What is mid-ocean ballast water exchange?

Container ships at Port of Oakland, San Francisco Bay

Ballast water is forced  out an opening at the  top of the ship in  flow-through exchange.

In general, ships can replace their coastal ballast in two ways:

• In empty/refill exchange, the ballasted tank is emptied by pumps until the pumps lose suction, and then refilled via gravitation and pumping of ocean water.  100% of the ballast water must be emptied from the tank before refilling to complete an empty/refill exchange.
  
  
• In the flow-through method of exchange, mid-ocean water is pumped into a full tank or hold from below while the existing coastal water is forced out an opening at the top. A volume of water three times the ballast tank capacity (300%) must be pumped out to complete a flow-through exchange as mandated by the US Coast Guard. 

Correctly completed mid ocean ballast water exchange can replace up to 99% of the volume of initial coastal waters with ocean waters and can remove over 90% of the coastal zooplankton trapped within the ballast tank, depending on ship type and ballast tank design (Minton et al, 2005; Ruiz et al, 2005). 

How can mid-ocean ballast water exchange prevent or mitigate the spread of non-indigenous species (NIS)?

Ship undergoing mid- ocean ballast water exchange

Since mid-ocean ballast exchange replaces most of the coastal water with open ocean water,  it also removes most of the coastal organisms that were originally entrained in the  ballast tank.  Although oceanic organisms can be captured in ballast water tanks during this exchange, these organisms are considered less likely to become established than biota of coastal origin when discharged into a coastal ecosystems, due to a mismatch among open ocean and coastal habitats (Murphy et al, 2001; Minton et al 2005; Wonham et al, 2001).  Conversely, coastal organisms discharged into the ocean are considered less likely to survive in open ocean habitat. 

In some cases, replacing low salinity or fresh water with open ocean, high salinity water during ballast water exchange will stress or kill residual coastal organisms. These processes may also reduce the likelihood of successful invasion where water is deballasted.

What are the limitations of mid-ocean ballast water exchange?

Zooplankton can hitch  rides inside ballast water  tanks.  From top to  bottom:  the copepod  Acartia; a cyprid barnacle  larva; a snail veliger larva.

Although mid-ocean ballast water exchange is currently the most widely available method to mitigate the spread of NIS in ballast water, it is a stopgap measure.  In some circumstances, ships may not be able to undergo exchange at sea due to severe weather or stability issues.  Furthermore, mid-ocean ballast water exchange is not 100% effective at removing all coastal organisms from ballast tanks (Ruiz et al, 2005). Percent reduction of zooplankton by ballast water exchange is dependent on initial planktonic densities inside the tanks.   Zooplankton density in ballast water tanks is dependent on a number of factors, such as place and season of ballast water uptake, the environment, population dynamics of ballast water biota, voyage length, exchange efficiency, and ship and tank design (Ruiz and Carlton, 2003; Ruiz et al, 2005; Verling et al, 2005). The greater the starting density of zooplankton in a ballast tank and the shorter the voyage, the more likely there will be some survivors remaining inside the tank.  Additionally, benthic biota can accumulate in tank sediments, which are not necessarily removed during ballast water exchange. Some species of marine organisms can form hardy resting stages that are resistant to extreme changes in ballast water tanks, and can survive in a small volume of water that might be left even after a ballast tank is fully pumped out. Ultimately, the least risky ballast water discharged into ports is water in which organisms are completely eliminated.  Because mid-ocean ballast water exchange is not 100% effective, alternative treatment strategies for ballast water are being tested.  In the United States, these technologies must meet the Coast Guard treatment criteria within its Shipboard Technology Evaluation Program (STEP).