Evaluating the acute and chronic toxicity of potassium carbonate and the interactive effects between sodium and potassium on rainbow trout (Oncorhynchus mykiss)

Abstract

Capture-and-reuse of recirculating aquaculture system (RAS) effluents as hydroponic fertilizer can mitigate the high costs of waste treatment and disposal. However, RAS operators must address limitations in macro- (K, Ca) and micro-nutrients (Fe, Zn, Mo, Cu) and the phytotoxic sodium concentrations (\u3e50 mg/L Na) present in solution. One option is to replace sodium-based pH buffering salts such as sodium bicarbonate with potassium-based derivates such as potassium carbonate or bicarbonate. However, findings in previous studies demonstrate adverse sublethal impacts on fish health due to elevated potassium concentrations. Lethal effects of potassium salts (KCl, KHCO3, and K2SO4) have been demonstrated in fathead minnows (Pimephales promelas) and bluegills (Lepomis macrochirus). Research findings indicate that potassium toxicity in fishes may be mitigated by concentrations of other cations such as sodium, magnesium, and calcium. However, published research on potassium toxicity and interactive effects between sodium and potassium on fish health is limited. The objectives of this research were to determine the acute toxicity of potassium carbonate (K2CO3), the interactive acute toxic effects between two salts (K2CO3 and NaCl), and the chronic toxicity of K2CO3 on growth performance of rainbow trout (Oncorhynchus mykiss) cultured in RAS. The acute toxicity of potassium carbonate (K2CO3) was determined using a median lethal concentration (LC50) study. Five concentrations (707, 1502, 2298, 3093, and 3888 mg/L K2CO3) were tested with ten replicates per treatment. Percent mortality was recorded over a 96-hour period and LC50 concentrations were determined at 24, 48, 72, and 96 hours using Probit regression. Two rounds of preliminary mortality studies with three replicates per treatment were used to determine the interaction between sodium and potassium. The first round of studies tested the effect of K: Na ratio and the second round tested the effects of a balanced (1:1) K: Na ratio using the same potassium concentrations (from K2CO3) used in Experiment 1. Percent mortalities were recorded over a 96-hour period and the results of round two were compared to Experiment 1. A grow-out study was conducted to determine the effects of increased potassium concentrations on the growth performance of rainbow trout. Growth data were collected over 79 days in a replicated study where potassium carbonate was used as the pH buffering salt. Feed conversion ratios (FCRs) and specific growth rates (SGRs) were calculated after the study was concluded. The growth data obtained in this study were compared to growth data obtained in Spring 2018. Median lethal concentrations (LC50) of 3470, 3016, 2831, and 2672 mg/L K2CO3 were obtained at 24, 28, 72, and 96-hours, respectively. However, interactive effects between sodium and potassium on acute toxicity were not observed. In the grow-out study, the SGR was significantly higher (p=0.0093) in the potassium treatment (SGR-1.32 ± 0.02 %/day) than in the control (SGR-0.87 ± 0.06 %/day) and the FCR was significantly lower (p=0.013) in the potassium treatment (FCR-1.18± 0.04) than in the control (FCR-2.96 ± 0.22). The results of this research displayed low acute toxicity to elevated potassium concentrations in rainbow trout. Growth performance was also greater in the potassium treatment than in the control. Furthermore, no interaction between sodium and potassium was demonstrated. This potentially indicates that the replacement of potassium carbonate as the sole buffering salt will not negatively impact fish health. However, there are sources of error including the use of different cohorts and the fluctuations in culture water potassium concentrations. Therefore, another grow-out study should be run using the same cohort of fish reared at the same time under different conditions to obtain a more valid understanding of the effects of elevated potassium on fish health