Effects of long term sublethal Cd exposure in rainbow trout during soft water exposure: implications for biotic ligand modelling

The objectives of the study were to determine the physiological and toxicol ogical effects of chronic cadmium exposure on juvenile rainbow trout in sof t water. Particular attention focused on acclimation, on comparison to an e arlier hard water study, and on whether a gill surface binding model, origi nally developed in dilute soft water, could be applied in this water qualit y to fish chronically exposed to Cd. Juvenile rainbow trout, on 3% of body weight daily ration, were exposed to 0 (control), 0.07, and 0.11 mu g l(-1) Cd [as Cd(NO3)(2). 4H(2)O] in synthetic soft water (hardness = 20 mg l(-1) as CaCO3, alkalinity = 15 mg l(-1) as CaCO3, pH 7.2) for 30 days. Mortalit y was minimal for all treatments (up to 14% for 0.11 mu g l(-1) Cd). No sig nificant effects of chronic Cd exposure were seen in growth rate, swimming performance (stamina), routine O-2 consumption, or whole body/plasma ion le vels. In contrast to the hard water study, no acclimation occurred in eithe r exposure group in soft water, with no significant increases in 96-h LC50 values. Cadmium accumulated in a time-dependent fashion to twice the contro l levels in the gills and only marginally in the liver by 30 days. No signi ficant Cd accumulation occurred in the gall bladder or whole body. Cadmium uptake/turnover tests were run using radioactive Cd-109 for acute (3 h) exp osures. Saturation of the gills occurred for control fish but not for Cd-ex posed fish when exposed to up to 36 mu g l(-1) Cd for 3 h. Cd-exposed trout accumulated less 'new' Cd in their gills compared to controls and they int ernalized less Cd-109 than control fish. This effect of lowered Cd uptake b y the gills of acclimated trout was earlier seen for the fish acclimated to 10 mu g l(-1) Cd in hard water. The affinity of the gill for Cd was greate r in. hard water (log KCd-gill = 7.6) than in soft water (log KCd-gill = 7. 3) but the number of binding sites (B-max = 0.20 mu g g(-1) gill) was simil ar in both media. Is addition, there was a shift in affinity of the gill fo r Cd (i.e. lowered log KCd-gill) and increased B-max with chronic Cd exposu re in both soft water and hard water. We conclude that the present gill mod elling approach (i.e. acute gill surface binding model or Biotic Ligand Mod el) does work for soft and hard water exposures but there are complications when applying the model to fish chronically exposed to cadmium. (C) 2000 E lsevier Science B.V. Ail rights reserved.