Assessing ballast water treatments: Evaluation of viability methods for ambient freshwater microplankton assemblages

In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: a b s t r a c t a r t i c l e i n f o Index words: Ballast water Invasive species Phytoplankton Fluorescein diacetate Algae Viability For decades the Great Lakes have been subject to invasive species introductions through the discharge of ships' ballast water. Several treatment technologies involving physical, chemical, and biological processes have been developed to remove or inactivate organisms in this discharge. Assessing the efficacy of these technologies involves estimating the number of viable propagules in treated discharge relative to untreated controls. For organisms in the 10–50 µm size range, for example, the International Maritime Organization (IMO) mandates that fewer than 10 viable organisms per milliliter may be discharged. To date, however, there is no standard method to assess viability of natural assemblages of organisms in this size group (largely phytoplankton and protozoans) in freshwater environments. We report here on a process of assemblage concentration, staining with fluorescein diacetate (FDA), and microscopic observation as a reliable and efficient method to assess densities of viable freshwater organisms in this size category in ballast discharge. A number of other methods, including digestion with enzymes, flow cytometry, and a variety of vital and mortal stains, were tested and discarded during this vetting process due to inconsistent or ambiguous results. Introduction Commercial vessels are the primary vector for transport of non-native aquatic species to the Great Lakes–St. Lawrence River system (Mills et al., 1993; Ricciardi and MacIsaac, 2000). More than 180 aquatic invasive species have been confirmed as introduced and established in the Great Lakes, and ballast water is likely responsible for approximately 70% of these species (Grigorovich et al., 2003). These organisms threaten not only ecological stability and diversity and abundance of native taxa but commercial activities as well. Over the last two decades, researchers have described the vectors, ecology, and impacts of invasive species such as zooplankters (including the spiny water flea [Bythotrephes] and the zebra mussel [Dreissenia While vertebrate and large invertebrate invaders garner the most public attention, several other worrisome microscopic taxa have been observed in ballast tanks of vessels entering the Great Lakes. Johengen et al. (2005) recorded phytoplankton that are known …