Effects of metals on aquatic assemblages: What do we really know?

Most knowledge of the effects of metals on benthic organisms comes from laboratory studies with subsequent extrapolation to field scenarios. Here, we critically reviewed studies on effects of metals on aquatic organisms, with an emphasis on marine systems to determine the “real” state of our current knowledge about effect of metals on assemblages. These studies include experiments in the laboratory and/or micro- and meso-cosms and those done in the field. Laboratory studies have shown lethal and sub-lethal effects of metals on organisms. There is, however, little evidence that these effects occur in natural environments. Conditions in the laboratory are very different from those in natural habitats. It is sensible, then, to assume that organisms may respond differently under field conditions, but how representative laboratory studies are of field conditions is rarely tested. In contrast, many field studies use patterns to infer processes, without any experimental verification. Of the descriptive papers reviewed here, nearly 50% found differences in the assemblages between contaminated and reference areas. Often, however, the differences were definitely not attributable or could not be unambiguously attributed to the presence of metals, because most of the studies lacked proper controls and/or appropriate temporal or spatial replication. Relatively few experimental studies in the field found clear effects. Where they were found experimentally, differences were generally decreased abundances of populations of some species or reduced diversity of species. Indirect ecological interactions can also occur, where abundances of a superior competitor are reduced by metals, leading to increases in abundances of other species. Properly designed experimental field studies are needed to evaluate the effects of metals on organisms and/or assemblages in natural habitats. This is essential for developing realistic environmental guidelines and managerial strategies for protecting aquatic habitats.