Zooplankton community dynamics
Ballast water transport as a taxonomic and numeric 'filter'
Zooplankton data collected from ballast tanks at the beginning and end of 25 voyages showed an overall decline in total zooplankton abundance during a voyage. Mortality within tanks could be caused by a number of factors, including:
- Lack of settlement substrate
- Exposure to toxic substances
- Physiological stress caused by changes in physical conditions
In addition, the duration of a voyage or the age of the ballast water is also an important factor, as mortality within tanks appears to increase with time.
For example, our data indicated that:
• On short voyages (<10 days) survival of zooplankton is unpredictable, but typically high (both increases and decreases in abundance were recorded) .
• On long voyages (16-22 days) there were large declines in survivorship (>95% decrease in abundance in all cases.
This an important finding because in order to survive, establish, and thus achieve a successful ballast-mediated invasion, an organism must be delivered in adequate densities to increase the chances of encountering a mate and reproducing. This means that shorter coastwise voyages, where survivorship is more variable and the final densities are relatively higher pose a greater 'invasion threat' than do longer voyages.
Though these data bring us closer to identifying predictors of overall zooplankton survivorship, identifying individual taxonomic groups that are more likely to become successful invaders remains elusive. Survivorship varied both between taxonomic groups and within taxonomic groups for different voyages. Thus, we still do not fully understand the extent to which particular taxonomic groups are better able to survive transport in ballast tanks than others.
These data strongly point to the need for rigorous ballast water management policies across the board to effectively handle the release of domestic ballast water, particularly that portion of it which has only been subject to short-term transport.