Distribution of Heavy Metals in Marine Fish, Oysters and Coastal Sediments in the Gulf and Gulf of Oman
Introduction
Maintaining good marine environmental quality in the Gulf and Gulf of Oman is crucial for several socio-economic reasons. The region relies heavily upon the sea water as a source of fresh water through desalination. The seafood is of value for both local consumption and export revenue. The relatively fragile ecosystem experiences high temperatures, salinity and UV exposure. Thus, many species functioning close to physiological limits, and the added stress imposed by pollutants may have severe consequences. Such a problem can be exacerbated, given that contaminant inputs undergo more limited dilution and slower dispersion than would occur in open marine systems because the Gulf is a relatively shallow, semi-enclosed sea with poor flushing characteristics and very high evaporation rates. Together with ROPME (Regional Organisation for the Protection of the Marine Environment), the IAEA has undertaken surveys of heavy metals and organic contaminants have taken place in the Gulf and Gulf of Oman since the early 1980s. The pre-war baseline studies proved invaluable for the identification of pollution hot spots and assessing the consequences of the 1991 Gulf War.
Marine pollution of metals was assessed in the coastal zone of the eastern Gulf and Gulf of Oman firstly based on sediments from coastal locations in Bahrain, Oman, Qatar and the UAE during 2000-2001 (see Figure 1). A wide range of elements was determined for pollution assessment and to facilitate interpretation of the origins of potential contaminants. Secondly, the quality of local seafood with respect to metal contamination was investigated, focusing on two fish species of commercial importance, namely the orange spotted grouper (Epinephelus coioides, known locally as hamoor) and the spangled emperor (Lethrinus nebulosus, known locally as sha'ri, shaeri or sheiry), together with various bivalves, notably oysters.
Study Location
Figure 1 Location of all sampling sites for sediments (O), fish (X) and bivalves (+) in Bahrain, Qatar, UAE and Oman.
Sediments
Only two locations stood out in terms of elevated levels of trace elements in sediments: off the BAPCO (Bahrain Petroleum Company) refinery in Bahrain and at Akkah Beach on the east coast of UAE (Figure 2). Whereas the elevated levels at the BAPCO site signaled localised anthropogenic inputs, the Akkah Beach location reflected the metal-rich mineralogy of the region. The geology of Oman and eastern UAE is rich in ophiolites, which contain chromite and various nickel sulfide minerals. Concentrations of trace elements at locations other than these hot spots were for the most part substantially lower, and fell within the ranges reported previously for these elements in the region.
Figure 2 Concentrations of selected heavy metals in sediments of the ROPME Sea Area. Data are arranged in order along the coast from western Bahrain to southern Oman. Concentrations on a dry weight basis are given in µg g-1 for all metals, except Hg, which are presented as ng g-1.
Fish
Two economically important species in the RSA were investigated, namely the orange spotted grouper (Epinephelus coioides, hamoor) and the spangled emperor (Lethrinus nebulosus, sheiry). Although both species are demersal carnivores, hamoor feed preferentially on fish and crustaceans whereas sheiry tend to eat echinoderms, worms molluscs and crustaceans.
Mercury bioaccumulates in fish, such that relatively high concentrations can be attained in top predators, like hamoor. The total Hg concentrations generally increase with age, and thus size, of the fish. Nevertheless, Hg concentrations were generally very low and the total Hg levels in top predator fish commonly consumed in the RSA were found to be below the 0.5 µg g-1 wet threshold safety value set by many countries. The Hg content was similar to levels measured in the same species during earlier years.
The most unusual findings related to Cd measurements. Cd concentrations in muscle for both species were quite low, and typical of levels that have been found in various fish species from pristine areas. However, the Cd content in liver, especially for spangled emperor, was much higher than observed in the muscle. Of particular note were the extremely high concentrations (109 and 195 µg g-1) in liver of fish from southern Oman. To our knowledge, such high Cd concentrations in fish liver have not been reported previously. These very high Cd concentrations may result from food-chain bioaccumulation of elevated Cd levels brought into the productive surface waters by prominent upwelling in the region.
Oysters
Several composite samples of oysters were obtained in the Gulf and Gulf of Oman, comprising 5 samples of pearl oysters (Pinctada radiata) and 6 samples of rock oysters (Saccostrea cucullata). All pearl oysters were collected in the Gulf, except for the sample from Akkah Head. Very high concentrations of Zn (4290 µg g-1), V (7.3 µg g-1) and Pb (3.92 µg g?1) were found in pearl oysters near the BAPCO site in Bahrain. The pearl oysters from Abu Dhabi contained relatively high concentrations of Cr, Fe, Co, Ni, Cu and Ag. The pearl oyster sample from Akkah Head, the only one collected in the Gulf of Oman, actually contained the lowest levels measured for several elements, namely V, Cr, Fe, As, Hg and Pb. In general, the metal contents agree well with previous studies, but the high Zn level found near the BAPCO refinery was exceptional.
The content of several metals in rock oysters from the Gulf of Oman were comparable to previous observations. Surprisingly, metal concentrations in the pearl and rock oysters from Akkah Beach were not enhanced in the same trace metals (notably As, Co, Cr and Ni) that were greatly elevated in the local sediments. The rock oysters from Ras Al Yei on the seaward side of Masirah Island, Oman, contained the highest levels of many of the trace elements analysed. During the monsoon months, many of these trace metals are possibly enhanced in the water and the filterable suspended particulates when upwelling and rough sea conditions are predominant.
Figure 3 Rock oysters along the shore at Masirah Island, Oman.
Most interesting, and as yet unexplainable, is the observation of very high arsenic concentrations in certain bivalve species (clams and sea pens) from the region when compared to those from other regions in the world. Again, it is not clear whether this is related to point sources of contamination (unlikely) or to natural biogeochemical processes in the region (more likely). It is evident that to interpret sources of possible metal contamination properly, it is imperative to understand the natural bioaccumulation potential and natural background levels of elements like As in the species under study since content and ratios of heavy metals vary greatly among the bioindicator species (particularly bivalves) used in the RSA.
Reference
Stephen de Mora, Scott Fowler, Eric Wyse, Sabine Azemard (2004) Distribution of Metals in Marine Biota and Coastal Sediments in the Gulf and Gulf of Oman Marine Pollution Bulletin. 49, 410-424
