Costs of chronic waterborne zinc exposure and the consequences of zinc acclimation on the gill/zinc interactions of rainbow trout in hard and soft water

Juvenile rainbow trout were exposed to zinc in both moderately hard water ( hardness = 120 mg CaCO3/L, pH = 8.0, Zn = 150 mu g/L or 450 mu g/L) and sof t water (hardness = 20 mg CaCO3/L, pH = 7.2, Zn = 50 mu g/L or 120 mu g/L) for 30 d. Only the 450 mu g/L zinc-exposed fish experienced significant mor tality (24% in the first 2 d). Zinc exposure caused no effect on growth rat e. but growth affected tissue zinc Levels. Whole body zinc levels were elev ated, but gills and liver showed no consistent increases relative to contro ls over the 30 d. Therefore, tissue zinc residues were not a good indicator of chronic zinc exposure. After the 30-d exposure. physiological function tests were performed. Zinc was 5.4 times more toxic in soft water (control 96 h LC50s in hard and soft water were 869 mu g/L and 162 mu g/L, respectiv ely). AII zinc-exposed trout had acclimated to the metal, as seen by an inc rease in the LC50 of 2.2 to 3.9 times over that seen in control fish. Physi ological costs related to acclimation appeared to be few. Zinc exposure had no effect on whole body Ca2+ or Na+ levels, on resting or routine metaboli c rates, or on fixed velocity sprint performance. However, critical swimmin g speed (U-Cru) was significantly reduced in zinc-exposed fish, an effect t hat persisted in zinc-free water. Using radioisotopic techniques to disting uish new zinc incorporation, the gills were found to possess two zinc pools : a fast turnover pool (T-1/2 = 3-4 h) and a slow turnover pool (T-1/2 = da ys to months). The fast pool war much larger in soft water than in hard wat er, but at most it accounted for <3.5% of the zinc content of the gills. Th e size of the slow pool was unknown, but its loading rate was faster in sof t water. Chronic zinc exposure was found to increase the size of the fast p ool and to increase the loading rate of the slow peal.