Empact of dredging in a shallow coastal lagoon: a sediment quality triad approach. II. Microtox® basic solid-phase test

The aim of this work was to compare toxicity responses of the marine bacteria Vibrio fisheri with surface sediment samples collected from a shallow coastal lagoon (Pialassa Baiona, northern Adriatic italian coast) before and after dredging activities. Natural and man-made changes over the time, lead to existing physiographic features including shallow brackish ponds, isolated or semi-isolated by embankments, and crossed by a network of artificial channels dredged since 1850. The shallowness of the basins, the pollutant and nutrient loads, and seasonal algal blooms have severely affected the lagoon in the past decades. In an attempt to overcome the silting up of channels and restore the water circulation, a Channel Deepening Project (CDP) has been set for the main inner channel of the lagoon. According to the CDP, four replicates sediment samples were collected with a Wildco® box corer at each of the twelve stations in channels and ponds of the lagoon selected as control and impact areas during the pre-dredging phase. Grain size and organic carbon matter content as loss of ignition were measured. Parallel sediment toxicity testing with V. fisheri was carried out on bulk sediments with a Microtox® 500 analyser according to the Microtox® Basic Solid-Phase Test (BSPT) protocol. The performance of the Microtox® BSPT was tested in order to (a) determine if sediment grain size and organic carbon effects were significant enough to bias the interpretation of toxicity results; (b) evaluate if this sediment bias could affect the interpretation of results by overestimating the potential for contaminant effects especially in black anoxic sediments; (c) evaluate an hypothesis regarding the cause of low light outputs with silty sediments.