Summary

Institutt for plante- og miljøvitenskap

Aluminium (Al) is present in different physico-chemical forms in natural freshwaters. In acid Al rich freshwaters monomeric cationic Al-species (Ali) are the key toxic components for fish due to the accumulation on gills leading to physiological failure. To counteract the Al toxicity towards fish in acidified freshwater ecosystems, widespread liming is carried out in rivers and lakes. However, very toxic transient Al-species can form in the mixing zone immediately after liming. Thus, knowledge of Al-speciation and transformation processes influencing the gill reactivity of Al-species in unstable mixing zones is essential for management of acidified freshwater systems.

In the present work improved knowledge of Al speciation is gained from mixing zones field experiments performed in several moderately acidified rivers as well as in TOC enriched rivers, located in south and southwest of Norway and in the south of Sweden. Artificial mixing zones were created by continually adding neutralization agents to acidified Al-rich waters in entries of flow-through channel-tank systems. Using in situ Al fractionation techniques in mixing zone experiments, results demonstrated that immediately after liming, low molecular mass (LMM) Ali-species are transformed to high molecular mass (HMM) Ali-species being very reactive towards gills. These transient HMM Ali-species are formed due to hydrolysis and polymerization of LMM Ali-species. The HMM Ali-species formed have a certain lifetime and are transformed to non gill reactive high molecular mass neutral and colloidal Al-species (Alo and Alc, respectively), with time after liming until equilibrium is attained. Al concentration in gills of Atlantic salmon (Salmo salar L.) was used as a bioindicator. Results demonstrated that Al accumulation on gills of fish exposed immediately after liming was significantly higher than for fish exposed downstream the channel (increasing time after pH adjustment) and for fish exposed to the input acidic water. Thus, the transient nature of the toxic HMM Ali-species formed in unstable mixing zones was experimentally demonstrated in the field.

Based on several mixing zone field experiments, a dynamic model describing transformation processes influencing the Al speciation in mixing zones following a sudden increase in pH, is suggested:



The concentration levels of transient Al-species formed in mixing zones (HMM Ali, HMM Alo and HMM Alc) and the rates of transformation (k1, k2 and k3) depended on the input concentration of LMM Ali, as well as the presence of Al complexing ligands (DOC and Si) and pH in mixing zones. Results showed that k1 > k2 > k3 (half lives <30 s, 2-5 min, and >30 min, respectively), and that the ratio k1 : k2 was controlling the concentration of the toxic HMM Ali in mixing zones. For fish exposed to mixing zones, transient Al-species accumulated on gills. Results showed that deposition rate of HMM Ali on gills was significant higher (factor of 2) than for LMM Ali-species, while HMM Alo and HMM Alc were of no biological significance. The Al accumulation on fish gills increased exponentially with the concentration of transient Ali-species in mixing zones (R2 = 0.5).

Based on two field experiments performed at the west coast of Norway, the link between sea salt deposition, Al-speciation and Al toxicity towards fish in rivers was demonstrated. High sea salt deposition during storms increased the sea salt concentration (5-20 mg Cl L-1), decreased pH (from pH 5.3 to 4.6) and increased the concentration of LMM Ali (from 20 to 80 µg L-1) in rivers draining the acidified catchment. For fish exposed to the river waters the Al accumulation on gills increased exponentially to high concentrations (from 200 to 500 µg g-1 dw) with increasing concentration of LMM Ali in water (R2 = 0.7) leading to respiratory and ionoregulatory disturbances as well as fish mortality (up to 50 %). The concentration of LMM Ali in water and Al accumulation on fish gills correlated with the sea salt concentration (mg Cl L-1) in water (R2 = 0.9 and R2 = 0.6, respectively). These variables increased linearly as the concentration of deposited sea salt increased. Episodic events with enhanced concentrations of LMM Ali in water lasted for short periods, while the subsequent Al accumulation on fish gills remained high relatively long time after the episodes. Furthermore, the “memory effect” of the physiological stress response in fish remained high for a longer period of time after the episode (> four weeks).

Based on estuarine mixing zone field experiments performed at the west coast of Norway, results showed that Al-species associated with colloids (10 kDa –0.45 µm) in the river water were mobilized upon contact with seawater. Thus, the concentration of LMM Ali-species in the estuarine mixing zone was higher (factor of 2-18) than in input river water, and increasing during flooding events with high river transport of colloidal Al-species. The pH in moderately acid river waters increased upon contact with seawater and initiated polymerization of mobilized LMM Al-species. For fish gills the Al accumulation was also higher (factor of 2-18) in the estuarine mixing zone than in the input river water, and increased exponentially with the concentration of Ali in the estuary water. During high flow and flooding events in rivers high mobility of Al and increased Al accumulation on gills (from <10 to >200 µg g-1 dw.) were the probably cause of massive mortality of farmed Atlantic salmon in a Norwegian fjord. With increased time after mixing seawater into freshwater the concentration of accumulated Al on fish gills was reduced due to transformation of gill reactive Ali-species to HMM Alo-species being of no biological relevance.

Based on these experiments the present work demonstrated that sea salts deposited in moderately acidified catchment can mobilize Al from soils and that sea salt in estuaries can mobilize Al from colloidal Al-species transported by moderately acidified rivers. Furthermore, the mobilized Al accumulated on fish gills and effects such as fish mortality was observed in freshwater and estuarine mixing zones.

Mixing zone field experiments were also used in the study to test diluted sodium silicate solution (silica lye) as an alternative to liming to counteract the toxicity of transient Al polymers. Results showed that the transformation of LMM Ali and formation of HMM Ali in water, as well as Al accumulation on fish gills and fish mortality were lower using diluted sodium silicate than when using lime. Furthermore, detoxification of Al in fresh water mixing zones was faster using diluted sodium silicate than lime. For estuarine mixing zones the concentration of mobilized LMM Ali that hydrolyzed and polymerized and the concentration of accumulated Al on fish gills decreased significantly by addition of sodium silicate to acidic river water prior to mixing with seawater. Thus, sodium silicate was also observed to detoxify Al in estuarine mixing zones. For sea salt effects in acidified catchments, however, the use of catchment liming was a convincing countermeasure. Due to catchment liming the concentration of LMM Ali and the Al accumulation on gills were significant lower than in river water draining the acidic reference catchment, and due to catchment liming no fish mortality was observed during the sea salt episodes.

In the present work in situ Al fractionation techniques were demonstrated to be essential for obtaining information on Al-speciation in unstable water qualities, and Al-gill concentrations were demonstrated to be useful bioindicators for gill reactive Al-species in waters.


Hans-Christian Teien
“Transformation of Aluminium Species in Unstable Aquatic Mixing Zones Mobility and Bioavailability towards Fish”
Dissertation for the degree of dr. philos. 2005
Norwegian University of Life Sciences, Ås


Oppdatert: 16.08.05
Utskriftsvennlig versjon : Arkiv : Tips en venn