Mobilization of river transported colloidal aluminium upon mixing with seawater and subsequent deposition in fish gills

During flooding events, increased concentrations of gill-reactive aluminium (Al) have been identified in estuarine water causing high Al accumulation in fish gills. By in situ filtering and ultrafiltering river water (pH 5.5–6.4, 3–5 mg L− 1 DOC) and continually mixing the size fractioned river water with seawater (30 in salinity), Al mobilization was studied in flow-through tank systems (6 in salinity) during a six-week field experiment in Western Norway. Atlantic salmon (Salmo salar L.) kept in tanks were continuously exposed to the mixtures. Based on in situ Al fractionation of the experimental waters and sampling of gills from exposed fish, results showed that Al associated with river transported colloids, probably organic material (≤ 0.45 μm and > 8 kDa) was mobilized to low molecular mass cationic Al-species (LMM Ali) upon contact with seawater. Mobilized Ali-species deposited immediately on fish gills. During high flow conditions with increased amounts of colloidal material, the concentration of mobilized LMM Ali and the Al accumulation in gills of fish exposed to river water–seawater mixtures increased by a factor of 5 and 10, respectively, compared to the input river water. The concentration of mobilized LMM Ali and the Al accumulation in fish gills decreased with time after mixing (from 1 to 30 min) and as high runoff subsided.