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Elements and elemental compounds in waters and the aquatic food chain

Understanding the distribution of chemical forms of metals within certain water type, and their uptake into biota, is based on the electronic configuration of elements and the empirical classification of electron acceptors (metals) and donors (ligands) to "hard" and "soft" categories (Morgan and Stumm 1991 ; Raspor 1991). The relationship between the chemical properties of elements, their uptake and accumulation, which has implications on detoxification and food chain transfer, will be considered. Classification of trace metals to essential (Fe, Cu, Mn, Zn, Co) and non-essential (Hg, Cd, Ag, Pb) should be done with caution bearing in mind that the former can exert beneficial effects at low concentrations and harmful ones at higher levels. There are numerous biotic and abiotic parameters, which influence metal uptake and accumulation. The abiotic factors (salinity, temperature, light, pH, Eh and ligand concentration) influence the relationship between metal species distribution and organisms. Using the available speciation techniques (this Vol. Chap.I.1 ; Raspor 1980), biological responses can often be predicted from a knowledge of the chemical properties of the metal and the complexation capacity of the surrounding media. In many instances the major features influencing biological responses can be identified despite the difficulties in quantifying the heterogenous mixtures of ligands present in natural waters (Langston and Bryan 1984). Except for Hg, other metals are not biomagnified along the food chain. The reason for that is reduced bioavailability of metals. Concentration factors tend to be the highest in the primary producers and in the organisms at lower trophic levels, for which the dominant source of metals is the uptake from water. Organisms counter the reactivity of metals and their potential toxicity by ligand binding and compartmentalisation. The bound forms include insoluble phosphates and sulphur compounds which are formed and accumulated within membrane-limited vesicles in specialised tissues like the liver and kidney. These metals are also unavailable to predators that consume the tissues because they are not absorbed by the digestive system and therefore are not transferred along the food chain (Nott 1998).

Chemical Forms of Metals, Aquatic Food Chain

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