Soft waters are characterized by low Ca concentrations, and the distribution and relative success of Ca‐demanding invertebrates could be limited by low Ca in extreme softwater localities. A further Ca depletion caused by reversed acidification could thus seriously affect freshwater crustaceans. Experimental studies on the calcification and Ca content of Daphnia magna clearly suggested the potential of a Ca limitation. Saturated calcification was reached at Ca concentrations >0.13 mM. Individuals reared in media with lower Ca concentrations were unable to compensate by increasing the period of postmolt Ca uptake and thus had a lower specific Ca content. Specific Ca content decreased from 4.2% to 1% of dry weight over the range 0.25–0.013 mM Ca. Even at the low Ca concentrations, only 10% of total Ca was reclaimed upon molting, the rest was lost with the old exuviae (∼40%) or to the ambient medium (∼50%). This incomplete calcification under low ambient Ca concentrations could represent a competitive drawback in Ca‐poor waters.

This study focuses on the effect of acidic water and aqueous aluminium on the monogenean ectoparasite Gyrodactylus salaris, infecting Atlantic salmon (Salmo salar) parr. G. salaris-infected salmon were exposed to various combinations of acidity and aluminium concentrations. The most pronounced effect was the elimination of parasites after 4 days when 202 micrograms Al/l was added to the water. The effect of aluminium was concentration dependent, but was relatively independent of pH (5.2, 5.6 and 5.9). At the lowest pH of 5.0 the effect of aluminium was enhanced. Acidic aluminium-poor water had no or minor effects on the G. salaris infections except at pH 5.0 where all parasites were eliminated within 9 days. The G. salaris populations increased exponentially in untreated control water. The results show for the first time that aqueous aluminium can, to a limited extent, have a positive effect on fish health. This study emphasizes that basic knowledge about abiotic environmental factors is of importance in order to understand the population dynamics, range extension and dispersal of ectoparasites such as G. salaris. Finally, our results suggest that aluminium treatment could form an effective disinfection method against ectoparasites in hatcheries and laboratories, as well as complementing the controversial rotenone treatments used against natural populations of G. salaris.

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