Biomagnification of DDT and its metabolites in four fish species of a tropical lake
Introduction
DDT is an organochlorine pesticide that has been used for vector and pest control since World War II (Foreman and Gates, 1997). This pesticide is grouped under the persistent organic pollutants (POPs) due to its toxic, lipophilic and persistent nature (Burreau et al., 2004, Holden, 1966, Jones and de Voogt, 1999). Coupled with its ability to bioaccumulate and magnify in the food chain (Alexander et al., 2007, Burreau et al., 2004, Mackay and Fraser, 2000, Rognerud et al., 2002), it has potential impact on top predator species, including humans. DDT and some of its metabolites are reported to affect the nervous and reproductive systems, and cause cancer (Beard, 2006, Kelce et al., 1995, McBlain, 1987). While the use of DDT has been limited internationally, some countries continued to use it in disease control programs as it was found a valuable short term line of attack for controlling malaria (Goldberg, 1991). In recognition of this pressing need, SC permits the production and use of DDT for disease vector control only by notifying the convention secretariats, provided that no safe, effective, and affordable alternatives are locally available (Stockholm Convention on Persistent Organic Pollutants, 2008).
Technical grade DDT is mainly a mixture of 4,4′-DDT (85 percent) and 2,4′-DDT (15 percent). Both DDE (1,1-dichloro-2,2-bis(4-chlorophenyl)ethylene) and DDD (1,1-dichloro-2,2-bis(4-chlorophenyl)ethane) exist as impurities in commercial DDT formulations. In the environment, DDT breaks down to form DDE or DDD (Foght et al., 2001, Pirnie et al., 2006, Sayles et al., 1997). DDT may enter rivers and lakes mainly through industrial release point sources, as runoff from agricultural fields, as well as from atmospheric deposition due to volatilization (Binelli and Provini, 2003, Schwarzbauer et al., 2001). DDT, DDD and DDE are strongly retained by soils, sediments, and biota lipids due to their low aqueous solubility and high octanol–water partitioning coefficients. 4,4′-DDE can often account for 70 percent of the total DDT in fish (Schmitt et al., 1990). The higher accumulation of 4,4′-DDE than the other metabolites is attributed to mixed-function oxidases that may have induced the dechlorination of 4,4′-DDT to 4,4′-DDE. The ratio of 4,4′-DDE to 4,4′-DDT is a helpful tool in revealing the significance of the degradation of DDT and to evaluate the current use of DDT in the given region (Strandberg and Hites, 2001). However, this method is limited in regions where dicofol is used. This is because dicofol contains high levels of DDT as an impurity (Qiu and Zhu, 2010).
There are two main routes by which fish can bioaccumulate chemicals in their natural aquatic habitat: (i) from water via body surfaces (e.g. gills) and (ii) through the diet (Burreau et al., 2004, Campbell et al., 2000, Holden, 1966). Stomach content analyses and stable nitrogen isotope ratio (δ15N) provide complementary information which can be used in analysis of the trophic transfer and biomagnification rate of persistent contaminants, including DDTs (Rognerud et al., 2002, Sharma et al., 2009). It is well established that lipophilic compounds such as DDT preferentially accumulate in lipids of fishes and other animals (Mackay and Fraser, 2000), and the extent of accumulation is greater in fish that have high lipid content (Muir et al., 1990).
The Ethiopian Rift Valley Lakes (ERVLs) are the most northern part of the East African Rift Valley Lakes. In Central Ethiopia, the Great Rift Valley splits the Ethiopian highlands into northern and southern halves, and the ERVLs occupy the floor of the rift valley between the two highlands. Most lakes are highly productive and well known for their aquatic diversity and indigenous populations of edible fish species (Golubtsov et al., 2002). With changing environmental conditions under increasing anthropogenic influences, the nature of the Ethiopian Rift Valley Lakes is also changing.
Due to the intensive agricultural and deforestation activities in the catchments of the Ethiopian Rift Valley Lakes (Zinabu, 2002, Zinabu and Elias, 1989), there is a risk of chemical pollution from fertilizers and pesticides. Run off and erosion from the surrounding catchment could release pesticides (e.g. DDTs) sequestrated in the soil to the lake may enter the food chain, and reaching out to the fish. Hence, the abundance and quality of commercially important fish species, an important ecosystem service of the Rift Valley Lakes, may be at risk. Furthermore, consumption of contaminated fish is one of the main exposure routes to toxic organic chemicals like DDT for humans (e.g. Han et al., 2000, Svensson et al., 1995). However, the level and extent of DDTs contamination in fish species at various trophic levels, has not been studied. The objective of this study is therefore to determine the concentration levels of DDTs and their biomagnifications in relation to the trophic position of fish species from Lake Ziway, which is one of the important Ethiopian Rift Valley Lakes.
Section snippets
Description of the study area
The study area, Lake Ziway, is located in the Rift Valley in the southeastern part of Ethiopia, at 1636 m a.s.l. (coordinates 7° 52′N and 38°4 5′E) (Fig. 1). It is part of the Ziway–Shala basin and has a catchment area of about 7000 km2, and an average surface area of 490 km2. The lake has an average volume of 1.8 km3, and a maximum depth of 9 m (Vallet-Coulomb et al., 2001). There are two inflowing rivers, the Meki River from the north-west, and the Ketar River from the east. The lake drains
Fish diets
Mean volume (percent) of food items for C. gariepinus, C. auratus, T. zillii, and O. niloticus are presented in Fig. 2. Their diet was composed of plants, invertebrates, fish and detritus, with variations among the species. Consumed plants consisted of both phytoplankton (blue green algae, green algae and diatoms) and macrophytes.
Algae were the most important food group of O. niloticus in Lake Ziway, and formed 34.2 percent (by volume) of the total stomach content, of which blue green algae,
Discussion
The concentrations of DDTs found in fish from Lake Ziway (Table 3) are generally lower than those found in Lake Nubia, between Egypt and Sudan (Elzorgani et al., 1979) with a mean concentration of 184 ng g−1 ww and fish from Lake Victoria, Kenya with a wide range of concentrations, from 3–460 ng g−1 ww (Mitema and Gitau, 1990). However, they are comparable with fish from the Densu River Basin, Ghana, which had a mean concentration of 8.0 and 4.0 ng g−1 ww for 4,4′-DDE and 4,4′-DDT, respectively (
Conclusion
Although the low concentrations of DDT found in the fish species from Lake Ziway may indicate that the fish is safe for human consumption, the presence of at least one metabolite of DDT in all fish sampled from the lake, and the recent exposure of some of the fish indicate the current use of DDT in the study area. Paired stomach content and stable isotope analysis provides a better understanding of the trophic positions of the fish species studied from Lake Ziway that neither method can deliver
Acknowledgments
This study was financially supported by the Norwegian Program for Development, Research and Higher Education (NUFU); Project ID: NUFU PRO 2007/10115. The lab work was assisted by staff members at Bioforsk, Pesticide Chemistry Section.
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