Small mammal - heavy metal interactions in contaminated floodplains. Bioturbation and accumulation in periodically flooded ebvironments

Abstract

A better understanding of interactions between biota and contaminants in floodplains is needed as it is uncertain whether ecological rehabilitation of floodplains is possible at the current contaminant levels. This study investigates where and when contacts between small mammals (voles, mice, shrews and moles) and heavy metal contaminants take place. The impact of bioturbation on the fate of heavy metals, and the risk of accumulation is also studied. Monitoring shows that habitat suitability and connectivity can predict the presence of small mammals. Large parts of floodplains are unsuitable, whereas small parts harbour large numbers of several species. The dominant process permanently regulating densities and distributions in floodplains is flooding. Populations are reduced by floods and survivors concentrated in non-flooding areas. Densities are always highest near non-flooding parts as recolonisation is slow. Bioturbation by small mammals and risks on accumulation and toxic effects are concentrated in space and time. Microcosm experiments show that bioturbation results in percolation of metals to deeper soil layers under wet conditions. This occurs in floodplains as numbers of flood tolerant earthworms and burrowing activities of small mammals are plenty. Surfacing of contaminated substrate by animals is even more significant towards the fate of heavy metals. Bioturbation is nowadays even more important for the presence of metals in top layers than deposition during floods. Ecotoxicological risk assessment and floodplain management traditionally focuses on flooding areas where total metal concentrations are highest. Taking availability, concentrations in foods, and animal distributions into account, risks of accumulation of metals in floodplain food webs are more related to non-flooding areas. Some individual mammals might suffer toxicological effects. Due to short 'natural' life expectancies in floodplains, no effects at population level are expected. Findings should be incorporated in spatial planning and management strategies to favour small mammals and reduce ecotoxicological risks.