This field study measured total mercury, methylmercury and total selenium in water, sediment, invertebrates and fish from three reservoirs in Burkina Faso and used stable isotopes to trace food web position. Mercury increased up the food chain, but selenium did not. Fish muscle selenium was low and every fish had more selenium than mercury on a molar basis, although the selenium to methylmercury ratio fell in top predators.
Key numbers
- Mean (SD) total selenium in fish muscle: Loumbila 147 (100) ng/g wet weight; Ziga 264 (82) ng/g; Kompienga 167 (168) ng/g (Results, p. 9).
- Fish analyzed for selenium: 22, 25 and 49 per reservoir (Table 3).
- All reservoir means were below the 3 µg/g wet weight selenium threshold the authors cite (Results, p. 9).
- Method detection limit for solids: 0.022 µg/g dry weight (Methods, p. 4).
Methods (brief)
Samples were collected in the 2010 rainy season. Tissues were microwave-digested with nitric acid and hydrogen peroxide and total selenium measured by hydride generation atomic fluorescence spectrometry, with two marine reference materials. Trophic magnification was assessed by regressing log concentrations on nitrogen isotope ratios.
Evidence fitness
Supports wild freshwater fish selenium concentrations in West Africa and the finding that selenium did not biomagnify. Fish were not sampled from markets and intake was not estimated.
Limitations
Three reservoirs, one season; species mix varies by site; the selenium threshold used for comparison is cited, not derived.
Related evidence
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