THE INFLUENCE OF WATER HARDNESS, pH, AND SUSPENDED SOLIDS ON NICKEL TOXICITY TO LARVAL FATHEAD MINNOWS (PIMEPHALES PROMELAS) G. G. PYLE 1∗ , S. M. SWANSON 2 and D. M. LEHMKUHL 1 1 Department of Biology, University of Saskatchewan, 112 Science Place, Saskatoon, SK, S7N 5E2, Canada; 2 Golder Associates, 10th Floor, 940 – 6th Ave. S.W., Calgary, AB T2P 3T1, Canada ( ∗ Author for correspondence. Present address: Department of Biology, Laurentian University, Ramsey Lake Rd. Sudbury, ON P3E 2C6, Canada. E-mail: gpyle@nickel.laurentian.ca) (Received 13 July 2000; accepted 21 December 2000) Abstract. Nickel (Ni) is an ubiquitous, naturally occurring metal that is associated with metal min- ing and other industrial activities. Despite elevated Ni concentrations reported for many industrial receiving waters, Ni receives little research attention addressing factors influencing its toxicity to freshwater fish. This study examined the influence of water hardness, pH, and total suspended solids (TSS) in soft, reconstituted water on Ni toxicity to larval fathead minnows (Pimephales promelas). Increasing water hardness from 20 to 140 mg L -1 (as CaCO 3 ) reduced acute Ni toxicity by 5-fold (96-h LC50s 0.45 and 2.27 mg Ni L -1 , respectively). Low pH had a slight protective effect against Ni toxicity relative to neutral pH conditions. At pH 5.5, the 96-h LC50 was 0.69 mg Ni L -1 , compared to 0.54 mg Ni L -1 at pH 7.0. However, Ni toxicity was significantly reduced at pH 8.5 where the 96-h LC50 was 2.21 mg Ni L -1 . These results were explained on the basis of Ni speciation. Total suspended solids also reduced Ni toxicity (expressed as 96-h LC50s) from 0.35 to 1.12 mg Ni L -1 over a TSS range of 10 to 100 mg L -1 . This reduction of toxicity due to TSS is significant because mine effluents often have a combination of elevated TSS and metals. The ameliorative effect of TSS was not as significant as high hardness or pH probably because there is a TSS threshold, after which physical irritation to fish gills counteracts any protective effect conferred by TSS. This finding is relevant to choices made in design of mine effluent treatment systems; i.e., there may be an optimum range of TSS concentrations that protect aquatic biota against effects of metals that remain after treatment. Keywords: fathead minnow, fish, nickel toxicity, pH, total suspended solids, water hardness 1. Introduction Cadmium (Cd), copper (Cu), zinc (Zn), and silver (Ag) toxicity to fish has received considerable research attention. However, nickel (Ni) has received much less at- tention, and yet it has long been known as an important aquatic contaminant, espe- cially in water bodies near industrial operations (Nriagu, 1980). Elevated Ni con- centrations in aquatic systems are associated with steel manufacture (Krantzberg and Boyd, 1992), electrical battery manufacture (Greenwood and Earnshaw, 1984), pesticide formulations (Galvin, 1996), and metal mining (Rutherford and Mellow, 1994; Nriagu et al., 1998). Nickel has been identified as one of the most important aquatic contaminants Water, Air, and Soil Pollution 133: 215–226, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.