TBT and Imposex
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This article describes the use of Tributyltin (TBT) in aquatic antifouling paints, its behaviour in the marine environment and one of its powerful negative effects in non-target species - the phenomenon of imposex in marine gastropods, which has led to the partial ban of this coumpound.
Contents
Introduction
Tributyltin (TBT) is a compound which integrates certain anti-fouling paints used on the hulls of vessels to prevent biological fouling - a phenomenon which has considerable economic costs and environmental risks. Although very efficient, TBT has been subject to restrictions due to its toxic effects in non-target species, detected at the end of the 1970s. One of this harmful effects is imposex – the masculinisation of females of certain marine sanils in response to exposure of TBT concentration, in the magnitude of ng.l-1. So far this phenomenon has been described for over 150 species. The sensitiveness and high correlation between the intensity of this phenomenon and the environmental concentrations of TBT allow the use of certain neogastropod species as indicators of the degree of contamination in coastal zones. Though the use of TBT has been forbidden in many countries for vessels smaller than 25 m, the contamination levels are still a concern, particularly close to areas of intense boating and associated activities, such as fishing and commercial ports, marinas and dry-docks.
The Problem of Fouling in Vessels
Any submersed rigid structure can work as substrate and be colonized by several marine organisms. It is estimated that there are over 4000 marine fouling species. In the case of vessels, the degree of fouling of the hull depends on the time of submersion, the time the vessel is immobilized or its speed, but mainly on the features of the marine environment. Without an antifouling protection, the fouling can reach 150 kg per square meter, in less than 6 months. This phenomenon leads to an increase in the weight of the vessel, the drag resistance of the hull surface and therefore a decrease in speed, a consequent increase in the fuel consumption (up to 40%) but also more frequent maintenance operations. Additionally, the hulls can work as vectors of translocation of organisms form one place to the other, which poses the risks of introducing non-native invasive species.
Antifouling strategies
The problem of fouling in vessels was recognised since the beginning of navigation. The ancient Phoenicians and Carthaginians were thought to have used copper sheathing and the Greeks and Romans both used lead sheathing on their ships’ hulls. More recent methods included the usage of organic compounds of lead, arsenic, mercury and halogens (e.g. DDT) and copper oxide. The later is still widely used nowadays. The first antifouling paints using organic compounds of tin started to appear in the second half of the XX century and quicky dominated the markets during the following decades. Even today, TBT is globaly considered the most effective solution developed so far to prevent fouling.
Effects in non-target species
The case of the Bay of Arcachon (France)
During the period when TBT was being widely used as antifouling, the production of oisters in the Bay of Arcachon (France) almost collapsed. This coastal area is sumultaneasly a place of production of this shellfish and an area of intense recreative boating. Although the knowledge of TBT was very limited at the time, the French Authorities restricted the use of the compound in antifouling paints in the region, as a rare example of adopting the precautionary principle. Later on, it became clear that TBT was responsible for the reproduction failures and abnormal shell development of the oisters.
Imposex in marine snails
Also in the beginning of the 70’s certain reproductive abnormalities in other molluscs were discovered, which were later proved to result from exposure to TBT. In certain species of gastropods with separate genders, the females presented a penis and/or vas deferens. The term “imposex” was given as “a superimposition of male features in females” and was first described in dog whelk (Nucella lapillus). Soon it was clear that this was a generalised phenomenon – not only all the populations of dog whelk analysed in southwest England were affected but worldwide the same phenomenon was reported and for different species of snails, particularly in areas of intense maritime traffic. So far, imposex and intersex (a similiar phenomenon) have been described in over 150 species of marine snails. More developed stages of imposex can lead to the sterilization and premature death of the females, affecting the entire population. However, the most dramatic aspect of this endocrine disruptor is the fact that TBT can act at extremely low concentrations: a few nanograms per litre is enough to trigger imposex in marine snails - the equivalent of 1 g of salt dissolved in a square pool of 100 m side and 100m depth!
Effects in other species
The knowledge of TBT, its toxicity and risks to non-target organisms, including humans, is still limited. However, studies suggest several harmful effects on the imune and neurological systems and embrios in mammals and described toxicity to plankton, algaes, fish and marine birds. It is known that top predators from marine ecossystems can accumulate significant amounts of pollutents. TBT is not an exception and has been already detected in cetaceans and seals, sharks and tunas.
Restrictions to TBT
Since 1988 the International Maritime Organization (IMO)IMO, through the Marine Environment Protection Committee (MEPC, has recognised the harmful effects of the antifouling systems, particularly TBT. In 1990 the MEPC recommended the IMO Member States to restrict the usage of TBT in boats smaller than 25m (as the recreation boating was considered to be the main direct input) and to establish maximum bleaching rates for the antifouling paints. As the evidences of the negative impacts and toxicity of TBT increased, IMO adopted the International Convention on the Control of Harmful Anti-fouling Systems on Ships with the intention to globally ban TBT, starting in 2008. The ratification of this proposal was slow, though the number of joining countries has increased, as the goals haven't been met. France, in 1982, was the first country to forbid the use of TBT in boats smaller that 25m, followed by the UK in 1987. The rest of the EU gradually joined the action. Japan has banned the organic compounds of tin from antifouling paints in 1990 and has called for a global ban. Other countries like Switzerland, Austria and New Zealand have followed the IMO suggested deadline. Most developed countries have adopted legislation partially prohibiting the use of TBT but the necessary global ban seems far from being achieved.
Monitoring of TBT contamination
Imposex as an indicator of TBT contamination
Some species of snails have been used as indicators to evaluate and compare the degree of TBT contamination in aquatic environments. They are suitable species since:
- the stage of imposex reflects the amount of TBT present in the tissues of the organism and in the surrounding environment
- the imposex is triggered by extremelly low concentrations – close to the level of detection of measuring instruments
- marine snails can be very commun in certain habitats and have restricted mobility
See also
Internal Links
- Coastal pollution and impacts
- Endocrine disrupting compounds in the coastal environment
- Pollution laws and regulations
- Pollution indicators
- Differentiation of major algal groups by optical absorption signatures
External Links
References
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