The ecotoxicologists from the Netherlands Institute for Sea Research investigated the relationship between shipping and imposex in snails. Imposex is an abnormality in snails of the seabed, resulting in male sexual characteristics in females. Imposex is most prevalent in and around harbours. In 1991, the Netherlands Institute for Sea Research demonstrated that the phenomenon also occurs on a worldwide basis in open seas, particularly near busy shipping lanes.
However, the clear relationship between pollution from shipping and damage to local seabed life does not appear to apply when the water is stratified year round. To the north of the Dogger Bank and in the Atlantic Ocean near Spain and Portugal, the researchers did indeed find imposex but not always precisely where the shipping was busiest.
The ecotoxicologists concluded that the harmful substance in antifouling paint, TBT (Tributyl Tin), was not able to sink though the boundaries between seawater layers. The boundary between two layers is clear-cut and can be seen underwater as a reflecting surface. The organotin compound TBT is probably not the only toxic substance whose distribution to the seabed is disrupted by such boundary layer between two seawater layers. Researchers studying the effects of pollution in the sea surface on seabed life will need to take this into consideration.
Stratification most often occurs as a result of warm water floating on top of cold water. Less saline water from the Baltic floats on top of heavier, more saline Atlantic water. Near Spain and Portugal the water is composed of several layers, which differ in temperature and salt content, including near Gibraltar an in-between stratum of warm very salty water from the Mediterranean.
The United Nations' International Maritime Organization (IMO) has recently adopted a convention that prohibits the use of antifouling paints based on organotin compounds such as TBT, which cause imposex. From 2003 onwards they may no longer be applied to vessels and from 2008 onwards they may no longer be present on vessels. In the meantime the chemical industry has developed several effectif alternatives that are less harmful to the maritime environment.
The research was funded by the Netherlands Organisation for Scientific Research (NWO).