Viability of invertebrate diapausing eggs collected from residual ballast sediment

Natural or anthropogenic movement of sediments may be an important vector for the dispersal of invertebrate resting stages between water bodies. Here we record the presence of invertebrate diapausing eggs in residual sediments from transoceanic vessels and explore whether these may pose an invasion risk. Viability of diapausing eggs was explored under light and dark conditions using sediment collected from eleven tanks on nine vessels operating on the Great Lakes. Seventeen cladoceran, copepod, and rotifer taxa were identified. Four of the species hatched have not yet been reported as established in the Great Lakes. Egg viability for individual species varied from 0% to 92%. Exposure to saline water may impact egg viability of some freshwater species. Generally, the proportion of eggs hatched in light and dark treatments did not differ significantly, indicating that light was not required to terminate diapause. As a result, eggs could potentially hatch in dark ballast tanks when immersed in freshwater loaded as ballast during operation on the Great Lakes. Viability of diapausing eggs differed among ballast tanks on a single vessel, indicating that tanks with independent ballast histories have different invasion risks. While additional work is needed to quantify risk, results from this study indicate that vessels entering the Great Lakes with only residual ballast are a potential vector for the introduction of new nonindigenous species during multiport operations. Freshwater invertebrates achieve dispersal via transport of their desiccation-resistant dormant stages in flowing waters, wind, or by ectozoochorous or endozoochorous animal vectors (see Bilton et al. 2001; Cáceres and Soluk 2002; Figuerola and Green 2002). Water currents are likely responsible for most short-distance dispersal events, particularly of stream-dwelling taxa (Bilton et al. 2001). Wind and animal vectors also disperse invertebrates (Figuerola and Green 1 Corresponding author ([email protected]). Acknowledgments We are grateful to F. Dobbs for providing pore-water salinity data, H. Limén for diapausing egg extraction protocols, and D. Gray for laboratory assistance. The Shipping Federation of Canada, ship agents, masters, officers, and crew were all very accommodating with respect to boarding and sampling ships. T. Zemlak, P. Hebert, I. Grigorovich, and R. Shiel confirmed identity of some zooplankton species. This project was supported by an NSERC Industrial Postgraduate Scholarship, in partnership with the Shipping Federation of Canada, an Ontario Graduate Scholarship, and an Ontario Federation of Anglers and Hunters Conservation Grant to S.A.B., and by a Premier’s Research Excellence Award to H.J.M. This work was conducted under the multi-institutional Great Lakes NOBOB Project funded by the Great Lakes Protection Fund, the National Oceanic and Atmospheric Administration (NOAA), the U.S. Environmental Protection Agency, and the U.S. Coast Guard. The project was comanaged by the Cooperative Institute of Limnology and Ecosystems Research and the NOAA Great Lakes Environmental Research Laboratory and sponsored under cooperative agreement NA17RJ1225 from the Office of Oceanic and Atmospheric Research, NOAA. 2002), although recent work indicates that these vectors may be species-specific or relatively unimportant (Jenkins and Underwood 1998; Cáceres and Soluk 2002). Movement of sediment represents an alternative dispersal medium that could transport resting stages of many taxa. As an example, sediment associated with heavy machinery has been linked to dispersal of both rotifer and copepod species (Koste and Shiel 1989; Hairston et al. 1999). Hebert and Cristescu (2002) estimated that the rate of human-mediated dispersal of crustacean zooplankton to the Laurentian Great Lakes exceeds the natural rate by up to 50,000 fold. Transoceanic ships have been the dominant transport vector of nonindigenous species (NIS) to the Great Lakes for most of the 20th century (Mills et al. 1993; Ricciardi and MacIsaac 2000; Ricciardi 2001). Regulations enacted in 1993 effectively require open-ocean ballast exchange for vessels inbound to the Great Lakes with freshwater or brackish water if the water is to be discharged into the lakes (United States Coast Guard 1993). Open-ocean exchange purges most freshwater organisms, while remaining individuals should be killed when tanks are refilled with saline water. The invasion risk posed by these vessels appears to be much lower than that posed by ships entering without ballast water (MacIsaac et al. 2002). The latter vessels, officially classified as no ballast on board (NOBOB), are exempt from current ballast water regulations (United States Coast Guard 1993). NOBOB vessels, which dominate trade into the Great Lakes, cannot completely empty their ballast tanks due to structural and operational limitations and carry an average