Some general considerations

Soft sediments have long been considered to be a critical compartment of aquatic ecosystems, where natural and anthropogenic chemicals accumulate. This accumulation of particulate and dissolved, organic and inorganic materials makes sediments a centre of biogeochemical cycling and a base of the food web. The benthic microbial, meiofaunal and macrofaunal communities can reflect sediment quality within a given system, which in turn reflects the quality of the watersheds that contribute to the system (Burton, 2002). Therefore, a tremendous amount of effort has been focused on how to properly assess the quality of sediments, particularly in environmental risk assessments of contaminants, such as trace metals, contained in the sediments.

The behaviour of trace metals in aquatic sediments is controlled by a complex of physical, chemical and biological factors that might transform their speciation under changing redox conditions below the sediment-water interface. Bioavailability of trace metals in sediments is determined, on one hand, by the sediment properties and by the chemistry of pore-water and overlying water and, on the other hand, by the physiology and feeding behaviour of benthic organisms (Chapman and Wang, 2001).

In oxidized sediments, iron and manganese oxyhydroxides and particulate organic carbon control the bioavailability of trace metals, while in anoxic sediments, the most important regulating factors are pH, redox potential and sulphides. In most undisturbed silty sediments there is a sharp gradient in redox potential from the sediment surface downward to deeper layers. While the sediment surface may be well oxidized, there may be no molecular oxygen present at a sediment depth of 2-5 mm (anoxic conditions), in particular in sediments rich in organic matter. Thus, for bioavailability studies with deposit-feeding invertebrates, some of which have a burrowing behaviour, it is important to consider exposure not only to trace metals at the oxidized sediment surface, but also to lower sediment layers with suboxic or anoxic conditions (Eriksson Wiklund and Sundelin, 2002).

As was discussed in detail in section 5.4.3 of this report, one of the most popular among the recent approaches to predicting metal bioavailability and toxicity in sediments has been based on the SEM/AVS concept. Lately, the significance of this tool as the most important toxicity predictor has somewhat weakened and it is now rather considered as one among several tools in the toolbox (Chapman and Wang, 2001). However, the tool seems to be more efficient in predicting non-toxicity of sediments. Since bioavailability of sediment-bound metals is not necessarily attributable only to the characteristics of sediment pore-water, bioaccumulation cannot be fully accounted for by the SEM/AVS model. In fact, some recent studies indicate that the bioavailability of sediment-bound trace metals may be more closely related to the digestive systems of certain groups of benthic organisms than to chemical extractability.

That the food route is important for trace metal uptake and toxicity in various invertebrates has also been demonstrated by, e.g., Thomann et al. (1995) who developed a steady-state model relating the ratio of metal concentrations in two species of bivalves (the oyster and the blue mussel) to sediment metal concentrations. Relevant components included sediment-water column partitioning, bioconcentration factor, depuration rate, metal assimilation efficiency from food, the bivalve feeding rate, and the growth rate of the organism. The model, calibrated with field data from the United States, indicated the food route of exposure to be significant for metals such as zinc, cadmium, copper and mercury. When LaBreche et al. (2002) used a similar model, they found for adult hard clam (Mercenaria mercenaria) that 94% of the copper intake was predicted to be accumulated via food ingestion. Of course, a key question that needs further investigation is to what extent the trace metals taken up via the food produce toxic responses to the same extent as metals taken up via the respiratory membranes.

Most of the present models that account separately for bioaccumulation via the dissolved phase versus food, however, have been developed predominantly for suspension feeders like mussels (partly for clams), which differ in feeding behaviour from the notorious deposit-feeding amphipods (e.g., Monoporeia affinis), which feed exclusively at the sediment-water interface. Amphipods obviously bioaccumulate trace metals from either the pore-water or the sediment phase and the extent to which bioaccumulation from the sediment phase occurs depends on both the binding strength of the metal with the sediment and the extraction capacity of gut fluids (Eriksson Wiklund and Sundelin, 2002). Again, the toxic properties of the trace metal species taken up from the sediment and extracted by the gut fluids are not yet sufficiently examined.

Nonetheless, during the last few years, a great number of studies have been conducted with the aim of testing the reliability and usefulness of the

SEM/AVS concept under more or less realistic field conditions (cf. section 5.4.3 of this report). Two of the most comprehensive among the recent efforts will be presented and commented on in the following:

• The first one used an approach consisting of spiking natural sediments from four aquatic habitats, two lakes and two riverine ecosystems (each one with low and high AVS), with zinc and then putting the spiked sediments in colonisation trays, which were returned to the original sites and allowed colonisation by benthic invertebrates for 37 weeks. The colonisation experiment was complemented with in situ toxicity tests.

• The second one used intact sediment cores from metal-polluted and reference sites in both freshwater and brackish-water habitats. The cores with overlying natural water were slowly oxygenated by means of a gentle flow of aerobic water over the sediment surface, which was also subjected to bioturbation by introduced amphipods. This approach allowed a long-term follow-up of the changes in vertical gradients in sediment chemistry as well as of bioavailability and toxicity of the metals originally occurring in the sediment cores.

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