The role of sediment-feeding oligochaeteTubifex on the availability of trace metals in sediment pore waters as determined by diffusive gradients in thin films (DGT)

Background  Available fractions of metal concentrations in sediment pore water are difficult to measure without disturbing the sediment core. The available fraction is an important parameter in understanding the risks for benthic organisms. Not much is known about the influence of the presence of benthic organisms to the available fraction. Objectives  The aim of this study was to discuss the dynamics of metal behaviour in natural sediments and the influence of the presence ofTubifex on sediment chemistry using DGT. Methods. DGT-probes were added to six beakers to cover a depth profile of 0 to 13 cm. The six beakers contained three different sediments. For each sediment, one beaker had 1 g ofTubifex added, the other beaker contained noTubifex. After two weeks of exposure, the probes were withdrawn and strips were analysed for their content of cadmium, copper and zinc. Results and discussion  Available concentrations were higher in the upper layer (0–1 cm) of the sediment core, where conditions are generally more oxic. The presence ofTubifex worms led to a decrease of the available concentrations in the upper layer, due to the competition of the worms with the DGT for the available metal fraction. On the other hand, the presence of the worms led, via bioturbation, to an increased oxygen penetration depth in the sediment, and, as a consequence, to an increase of the available fraction in the next-higher sediment layer. Conclusions  In sediment cores without organisms, very steep gradients of the available metal concentration in pore water have been found. The change from a high to low amount of available fraction corresponds to the change from oxidised conditions in the surficial sediment layer to reduced conditions in the lower sediment layers. The tunnelling behaviour ofTubifex worms leads to a penetration of oxygen to deeper sediment layers, and to an increase of the available metal fraction in the pore water.Tubifex competes with DGT for the available fraction, resulting in lower metal amounts accumulated in the DGT probe. Competition is the prevailing effect in the top 1 cm layer of all sediments. In the second 1 cm layer, competition, in most cases, is not strong enough to compensate the increase in metal availability due to the broadening of the oxidised zone. Recommendations  The effects of sediment organisms on the deepening of the oxidised layer and on the availability of metals in pore water need further investigations and quantification. DGT is recommended as an instrument for measuring available metal concentrations in these studies.