Sediments as a dispersal vector of aquatic invertebrates: An estimation of propagule pressure associated with 'no ballast on board' vessels.

摘要

Ballast water has been the primary vector of nonindigenous species (NIS) to the Laurentian Great Lakes over the past 45 years. Although ballast water exchange regulations were implemented in 1993 to reduce propagule loads, new NIS continue to be discovered. A possible explanation for this trend is the importance of alternative vectors, such as resting stages of invertebrates in residual ballast sediments of transoceanic ships claiming 'no ballast on board'. To determine the risk of invasion potentially associated with this vector, I collected sediment samples from 39 ships entering the Great Lakes and measured the density, diversity and viability of resting stages contained therein. Viable resting stages of NIS were found in 32% of ships, at a median density of 3.0 x 105 ship-1 . Twenty-one NIS were identified, consisting exclusively of rotifers and cladocerans. I subsequently conducted in situ experiments using emergence traps to assess the introduction potential of invertebrate diapausing stages present in ships' ballast sediment. Hatching was observed on all four ships, although not from all sediments. Overall hatch rates were very low (0.5 individuals per 500 g sediment), typically involving activation of 0.05% of total eggs present. While dormancy is a characteristic enabling enhanced survival during transportation, it becomes an impediment for introduction as resting stages that are buried in sediments appear to have little chance for expulsion from ballast tanks. Results from this study indicate, however, that diapausing eggs contained in ballast sediment of NOBOB ships are a potential mechanism for introduction of new NIS to the Great Lakes.