Organotin contamination in marine sediments and biota from the Gulf and adjacent region
Introduction
Organotin compounds exhibit varying degrees of toxicity towards a broad range of organisms and accordingly have seen widespread applications as biocides. Triphenyltin (TPhT) compounds, found in some marine paint formulations, have been widely utilized in agriculture as fungicides and insecticides. Tributyltin (TBT) compounds have been used most extensively as the main biocide in marine antifouling paints. Organotin-based paints have seen service on boats of all sizes, from small yachts to supertankers, thereby ensuring the global dispersion of TBT throughout the marine environment, from the coastal zone to the open ocean.
TBT accumulates in a variety of marine organisms, from plankton and fish to various marine birds and mammals. Numerous deleterious biological effects of TBT on non-target organisms have been observed. The most obvious manifestations of TBT contamination have been shell deformation in Pacific oysters and the development of imposex (i.e., the imposition of male sex organs on females) in marine gastropods. The latter effect demonstrates that TBT acts as an endocrine disrupter.
Organotin compounds are persistent in the marine sediments owing to their slow degradation rates and consistent flux. Sediments provide a valuable means to assess contamination and given the public health concerns, seafood analyses are prudent. Although there have been organotin investigations throughout the world, there are very few data for the marine environment of the Middle East. The Gulf, an enclosed sea, has quite dense shipping owing to the large number of oil tankers that transit the region. A survey was conducted to assess organotin contamination in sediments and biota in the coastal zone of Bahrain, Qatar and the United Arab Emirates, together with the first such data from the Gulf of Oman.
Sample collection
Material was obtained during the following three missions to the ROPME Sea Area (RSA), which comprises the Gulf and the Gulf of Oman: Qatar and the United Arab Emirates (March 24-April 4, 2000); Bahrain (November 23-25, 2000) and Oman (July 27-August 1, 2001). Sediment sampling sites are shown on a map of the RSA (Figure 1).
Figure 1 Sampling locations in the Gulf and Gulf of Oman.
Results and Discussion
Sediments
The concentrations of butyltin compounds in sediments are shown in Table 1. In the UAE, no organotin was detected other than a small amount (1.08 ng Sn g-1) of MBT found at the Jebel Ali port complex. Similarly, no butyltin was measured at one site in Qatar (Ras Al Nouf), and only MBT was observed at two other locations (Doha and Ras Laffan). In contrast, all three species were measurable at Umm Said and Dukhan. Dukhan was the only location that could be considered contaminated with respect to TBT (i.e. TBT>1.3 ng Sn g-1). As TBT degrades only slowly in marine sediments, the high relative percentage of MBT at all sites other than Dukhan indicates that there has been little recent input of TBT into the marine environment of the UAE and Qatar.
Table 1 Organotin concentrations (ng Sn g-1 dry weight) in marine sediments from UAE, Qatar, Bahrain and Oman
More variability was observed for organotin distributions in both Bahrain and Oman. There were two sites in Bahrain (Jasra and North of Meridien Hotel) and three in Oman (Al Sawadi, Mina Al Fahal and Mughsayl) without measurable butyltin levels. This observation for Mina Al Fahal, Oman, was surprising given that this location was very close to the oil terminal. In contrast, all three butyltin species were detected in sediments off the BAPCO industrial complex and at Askar in Bahrain. The comparatively high amounts of TBT near BAPCO reflect the importance of shipping at this site near the refinery. In Oman, only TBT was detected (3.3 ng Sn g-1 dry weight) at the Raysut Port Area in Oman, indicative of recent TBT inputs. Relatively high organotin concentrations were observed at Hilf on the west coast of Masirah Island (e.g. 60 ng Sn g-1 dry weight for TBT). The source of these organotin compounds is not immediately apparent. Hilf does have a ferry terminal for the small vessels that service the island and several fishing boats are based in the port. Both BAPCO in Bahrain and at Hilf in Oman are classified as moderately contaminated. Apart from these two contaminated sites, the relatively high percentage of MBT in most sediment suggests that there has been little recent input.
Fish
Concentrations of butyltin compounds were very low in most fish samples and in many cases can only be reported as 'less than' values.
Bivalves
Measurable amounts of organotin compounds were found in bivalves, with levels of TBT, DBT and MBT reaching 196, 229 and 63 ng Sn g-1 dry weight, respectively, in pearl oysters from Abu Dhabi (Figure 2 & Table 2). Likewise, pearl oysters collected at Askar, relatively near to the BAPCO industrial complex in Bahrain, also contained a relatively high amount of TBT (150 ng Sn g-1 dry weight). The high TBT/(MBT+DBT) ratio of 3.9 indicates the likelihood of fresh inputs of TBT near this location. Rock scallops from Abu Dhabi also displayed an elevated concentration of TBT (110 ng Sn g-1 dry weight), however, the corresponding DBT levels (24 ng Sn g-1 dry weight) were an order of magnitude less than were found in the pearl oysters from the same site. Both species were collected in areas near the port of Abu Dhabi where small and large boat traffic is common. This striking difference in DBT concentrations could reflect differing debutylation efficiencies in the two species of bivalves.
Figure 2 Concentrations of butyltin compounds in oysters from Bahrain, Qatar, UAE and Oman.
Table 2 Organotin concentrations (ng Sn g-1 dry weight) in bivalves from UAE, Bahrain and Oman
TBT concentrations in rock oysters from the Gulf of Oman were all relatively low except in those originating from Akkah Beach, UAE (25 ng Sn g-1 dry weight) and Hilf on Masirah Island in southern Oman (176 ng Sn g-1 dry weight). Both these locations are fairly remote. With respect to the high TBT concentration in oysters from Hilf, they closely reflect the high TBT levels measured in the surrounding sediments and clearly indicate fairly fresh inputs of TBT at this location, presumably due to boating activities at this minor port. Concentrations of total butyltins in oysters from the RSA ranged approximately two orders of magnitude from 6.5 to 488 ng Sn g-1 dry weight. If the two highest concentrations at Abu Dhabi and Hilf are excluded, the range (6.5-188 ng g-1 dry weight) is narrowed considerably. The levels in oysters and other bivalves from the RSA generally fall in the lower end of the range of typical concentrations.
Conclusions
In summary, the environmental levels of organotins found in coastal sediments from the RSA are relatively low by global standards. Only a few sites can be described as TBT-contaminated. Similarly, the organotin content of the marine biota is comparatively low. Based on the limited information concerning the dangers to public health from organotin compounds, organotin concentrations in edible fish and bivalves from the four countries investigated here pose no immediate public health problems.
Reference
S.J. de Mora, S.W. Fowler, R. Cassi & I Tolosa (2003) Assessment of Organotin Contamination in Marine Sediments and Biota from The Gulf and Adjacent Region. Marine Pollution Bulletin 46, 401-409.
