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1. Prevention - The #1 Priority
The first-line of defense and, over the long term, the only cost-effective strategy against aquatic invasive species is preventing them from being introduced or from becoming established.
Ballast Water and International Shipping
Ballast water transport poses the most significant threat as a vector for the potential introduction of new species to the Great Lakes and marine coastal waters; no other vector challenges our ecosystems with as many species, numbers of organisms, and numbers of inoculations or exposures. Ballast is used to provide stability and maintain the trim of vessels when traveling on open seas. Since 1900 ocean-going and coastal commercial vessels have employed water as ballast. In doing so, via the discharge of ballast water, they became the largest "single" vector for aquatic species introductions worldwide.
The opening of the St. Lawrence Seaway in 1959 also opened the Great Lakes to discharges of foreign ballast water and the species contained therein. In response to increasing concerns about the impacts of biological invasions, ballast exchange guidelines were introduced by Canada in 1989. U. S. regulations, which require ships inbound to the Great Lakes to use at least one of a suite of suggested ballast water management practices, were promulgated in 1993 by the U. S. Coast Guard (CG). Options include exchange of ballast water beyond the Economic Exclusion Zone (EEZ), retention of ballast water on board, use of a CG-approved treatment method or reception facility, or exchange of ballast water in a CG-approved alternate exchange area. Ballast water exchange is the only approved treatment to date.
Exchanged ballast water must have a salinity of >30 parts per thousand, which should kill most - but probably not all - freshwater organisms not flushed from the tanks during exchange. However, the record of new species invasions in the Great Lakes since the implementation of mandatory ballast exchange in 1993 shows that at least four new organisms can be attributed to an apparent ballast tank vector: the amphipod, Echinogammarus ischnus, the waterflea, Cercopagis pengoi, the ciliate Acineta noticrae, and the copepod Schizopera borutzkyi.
Ballast Water Exchange
Published studies of the effectiveness of ballast water exchange indicate that actual physical exchange of greater than 85% of the water carried in a ballast tank can be achieved. However, the commensurate removal of organisms is not necessarily the same and could be much lower, depending on the taxonomic groups examined and ballast tank designs used in the study (Rigby and Hallegraeff 1994; Smith et al. 1996; Dickman and Zhang 1999; Zhang and Dickman 1999; Taylor and Bruce 2000).
An important consideration for the Great Lakes is the effectiveness of open-ocean ballast exchange when the original ballast is fresh or low salinity water, which differs in density and biota from high salinity water. As noted above, the brackish and freshwater regions of Europe and especially the coastal regions of the Baltic and Black Seas have been implicated as source regions for many of the Great Lakes invaders found since 1985. Many of the aquatic organisms found in these regions are (a) euryhaline and can survive exposure to saline water and (b) form resting stages that accumulate in sediments and are difficult to remove with exchange. Therefore, the biological effectiveness of exchanging freshwater ballast from these regions for open-ocean saltwater is an important, largely unresolved question to consider when evaluating how well ballast exchange protects the Great Lakes from new invasions.
NOBOB Vessels
 | | Residual mud in the bottom of a ballast tank on a transoceanic vessel in the Great Lakes during 2001. (Great Lakes NOBOB Project Team) |
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Vessels carrying less than full cargo must load ballast water for trim and stability and are therefore subject to the Coast Guard ballast exchange requirement. Vessels fully loaded with cargo require no ballast water other than for adjusting trim, and generally their ballast tanks have been pumped out. Such vessels are called "NOBOBs" (NO-Ballast-On-Board). This is the most economical and therefore the most desirable operating condition for commercial vessels. NOBOB vessels are not subject to ballast exchange since, theoretically, they have no ballast water on board.
However, vessels cannot completely discharge all their ballast water because the pump-out port cannot be closer than a few inches off the bottom plating of the tank, and therefore a few inches (or less) of residual water and sediment will remain in the bottom after pump-out. In addition, ballast tanks accumulate fine sediment that settles to the bottom of the ballast tank and is deposited on ledges and in dead zones along structural supports. These residues can contain a wide assortment of live larval and mature plants, animals, and viable microorganisms, as well as so-called "resting stages" (Hallegraeff and Bolch 1992; Locke et al. 1993; Galil and Hülsmann 1997; Dickman and Zhang 1999; Hamer et al. 2000).
 | | The number of vessels entering the Great Lakes with ballast on board (BOB) vs. those entering with no ballast on board (NOBOB) from 1978 through 2000 (Coulatti et al. 2002) |
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Resting stages (variously called cysts, ephippia, resting eggs, or spores according to taxon) are dormant reproductive cells produced by many species. Resting stages are extremely resistant to harsh conditions such as lack of oxygen, exposure to toxic chemicals, low and high temperatures, and even survive passage through the digestive systems of fish and waterfowl. They may remain in sediment in a state of virtual suspended animation for decades or even centuries. Once exposed to the right combination of favorable environmental conditions, they can hatch or germinate to produce live organisms capable of reproducing.
Vessels entering the Great Lakes in ballast have decreased in both absolute and relative importance over the past 25 years, and constituted only ~10% of inbound traffic to the lakes during the 1990s. NOBOB vessels constituted the majority (~90%) of inbound vessel traffic, largely because they carry cargo in each direction to maximize economic efficiency.
Great Lakes water taken on as ballast by a NOBOB vessel to maintain trim and stability during operations can mix with residual ballast water, sediment, and any associated nonindigenous organisms, and later be discharged into the Great Lakes as the vessel moves between a succession of ports. Thus, ballast-water operations of NOBOB vessels present an invasion risk, but the magnitude of such risk remains unresolved.
Research is presently underway to evaluate the risk posed by NOBOB vessels. A mathematical model of risk posed by waterborne organisms suggests that ballast water exchange effectively reduces (but does not eliminate) risk as compared to unexchanged vessels. NOBOB vessels appear to constitute a significant threat today, largely because they are a major, if not dominant, component of in-bound vessel traffic to the Great Lakes, and because live freshwater organisms are contained in residual water and sediments when they arrive in the Great Lakes system.
Treatment Technology Development
Treatment of ballast water to reduce or eliminate the biota it contains has been the primary focus of efforts to address the ballast water issue. Many different technologies are being tested for their applicability, either alone or in combination, including filtration, ultracentrifugation, exposure to intense ultraviolet light (UV), ozone, heat, oxygen removal, ultrasound, and use of biocides such as hypochlorite, gluteraldehyde, and commercial products such as SeaKleenTM.
Ultra-centrifugation uses centripetal forces to separate solid particles from a spinning flow stream. It is most effective on particles whose density is significantly greater than water, such as the minerals that are components of mud. Biota, on the other hand, tend to have densities much closer to that of water, and thus are not easily or efficiently separated from the water stream by centrifugation. Therefore ultracentrifugation in combination with downstream exposure to intense UV radiation is being studied.
Ultraviolet radiation (UV) will kill even microbes if the dose is high enough. However, achieving a lethal dose of UV radiation in a ballast flow faces several difficult problems related to exposure time, power, and effectiveness across the broad range of organisms found in ballast water.
Filtration can remove particles down to micron sizes, but filters easily clog and require regular back-flushing. Larger filter openings require less back-flushing, but let more of the small biota through. At present the achievable flowrate, the minimum particle size that is removed, and the number and size of the filters/filtration units required to meet the ballast treatment needs of a full-size ship are serious issues that must be overcome before practical application is possible.
At least two physical methods to strip dissolved oxygen from ballast water are being considered, one based on vacuum removal and the other based on inert gas (nitrogen) stripping. There is some concern that the resulting anaerobic (oxygen-free) environment may promote the production of hydrogen sulfide, a toxic gas. The biological effectiveness, practicality, and economics of oxygen removal need to be determined and demonstrated at bench and then shipboard scale.
The use of ozone as a biocide (ozonation) has been tested in a full scale application on board a U.S. oil tanker with promising results. Other biocides such as hypochlorite and gluteraldehyde are known to be effective against many biota including many microbes. Gluteraldehyde and hypochlorite are likely impractical for treatment of the large volumes of water in full ballast tanks on board typical transoceanic ships. Biocides have more likely utility in treating NOBOB tanks where the residual water volume is small. However, laboratory-based studies have shown that these biocides may be less effective on biota buried in sediments, unless the sediment can be stirred-up and resuspended. The presence of organic matter also reduces the effective concentration of hypochlorite, thus requiring a higher initial concentration. In addition, the shipping industry has concerns about the effects of hypochlorite on the coatings and steel of ballast tanks. Chloride ion is very reactive with exposed iron and the concern is that regular use of hypochlorite would increase the rate of deterioration of the ballast tank structure. In addition, the environmental community, and some regulatory agencies are concerned, if not resistant, to the idea of discharging biocide-laden ballast water to the environment. In some cases, such as gluteraldehyde and ozone, the biocide will break down into non-toxic chemicals, mainly water and carbon dioxide, while still in the ballast tank or shortly after exposure to sunlight.
Although several of these treatment approaches show promise, no one technology appears on the horizon as a "silver bullet." All have limitations that make them less than 100% effective. Some presently have serious practical limitations that must be overcome or which may ultimately make them unusable, such as large size, high power requirements, costs, and maximum achievable treatment rate. With time and money, engineering solutions are likely for many of these problems, but perhaps one of the largest hurdles in the future may be finding adequate test platforms for technologies that are ready for on-board testing.
Ballast Water Treatment Standards
Progress on the development of ballast water treatment systems is impeded by the lack of a treatment standard or standards to serve as the basis for a program of enforcement and other initiatives. Yet development of a suitable treatment standard is one of the most difficult hurdles presently facing the scientific and regulatory community. The difficulties related to this issue are concisely discussed in the recent "Advanced Notice of Proposed Rulemaking" (U.S. Coast Guard 2002) and will not be detailed here. There is disagreement on what measures or treatment outcomes represent an acceptable standard, and whether a single standard is suitable nationwide, or if different standards and indicators should be developed for different ecosystems.
Part of this problem stems from the fact the aquatic invasion science is in its infancy and thus we have little confidence in our understanding of aquatic species invasions or our ability to predict them. We do not fully understand what makes some species successful invaders and others not, or what conditions are needed for successful invasion and establishment by an organism.
Related Research Issues
Effective treatments for ballast water have yet to be developed and applied at full scale on operating vessels. Research to determine their efficacy is in its infancy and there are many research needs and issues that must be resolved in order to eliminate the risks posed by ballast water, including:
- Compilation of patterns of shipping and ballasting in the Great Lakes;
- Ballast discharge standards that will prevent introductions of nonindigenous species to the Great Lakes;
- Development and implementation of technologies and ballast management practices that prevent or reduce the risk of ballast-mediated introductions of nonindigenous species;
- Introduction of new-ship designs that will prevent or reduce introduction of new species via ballast tanks;
- Design standards for new ship ballast systems that will permit mass production and installation of technologies that may be developed in the lifetime of ships now being built;
- Determination of relative risk posed by ballast water from ports in different areas of the world;
- Research on the vulnerability of different life stages (adults, larvae, resting eggs) to ballast water exchange and alternative treatments;
- The relative effectiveness of purging (dilution) and physiological stress in reducing the number of live organisms in exchanged ballast water;
- Data on species-specific survival rates in ballast water;
- Determination of differential vulnerability to ballast water exchange between benthic (i.e., bottom-dwelling) versus planktonic (i.e., waterborne) species.
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