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In recirculating rearing systems of aquatic food organisms, humic substances (HSs) accumulate along with toxic nitrogen species (nitrite, unionized ammonia). The aim of the present study was to find out whether or not HSs have the capability to modify the acute toxicity of nitrite and unionized ammonia; and whether different HS qualities cause similar effects in fish. 144-h-Embryo-Larval-Test (ELT) was used for these purposes. Two commercial HS isolates with high reactivity and one natural lake isolate with relatively low reactivity have been tested for their detoxification potential. Results revealed that the HSs of different qualities reduced toxicity of both nitrogen species to different degrees with the lake isolate being least effective. Ion exchange may be one prevalent mechanism with unionized ammonia. For nitrite, equivalent information is not available. Beside the ion exchange mechanisms, the weak pH-buffer capacity of HSs may stabilize the pH and, hence, prevent the formation of the toxic species. Further potential mechanism are discussed, namely the acquisition of multiple stress resistance of HS-exposed embryos. As a practical consequence, the addition of low-cost commercial HS preparation of high reactivity to recirculating aquaculture systems (RASs) is recommended. |
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