystem to which they were delivered. Such drastic ecological changes have also precipitated significant economic expenditures, as is the case with the zebra mussels that clog industrial water-cooling systems and must be removed.

In a parallel sense, are there inimical consequences to transporting and disseminating aquatic microorganisms in ships’ ballast tanks? We are certain only that scenarios envisioned by some have been of sufficient concern to promulgate regulations concerning ships’ handling of ballast water. What are those scenarios and how have they contributed to regulations facing global shipping?

Microorganisms in ballast water—can they be “non-indigenous”?

For more than 20 years, scientists have reported a variety of large, easily recognized phytoplankton (e.g., dinoflagellates and diatoms) and protozoa (e.g., ciliates and foraminifera) in ballast water. On the other hand, the diversity of smaller, less readily discerned forms, especially bacteria and viruses, is nearly unknown. We do know something, however, about their abundance. In lakes and oceans, every milliliter of surface water (that’s a small fraction of an ounce) contains about 100 protists, a million bacteria and as many as a billion viruses. It is inevitable, therefore, that hundreds of trillions of microorganisms enter a single ship’s ballast tanks during normal operations.

Although the overwhelming majority of these aquatic microorganisms are not harmful to humans, some species in ballast water do indeed represent potential risks to public health, notably pathogenic bacteria such as Vibrio cholerae, the etiologic agent of human cholera, and dinoflagellates, some species of which are responsible for harmful “red tides.” While to the best of our knowledge, no outbreaks of disease have been associated with ships’ ballasting activities, it is only on very, very rare occasions that ballast tanks are sampled for their microbiological inventories.

Are there microbes discharged with ballast water that could be classified as non-indigenous species? The answer is not as clear as it is for invertebrates, and microbial ecologists debate the biogeographic distribution of microbes. Some argue bacteria and protists must be distributed world wide, simply because their small size facilitates their dispersal. If microorganisms indeed are ubiquitous in their distribution, then they cannot be considered to be non-indigenous. Another school of thought disagrees and contends some microbes clearly do have a biogeography. If the second group of scientists is correct, then aquatic microorganisms can be non-indigenous, are therefore potentially invasive, and their presence in ballast water is indeed of concern.

Regulations and technological solutions

While microbial ecologists have not resolved the biogeography argument, many governmental and international agencies have, in a sense, already decided for themselves. Consider microbiological mandates proposed by the International Maritime Organization (IMO), which sets rules and standards for the global shipping industry. The IMO has issued an “International Convention for the Control and Management of Ships’ Ballast Water and Sediments,” now being considered for ratification by its member states. Numerous species of aquatic microorganisms, especially fresh- and saltwater phytoplankton and protozoa, fall within the convention’s purview. In the United States, there are no similar federal regulations and in their absence, several states are enacting their own. California, for example, has issued standards for discharged ballast water that call for orders-of-magnitude reduction in total bacteria and total virus numbers relative to their natural concentrations.

How will ships reduce the number of microorganisms discharged with their ballast water? The past decade has seen a flurry of technology development in this regard, and a cornucopia of techniques has been tested in the laboratory and in full-scale, land-based installations. In a few cases, treatment units already have been installed on ships. Proposed technologies range widely in their mechanisms, from filtration to biocides, ultrasound to ultraviolet irradiation, and more. It is imperative, of course, that the treatment not corrode the steel ballast tanks, and furthermore, it must yield water that meets any chemical-discharge standards for harbors. What a challenge it will be for technologies to effect such low numbers of microorganisms in enormous volumes of discharged ballast water! And what an equally daunting challenge for scientists to measure those numbers accurately!

As he roamed the mid-western portions of our country in the first half of the 19th century, Johnny Appleseed reportedly sprinkled apple seeds wherever he went. The result—apple trees proliferated and the region was characterized by an abundance of orchards. Are we playing Johnny Appleseed with aquatic microorganisms as global shipping inadvertently spreads them around the world in discharged ballast water? And if so, need we be concerned that some of those microbes are harmful? Certainly these questions are ones of considerable interest to microbial ecologists, but given the context, are pertinent to water-quality managers and regulatory agencies as well. Our understanding of the issues involved will increase as additional and more sophisticated studies are performed, especially those employing the tools of modern molecular biology.

Fred Dobbs does research worldwide as a marine microbial ecologist. He is a professor in the Department of Ocean, Earth and Atmospheric Sciences at Old Dominion University.