The effects of lead, water hardness and pH on oxygen consumption, plas- ma chlorides and bioaccumulation in the freshwater fish Tilapia sparrmanii

Abstract

Closed system respirometry was performed on captive juvenile Tilapia sparrmanii exposed for 96 hours to a range of Pb-acetate concentrations in hard and soft water to determine the effect of Pb in relation to water hardness and pH. For hard and soft water with a pH above 7.51 no change in the resting specific oxygen consumption rate (MO2) was found when the fish were exposed to nominal concentrations of 1, 5, 10, or 20mg Pb l–1 normoxic water. In soft, acidic normoxic water (pH 6.28) containing 10mg l–1 Pb-acetate, MO2 increased significantly (P < 0.05) above normal values from 4.38 to 6.36 ± 0.9mmol O2 kg–1 fish h–1. In progressive anoxic water (PO2 from 130mm Hg to 15mm Hg) resting MO2 decreased by nearly 85% compared with normoxic conditions. This decrease seemed not to be influenced by acute Pb-acetate exposure but to be due to acute oxygen deprivation. Handling (transport from maintenance tanks to respirometers) increased the MO2 by 35%, but this decreased to normal levels (4.38 ± 0.41mmol O2 kg–1 fish h–1) for a standardised (30 gram) fish four to six hours after handling. The percentage of Pb in solution 96 hours after application, decreased linearly in a range of soft to hard water types to less than 1%. Contrary to that in hard water, in soft water, with a pH of 6.28 a significant (P < 0.05) decrease in plasma chlorides (from 144.5mmol Cl to 111.9mmol Cl) was measured in 10mg Pb I–1 water while a significant increase in soluble blood plasma solids (from 5.68g 100ml–1 to 7.14g 100ml–1) was found. No change in bile solids was found for fish exposed to lead ranging from 1–20mg Pb l–1 in soft or hard alkaline water or soft acidic water. It is concluded that resting MO2 and blood chloride values are not influenced by acute Pb concentrations (10 and 20mg l–1) in hard water or soft alkaline water, but that this changes significantly in acidic soft water and may be used as a biomarker in short term (96h) exposure experiments. Progressive anoxia combined with lead exposure further reduces the ability of the gill epithelium to take up oxygen. Precipitated lead in water bodies may potentially be a health hazard for aquatic organisms if it dissolves from the sediment at low pH.