Gill reactivity of aluminium-species following liming

In acidified aluminium (Al) rich freshwater positively charged Al-species (Ali) are the key toxic components due to the accumulation in fish gills. As a countermeasure, liming is used to increase the pH and reduce the concentration of Ali-species; in particular low molecular mass (LMM) Ali-species by hydrolyses. However, very toxic high molecular mass (HMM) Al polymers can form in the unstable mixing zone immediately after liming. In the present work gill reactivity of LMM and HMM Al-species was studied under controlled conditions in eight channel–tank mixing zone systems in the field where Atlantic salmon (Salmo salar L.) kept in cages were exposed to defined mixing zone water. Mixing zones were created by continually liming acid river water (pH 5.0–5.7) high in LMM Ali to pH 6.0 and 6.4, respectively. Transformation processes affecting the Al-speciation as a function of time after liming were documented by in situ hollow fibre ultrafiltration interfaced with ion chromatography, while the Al accumulation in fish gills was used as bioindicator. For fish exposed to mixing zone water immediately after liming (1 min) the Al accumulation in gills (Ag Al g 1) was higher (factor of 2) than for fish exposed to acid water prior to liming, due to the formation of gill reactive HMM Ali-species. The Al accumulation in gills followed a first order kinetic expression reaching steady-state conditions after 24-h exposures. The deposition rate of Al in gills (Ag Al g 1 h 1) correlated with the water concentrations of HMM Ali (R2=0.80) in the mixing zones, and for LMM Ali in the acid water (R2=0.92). Due to the transient nature of HMM Ali the deposition rate of Al decreased from the point of liming with a factor of 10 downstream the channel–tank system (i.e. 100 min after liming). The concentration of gill accumulated Al was higher (factor of 3) immediately after high level liming (pH 6.4) than following low level liming (pH 6.0). However, high level liming was more efficient in detoxifying Al in downstream waters. Furthermore, the bioavailability of a given LMM Ali concentration as well as bioreactivity following liming was dependent on the TOC and silicon concentration in the acid water. Increased concentration of TOC (1.5 to 4.3 mg l 1) and silicon (0.3 to 1.0 mg l 1) reduced the gill reactivity of Ali by approximately 50%.