Trends of Organotin in Sediments in the Southern North Sea

D8 - Concentrations of Contaminants

D8.1 - Concentration of contaminants

Following bans on tributyltin, mean concentrations in sediment have measurably reduced in the Southern North Sea and are very low or undetectable elsewhere.

Area Assessed

Printable Summary


Tributyltin (TBT) and other organotin compounds are contaminants found globally, throughout the marine environment. Organotins have many applications, such as coatings, anti-odour/anti-fungal additives, pesticides, biocides in marine antifoulant paints, catalysts, wood treatments and preservatives. Extensive use in antifouling paints on watercraft (Figure 1) led to the widespread distribution of TBT in water, sediment and biota.

Figure 1: TBT has been used as an antifoulant in paint on ship hulls © US Navy

Figure 2: High concentrations of TBT in sediment are normally associated with commercial ports, harbours, shipyards, shipping lanes and marinas

High concentrations of TBT in sediment are associated with commercial ports, harbours, shipyards, shipping lanes and marinas (Figure 2).

Organotins are toxic to many marine organisms even at very low concentrations. High concentrations can cause shell deformities in oysters and impair reproduction. For example, some female marine snails develop male sexual characteristics due to hormone disruption by TBT. This has led to widespread declines in some snail populations (Imposex Indicator Assessment). However, the situation is slowly improving following legislation banning the use of TBT in antifoulant paints.

The OSPAR Hazardous Substances Strategy aims to achieve concentrations in the marine environment close to zero for man-made synthetic substances. TBT use was banned in the 1980s for vessels less than 25 m, and has been prohibited on all vessels and offshore installations since 2008. However, inputs of TBT to the aquatic environment are likely to continue, from countries not in compliance with the ban and from disused vessels or installations. Inputs of TBT may continue through the redistribution of already contaminated sediments. Wastewater treatment plants and landfills are another potential source of TBT to the marine environment, as organotin compounds are sometimes applied to consumer products.

A large number of organometallic derivatives in commercial use are based on tin (Batt, 2006). This has given rise to an increase of the worldwide production of organotin compounds over the past 50 years. A major application of organotin derivatives is their use as additives in the manufacturing of plastics in order to prevent thermal and UV decomposition of polyvinyl chloride (PVC) (Kawamura et al., 2000; Mersiowsky et al., 2001). Organotin derivatives also have biocidal properties which led to their use in several fungicides, miticides, molluscicides, nematocides, ovicides, rodent repellents, wood preservatives and antifouling paints, primarily containing tributyltin (TBT), triphenyltin (TPT) and tricyclohexyltin as toxic additives (Batt, 2006).

Owing to the wide range of commercial applications for organotins, considerable amounts have entered the environment (Strand and Jacobsen, 2005). As a result, TBT and some other organotin derivatives can be found not only in water and sediments, but also in various aquatic organisms and the tissues of some birds and mammals (Iwata et al., 1995; Guruge et al., 1996; Elgethun et al., 2000). While tin in its inorganic form is considered non-toxic, the toxicology patterns of the organotin compounds are complex (Ema et al., 1995; Vos et al., 2000; Jenkins et al., 2004). Depending on the type and number of the organic groups bound to the tin cation, some organotins show specific toxic effects in particular organisms even at very low concentrations (Vos et al., 2000; Whalen et al., 2002).

The effects of TBT are not limited to marine gastropods such as dog whelks (Nucella lapillus). The fertility of crustaceans, such as crabs, lobsters, crayfish, shrimp, krill and barnacles, is also affected and other effects have been observed in a range of species including birds and mammals (Waldock and Thain, 1983; Thain and Waldock, 1986; Vos et al., 2000). In spite of legislation regulating useof organotins in numerous countries, these compounds still represent a risk for aquatic and terrestrial ecosystems.

Legislation to ban the use of organotins on small boats was first introduced in France in 1982 and followed by the United Kingdom in 1987. Similar legislation has been introduced worldwide, leading to a significant decrease in organotin concentrations to the environment. A United States federal law was introduced early in 1989, banning the use of organotins on small boats but still permitting the use of paints containing organotins to be used on commercial vessels, provided the paint complied with a limited rate of release of organotins. Japan followed with a similar approach in 1990 and banned all use in 1997. In 1990, the International Maritime Organisation (IMO) adopted a resolution which recommended that Governments adopt measures to control the impacts associated with organotins in antifouling paints. In 1999, the IMO adopted a resolution to develop a legally binding resolution throughout the world, to address the harmful effects of anti-fouling systems used on ships. The resolution called for a global prohibition on the application of organotin compounds which act as biocides in anti-fouling systems on ships by 1 January 2003, and a complete prohibition by 1 January 2008.

The European Union has banned the use of certain organotin compounds in consumer products. Within the European Union, organotins are regulated by Decision 2009/425/EC (European Commission, 2009). Usage of tri-substituted organotin compounds such as TBT and TPT, dibutyltin (DBT) and dioctyltin (DOT) compounds in consumer products are all regulated by this decision. Bis(tributyltin)oxide (TBTO) is regulated under the REACH Candidate list of Substances of Very High Concern.

For each organotin at each monitoring site, the time series of concentration measurements was assessed for trends and status using the methods described in the contaminants online assessment tool The results from these individual time series were synthesised at an assessment area scale in a series of meta-analyses.

For trend analysis, data were used only from monitoring sites considered representative of general conditions. Data from monitoring sites impacted by point sources and data from baseline monitoring sites where trends would not be expected, were excluded. The analysis was also restricted to assessment areas with at least three monitoring sites with trend information and where those monitoring sites had good geographic spread. Only one assessment area fulfilled these criteria: the Southern North Sea.

The trend in each compound at each monitoring site was represented by the estimated annual change in log concentration, with its associated standard error. The annual change in log concentration was then modelled by a linear mixed model with fixed effects:

          ~ organotin

and random effects:

          ~ monitoring site + organotin: monitoring site + within-series variation

The ‘fixed effects’ measure the trend in each organotin in the Southern North Sea, while the ‘random effects’ measure variation in trends:

  • between monitoring sites common across organotins (monitoring site); and
  • residual variation (organo-meta: monitoring site + within-series variation)

There are two residual terms. Within-series variation is the variation associated with the estimate of the trend from the individual time series and is assumed known (and given by the square of the standard error). Organotin: monitoring site allows for any additional residual variation.

Evidence of trends in organotin concentrations in the Southern North Sea was then assessed by plotting the estimated fixed effects with point-wise 95% confidence intervals.

There are no assessment criteria for organotins in sediment, so a meta-analysis of status was not possible. However, a similar analysis was used to explore concentration profiles across compounds in the Southern North Sea. The summary measure was the fitted log concentration in the last monitoring year. Baseline monitoring sites were included in this analysis of concentration profiles.

Many more data series were used to assess the status of organotin concentrations in the Southern North Sea, than the Celtic Seas (Table a). Time series to assess temporal trends were only available for the Southern North Sea (Table b)

Table a: Number of monitoring sites used to assess organotin concentration status in sediment, by OSPAR contaminants assessment area and organotin compound. TBT: tributyltin, DBT: dibutyltin, MBT: monobutyltin, TPT: triphenyltin
OSPAR contaminants assessment areas Stations TBT DBT MBT TPT
Irish Sea 1 1 0 0 0
Irish and Scottish West Coast 1 1 0 0 0
Southern North Sea 30 19 24 27 1

The location of the monitoring sites in the Southern North Sea is shown in Figure a.

Figure a: Monitoring sites used to assess organotin concentrations in sediment in the Celtic Seas and Southern North Sea (white lines)

Differences in methodology used for the IA 2017 compared with the QSR 2010

For the IA 2017, a meta-analysis is used to synthesise the individual time series results and provide an assessment of status and trend at the assessment area level. Meta-analyses take into account both the estimate of status or temporal trend in each time series and the uncertainty in that estimate. They provide a more objective regional assessment than was possible in the QSR 2010, where a simple tabulation of the trend and status at each monitoring site was presented.


Most countries have stopped monitoring organotins in sediment, especially at offshore locations, because concentrations are now often so low that they are below the limit of detection. This means that a reliable assessment of organotin in sediment could only be carried out in the southern North Sea.

There are no environmental assessment values established for organotins in sediment. This means it is not possible to assess the environmental significance of the concentrations observed.

In the Celtic Seas, data are only available for one monitoring site in the Irish Sea and one monitoring site in the Irish and Scottish West Coast assessment area. TBT concentrations at both monitoring sites are very low.

The Southern North Sea is the only assessment area for which trend information is available for three organotin compounds (Figure 3): monobutyltin, dibutyltin and tributyltin. Trends in average sediment concentration for all three compounds in the Southern North Sea are decreasing, but still detectable. Annual average decreases are between 3.1% and 13.6%. These downward trends are also reflected in the reduction in biological effects associated with TBT exposure, which has been observed across the entire area assessed (Imposex Indicator Assessment). This indicates that the bans on the use of tributyltin are already having a positive effect on the marine environment.

There is high confidence in the assessment and sampling methodology and high confidence in the data used.

Figure 3: Percentage annual change in organotin concentrations in sediment by compound (with 95% confidence limits) in the Southern North Sea

Downward triangle: the mean concentration is statistically significantly (p <0.05) decreasing, MBTIN: monobutyltin, DBTIN: dibutyltin, TBTIN: tributyltin

Overall, organotin concentrations in sediment are declining in those parts of the OSPAR Maritime Area where monitoring is still ongoing. Most countries have stopped monitoring organotins in sediment, especially at offshore locations, because previous monitoring has shown that concentrations were consistently below the limit of detection. Therefore, a reliable assessment of current organotin concentrations in sediment is only possible based on the monitoring sites in the Southern North Sea (Figure a), where reducing concentrations of organotins in sediment are observed.

Individual Time Series Results per Monitoring Site

A summary of individual time series results per monitoring site (across the OSPAR Maritime Area) for organotin concentrations in sediment is available here In summary, none of the monitoring sites show an increase in organotin concentration in sediment over the assessment period. It should be noted that not all individual time series results are included in the regional assessments (Table a and Table b), due to the criteria set out in the assessment methods.

Confidence Assessment

There is high confidence in the quality of the data used for this assessment. The data were collected over many years using established sampling methodologies. The data were screened to ensure that only sites with sufficient spatial coverage and temporal data were included. Although synthesis of monitoring site data for the assessment area scale uses new methodology, this is based on established and internationally recognised protocols for monitoring and assessment per monitoring site, therefore there is also high confidence in the methodology.


Almost a decade after the use of organotins was prohibited in antifouling paints on ships, concentrations detected in marine sediments have fallen considerably, and are often below the limit of detection. As a result, only a few countries continue to monitor organotin in sediment.

The Dutch part of the Southern North Sea is the only area with sufficient monitoring data for an assessment. Data for this area show a decreasing trend in organotin concentrations in sediment.

However, because there are no background concentrations or assessment criteria for organotin concentrations in sediment, the ecological effects of organotin in sediment have not been established. Most countries have opted to monitor the biological effects of organotin pollution, rather than tributyltin itself (Imposex Indicator Assessment).

Monobutyltin and dibutyltin compounds are usually present in the environment as a result of the degradation of tributyltin, as well as from non-pesticidal industrial uses such as polyvinyl chloride (PVC) stabilisation. In the past, tributyltin (TBT) probably entered the environment mostly through their pesticidal uses. However, TBT may also enter the environment owing to their presence in other products and from the environmental breakdown of tetrabutyltin.

Knowledge Gaps

Although direct inputs of TBT to the marine environment have been banned, non-pesticidal use of TBT is still ongoing in some countries and thus further monitoring of TBT concentrations in the marine environment is warranted.

As there are no background concentrations or assessment criteria for organotin concentrations in sediment, OSPAR experts should consider establishing these.

There are only a limited number of monitoring sites in the OSPAR Maritime Area for which monitoring for organotins in sediment is still ongoing. In recent years, monitoring at several monitoring sites showed concentrations were below the limit of detection and as a result monitoring was discontinued. This assessment is limited to the Dutch part of the Southern North Sea, the only area with sufficient data for a temporal trend assessment. Although the use of organotins in antifouling paint is reducing, it is not known how harmful are the alternative antifouling options currently being used.

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