Post-exchange zooplankton in ballast water of ships entering the San Francisco Estuary

The San Francisco Estuary in California (CA), USA, has been heavily altered by invasions of nonnative zooplankton and benthic organisms, presumably by the discharge of ships' ballast water. Since 2000, ships entering CA have been required to exchange ballast water with oceanic water during the voyage to decrease the number of organisms discharged into the Estuary that had previously been taken aboard at foreign ports. We examined abundance of zooplankton in ballast water of 18 container ships and 48 bulk carriers. Asia dominated the sources of ballast water, which contained multiple nonnative zooplankton including species that have invaded and since become common residents in the Estuary. The abundance of zooplankton was significantly lower in ballast water that had been emptied and refilled with oceanic water than those that had continuously been flushed with oceanic water (about three times the volume of ballast water), suggesting that empty-refill is more effective in removing exotic zooplankton.