Influence of waterborne cations on zinc uptake and toxicity in rainbow trout,<i>Oncorhynchus mykiss</i>

Abstract

The effects of waterborne cations on 65 Zn 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 + , NH 4 + , and N-methyl-D-glucamine + and 0.5 mM Mg 2+ (divalent) reduced Zn uptake to a similar extent (approx. 50%), indicating a relatively nonspecific competition for anionic sites on the gill. Ca 2+ was an exception and was more potent at reducing Zn uptake, likely because only Ca 2+ would also compete for absorption. Although Na + and Mg 2+ were able to markedly reduce Zn uptake, they had no effect on Zn toxicity (measured with 96-h LC 50 tests), a result paralleled by their inability to restore Ca 2+ uptake that was inhibited by Zn. In contrast, Ca 2+ reduced Zn toxicity and restored Ca 2+ uptake. These results partially dissociate Zn uptake from Zn toxicity, implicate disturbed Ca 2+ uptake as the toxic mechanism, and have profound implications for water quality criteria where Ca 2+ and Mg 2+ (the two "hardness" cations) are traditionally considered to be equally protective.