All ships use ballast water to maintain stability and balance during cargo loading and unloading operations. Ships take up ballast water from the port and deposit it in special tanks; if needed, ships can release this ballast water during loading. The water and sediments contained in the ballast tanks may contain forms of resistance of marine organisms, bacteria and viruses, which can remain alive for weeks or months. When ballast water is discharged in the port, these living organisms and the forms of resistance that have survived in the tank are also released.
Ships' ballast waters are a recognised vector through which potentially invasive organisms are released into the environment. The introduction of allochtonous species into new ecosystems can displace established communities and cause very profound changes in the new ecosystem, causing severe losses of biodiversity and other natural and unnatural damage.
The International Maritime Organization adopted the International Convention for the Control and Management of Ships' Ballast Water and Sediments (Ballast Water Convention) in 2004, which requires all ships to implement a Ballast Water and Sediments management plan and to have a filtration or sterilisation system on board for discharging ballast water in other ports far from the water intake point. The Convention has not yet entered into force, as it is pending ratification by more member states.
In the Port of Barcelona, following a 1999 study by direct questionnaire to the captains of vessels stopping over in our port performed over a period of six months, the key data relating to ballast water are:
- Ships carry 41.5% of total capacity of their ballast tanks when they reach port.
- 6.6% of the capacity of ballast tanks is discharged in port waters.
- 740,000 tonnes of ballast water were discharged in port docks in 1999. Container ships account for more than 50% of discharged water. At present it is difficult to estimate the amounts discharged since this depends on the type of vessel and the type of traffic (import or export), but if we were to apply 1999 conditions in 2012, this would amount to 1,200,000 tonnes.
- The origin of 89% of ballast water was known and 11% could not be reasonably traced.
- Of the amounts with known origin, 58% were from the Western Mediterranean and 10% from the Eastern Mediterranean.
- Ballast water was kept in tanks for less than 30 days in 48% of cases, with 20% staying more than 30 days and the rest could not be traced sufficiently to calculate the figure.
A taxonomic and viability study of the organisms was also performed on ballast water. Samples were taken from the ballast tanks of five vessels from different origins, in which the water had been stored for 5 to 27 days. The conclusions were:
54 different taxa of phytoplankton were found, with a majority of diatoms and a smaller amount of dinoflagellates, which were nonetheless more resistant.
Storage times of more than seven days significantly reduced the number of viable cells.
The ballast waters analysed revealed 69 taxa of zooplankton, with 79% of copepods. Most of the individuals identified were viable.
In principle, the length of storage time reduces the number of viable organisms, but from the twentieth day there is an increase of zooplankton, due mainly to Harpacticoids (opportunistic organisms feeding, growth and rapid reproduction)..