Trace metals discharged into aquatic ecosystems are most likely scavenged by particles and removed to sediments, leading maybe to a situation, where the overlying water is free from elevated concentrations of trace metals, while the sediments have accumulated toxic metals up to levels which may harm biota. Although sulphides and organic matter have been identified now as one of the main factors for buffering the bioavailability of metals in sediments, toxicity may not be seen even if these substances become exhausted. This will imply the existence of still other binding phases, e. g. of dissolved or colloidal Fe or Mn oxides, which in addition contribute to the reduction of metal bioavailabilities. Metals entering the aquatic environment and associated with surface sediments are subsequently subjected to a multitude of transformation reactions occuring during early sediment diagenesis', leading among others to mineralization and the formation of secondary minerals (see section 5.5.6). As we also know that metal affinities to different sediment fractions vary greatly among different metals, locations and seasons, the prediction of bioavailability seems even more complex (ref. 13-15 cited in Fan et al. 2002). In particular sulphides have received much attention in recent years as major binding phases for metals, like Ag, Cd, Cu, Ni, Pb, and Zn, in anoxic sediments (see section 5.4.3).

Since the 1980s, various geochemically designed, analytical approaches have been used to relate the mobility and reactivity of metals to their potential bioavailability in sediments. For example, Fe-normalized metal concentrations in Fe-oxide fractions obtained by sequential chemical extraction are recognized as metal forms, which can positively correlate with metal bioaccumulation. Also increasing (SEM-AVS) values have been documented to correlate with sediment toxicity (see the following sections). Although known for their low selectivity and reproducibility, single and sequential extraction techniques are still widely used today to get a first estimate of the potential risk of metal-contaminated materials and ecosystems, like soils or aquatic sediments (see sections 5.4.4 and 5.5.3). But as all these approaches still produce rather inconsistent results, they have to be considered more as conservative measures, for example to stress the nontoxic nature of a particular sediment, rather than to indicate a particular toxicity level. Chemical speciation of metals increasingly includes also the use of thermodynamic data and calculations coupled with newly developed surface complexation models (see sections 5.4.7 and 5.5.4), which however depend on the accuracy of the dissolved metal measurements and on a proper estimate of available sorption sites. Moreover, the use of these numeric modelling approaches may be misleading, as natural systems often are out of equilibrium, but also because biological systems evolved mechanisms to respond to metal stress, which may in turn cause the respeciation of metals.

As the metal bioavailability we observe in sediments is the result of continuous multiple interactions existing between organisms, contaminants and the sediment, geochemical and biological approaches alone will always address just one aspect of these complex interactions.

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