Mobilisation of river transported colloidal aluminium in estuaries and subsequent deposition on fish gills - sodium silicate as countermeasure

During flooding events, increased concentrations of toxic Al and negative effects on fish have been identified in Norwegian fjords, in experimental flow through tank systems receiving mixtures of seawater and river water (6 ¿) and in landbased fishfarms using mixtures of freshwater and seawater. Sea salt effects can mobilize Al from poorly buffered soils, and Al associated with river transported organic material can be mobilised in estuarine waters. Subsequently increased Al deposition on fish gills can occur in estuarine water causing fish mortality and/or reduced homing of wild Atlantic salmon. As a countermeasure sodium silicate has been continually added to Al-rich river water prior to mixing with seawater in short and long term experiments. Based on in situ Al fractionation of waters and sampling of gills from exposed fish, results showed that the concentration of mobilized Ali in water and the concentration of accumulated Al on fish gills decreased significantly in waters containing excess silicates.