Potash mining effluents and ion imbalances cause transient osmoregulatory stress, affect gill integrity and elevate chronically plasma sulfate levels in adult common roach, Rutilus rutilus

Abstract

Secondary salinization is a growing global ecological issue. One cause is the discharge of effluents by the potash mining industry into surface waters such as the River Werra in Germany. Increases of major ions require various physiological responses of freshwater organisms to maintain the hydromineral balance of body fluids. However, only little is known about the acute and chronic effects of high concentrations and imbalances of ions on osmoregulation in freshwater teleosts. The present study aimed to elucidate the effects of potash mining effluents and different cation ratios on the osmoregulatory capacity and gill histopathology of a native fish species. Individuals of Rutilus rutilus were exposed to the currently allowed (HT) and intended future (LT) thresholds as well as to high concentrations of Mg²⁺ (Mg), K⁺ (K), and Mg²⁺ and K⁺ (Mg + K) for a period of 24 h, 7 d, 21 d and 8 wk. Plasma osmolarity, [Na⁺], [Mg²⁺], [K⁺], [Ca²⁺], [Cl^-] and [SO₄^2-] and branchial Na⁺/K⁺-ATPase activity were determined. Moreover, histological gill alterations after 21 d and muscle water content after 8 wk were examined. HT transiently (24 h) elevated plasma osmolarity, plasma [Na⁺] and [Ca²⁺], whereas [SO₄^2-] was chronically increased even after 8 wk. Exposure to LT, Mg and Mg + K led to increased [SO₄^2-] levels for at least 21 d. It seems that [SO₄^2-] is mainly disturbed by multiple ions at high concentrations and long-term effects are unknown. Hydromineral homeostasis was maintained as indicated by unchanged Na⁺/K⁺-ATPase activity and muscle water content. However, mild structural alterations of the gills were observed in all exposure groups suggesting adaptational responses but with the potential to affect gas exchange capacity. Hence, the current thresholds for potash mining effluents affect osmomineral regulation in roach and further investigations should address potential impacts on reproduction in native fish species and physiological effects of SO₄^2-.

Keywords: Electrolytes; Gill histology; Ion imbalances; Osmoregulation; Plasma sulfate.