a living benthic community on the tank bottom. A similar situation may exist on vessels that nominally are not carrying ballast. Inbound vessels that have released their ballast water prior to or during cargo loading, and outbound vessels with full cargo loads, may have relatively little ballast water remaining such that the mariner would report a ballasting condition of ''no ballast on board."

Sediment frequently accumulates on the bottom and on many horizontal surfaces in ballast tanks. Sediment may include the settled mud (silt and clay) of harbor, port, and estuarine waters, detrital and other flocculent material ubiquitous in shelf waters (but present to some extent in almost all waters), scale (rusted metal shedding off tank walls), and cargo residue. Of 343 cargo vessels sampled from 18 Australian ports, at least 65 percent "were carrying significant amounts of sediment on the bottom of their ballast tanks" (Hallegraeff and Bolch, 1992). Sediment is typically removed every three to five years when the vessel is undergoing special survey or refit work in a dry dock. Sediment is removed more frequently if the buildup warrants the expense.

In ballasted cargo holds, sediment typically is removed by hosing down at the end of each ballast leg before the next cargo is loaded; thus, a portion of the sediment is almost always directly released into the arrival port. The amount of sediment buildup is a function of ship design and operating practice.

REFERENCES

Hallegraeff, G.M., and C.J. Bolch. 1992. Transport of diatom and dinoflagellate resting spores in ships' ballast water: Implications for plankton biogeography and aquaculture. Journal of Plankton Research 14:1067–1084.


Peters, H. 1993. The Maritime Transport Crisis. Washington, D.C.: The World Bank.


Weathers, K., and E. Reeves. 1996. The defense of the Great Lakes against the invasion of nonindigenous species in ballast water. Marine Technology 33(2):92–100.



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