Invasion Pressure to a Ballast-flooded Estuary and an Assessment of Inoculant Survival

The relationships between invasion pressure, post-transport inoculant survival, and regional susceptibility to invasion are poorly understood. In marine ecosystems, the movement and release of ballast water from ocean-going ships provides a model system by which to examine the interplay among these factors. One of the largest estuaries in North America, the Chesapeake Bay, receives tremendous amounts of foreign ballast water annually and thus should be at high invasion risk. To date, however, few introductions in Chesapeake Bay have been attributed to ballast release. To understand better the dynamics of this invasion process, we (1) characterized and quantified the biota arriving to Chesapeake Bay in foreign ballast water, (2) compared temperatures and salinities of ballast water and harbor water in upper Chesapeake Bay, and (3) tested experimentally survival of organisms collected from ballast water in temperatures and salinities characteristic of the region. From 1993 to 1994, we sampled planktonic and benthic organisms from 60 foreign vessels arriving to Chesapeake Bay. Our data show that the estuary is being inoculated by a diverse assemblage of aquatic organisms from around the world. Furthermore, the short transit time (15d) for most vessels ensured that substantial numbers of larval and post-larval organisms were being deballasted alive. Most of the ballast water discharged into the upper Chesapeake Bay, however, was significantly higher in salinity (>20) than that of the receiving harbor. In laboratory tolerance experiments, ballast water organisms perished under such conditions. Thus, a mismatch in physical conditions between donor and receiver regions may explain the dearth of invasions in the upper Bay. It is likely that the lower Chesapeake Bay, which is more saline, remains at higher risk to ballast water invasion. Recognition of such intraregional differences should allow more focused predictions for monitoring and management.