You are here:
Density matters: Approaches to settling ballast water discharge concentrations
Citation:
LEE, II, H., D. REUSSER, AND M. R. FRAZIER. Density matters: Approaches to settling ballast water discharge concentrations. Presented at Sixth International Conference on Marine Bioinvasions, Portland, OR, August 24 - 27, 2009.
Impact/Purpose:
A consensus has evolved that invasion risk increases with propagule pressure.
Description:
A consensus has evolved that invasion risk increases with propagule pressure. However, translating this general principal into ecologically “acceptable” concentrations of organisms in ballast water has proven challenging. The treaty being promulgated by the International Maritime Organization (IMO) contains performance standards for different size guilds, with a standard of <10 organisms per m3 for organisms >50 microns. To put the IMO standards into context, we are evaluating other approaches to generating ballast water targets. The most protective is the zero discharge standard of organisms >50 microns being considered by California, though there is a question regarding its technological feasibility in the near term. An alternative California proposal is to set the invasion rate via ballast water discharges equivalent to the “natural invasion rate”. This approach, developed by Dr. Andrew Cohen, is purportedly ecologically protective but an initial analysis by experts generated a one-hundred fold range in natural invasion rates. Another low target is the 0.1 organisms per m3 that had been proposed in U.S. Senate bills, and which appears to be based on expert opinion. Modeling approaches include reaction-diffusion models that predict establishment of an invader as a “race” between the dilution of the population and population growth. Such models are most appropriate to passively distributed organisms that spend their entire life cycle within the water column. Application of this approach by Drake et al. generated acceptable ballast discharge volumes but these were not readily converted to organism concentrations. A more general approach uses population viability analysis (PVA) models that predict the likelihood of extinction (= non-establishment) as a function of the population’s growth rate and population variation. A practical limitation of the PVA models is the paucity of population data, in particular long-term population data to estimate population variability. A final approach is the per capita invasion probability that we are developing based on historic invasion rates. Assuming a linear propagule dose-response, the per capita invasion probability represents the likelihood that an individual will become established. These various approaches generate “acceptable” ballast water concentrations that vary by orders of magnitude, a consequence of differences in both assumptions and levels of acceptable risk.