Influence of waterborne cations on zinc uptake and toxicity in rainbow trout, Oncorhynchus mykiss

The effects of waterborne cations on ⁶⁵Zn uptake, Zn toxicity, and relationships with Ca uptake were examined in juvenile rainbow trout, Oncorhynchus mykiss, in soft water. Whole-body Zn uptake (waterborne [Zn] = 100 μg·L^-1 = 1.5 μM) was greatly reduced by a variety of cations. This reduction was directly related to the concentration of positive charges, regardless of which ion carried that charge. Thus, 1.0 mM Na⁺, K⁺, NH₄/⁺, and N-methyl-D-glucamine⁺ and 0.5 mM Mg²⁺ (divalent) reduced Zn uptake to a similar extent (~50%), indicating a relatively nonspecific competition for anionic sites on the gill. Ca²⁺ was an exception and was more potent at reducing Zn uptake, likely because only Ca²⁺ would also compete for absorption. Although Na⁺ and Mg²⁺ were able to markedly reduce Zn uptake, they had no effect on Zn toxicity (measured with 96-h LC₅₀ tests), a result paralleled by their inability to restore Ca²⁺ uptake that was inhibited by Zn. In contrast, Ca²⁺ reduced Zn toxicity and restored Ca²⁺ uptake. These results partially dissociate Zn uptake from Zn toxicity, implicate disturbed Ca²⁺ uptake as the toxic mechanism, and have profound implications for water quality criteria where Ca²⁺ and Mg²⁺ (the two 'hardness' cations) are traditionally considered to be equally protective.