Article pubs.acs.org/est
Food Quantity Affects the Sensitivity of Daphnia to Road Salt Arran H. Brown*,† and Norman D. Yan‡ †
York University, Department of Biology, 4700 Keele Street, Toronto, Ontario M3J 1P3, Canada Dorset Environmental Science Centre (Ontario Ministry of the Environment and Climate Change), 1026 Bellwood Acres Road, Dorset, Ontario P0A 1E0, Canada
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S Supporting Information *
ABSTRACT: Road deicing operations have raised chloride (Cl) levels in many temperate lakes in Europe and North America. These lakes vary widely in trophic status, but to date, no one has quantified the interaction between food quantity and road salt toxicity. We examined the effects of food quantity (particulate algal C concentration (C)) on the chronic toxicity of Cl to Daphnia in soft-water bioassays. There was a strong positive linear relationship (r2 = 0.92 for NaCl and r2 = 0.96 for CaCl2) between food quantity and Cl LC50. As food quantity increased from 0.2 to 1.0 mg C/L (levels characteristic of oligotrophic to eutrophic lakes, respectively), the chronic Cl LC50 increased from 55.7 to 284.8 mg Cl/L. Salt type (NaCl or CaCl2) did not affect the Cl LC50, Daphnia life history parameters, or the intrinsic rate of population increase (r). The life history parameter most sensitive to Cl was neonate production. Cl did not inhibit egg production, nor was the maternal lipid investment in eggs changed, but egg viability and the subsequent release of live neonates decreased as Cl levels increased and food decreased. Our results suggest the trophic status of lakes should be considered when assessing ecological threat from Cl.
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INTRODUCTION Elevated levels of chloride (Cl), attributable to road salt leaching from winter-maintained roads, now represent a widespread environmental threat to surface waters at temperate latitudes in the northern hemisphere. As of 2003, there were approximately 8 million km of roads in North America,1 implying that, on average, there is a road within 1 km of approximately 80% of North American lands. Since 2000, the amount of road salt (NaCl) added to roads in the U.S. has risen to between 10 and 15 million tonnes annually.2,3 Similarly, the amount of road salt added to roads in Canada has risen to 4.9 million tonnes annually.2,3 In Europe, the amount of road salt applied annually on Norwegian roads has more than tripled since 2000 from 70 000 to 238 000 tonnes annually.4 This NaCl does not stay put. Because of its high solubility, Cl is washed into neighboring water tables, thence, downstream. In consequence, Cl levels in freshwater lakes have increased dramatically in northern temperate lakes near winter-maintained roads.5 During the winter months in northern states in the U.S., 55% of 19 lakes and streams close to roads and urbanized areas examined in 2010 had Cl concentrations that exceeded the current U.S. Environmental Protection Agency (EPA) chronic water-quality criteria of 230 mg Cl/L.5 In contrast, the percentage of Cl concentrations in these same lakes and streams exceeding the current USEPA chronic waterquality criteria of 230 mg Cl/L drops to 16% during the summer months.5 Although the potential ecotoxicological threats of Cl to freshwater biota are well documented,3,6−8 © 2015 American Chemical Society
food availability may be an important modifying factor to Cl toxicity that has not as yet been considered. In general, the toxicity of a chemical to an aquatic organism is influenced by its food supply.9 The greater the exposure of an organism to a toxicant, the more energy, and thus the more food that organism requires to counteract toxicant damage. Most toxicity bioassay protocols suggest feeding test organisms above a threshold food concentration at which growth rates saturate for the test organism used; thus any effects of food are not quantified.10 However, ecotoxicologists cannot ignore this potential food effect. Most freshwater lakes in northern Europe (Switzerland, Scandinavia, and other Nordic countries) and North America, especially those in Canada, are oligotrophic, with pelagic particulate carbon levels of 0.1−0.5 mg C/L mainly as phytoplankton,11−13 well below the growth saturation thresholds commonly used in toxicological bioassays for Daphnia. In many culturally eutrophic lakes, the standing stocks of algae are now commonly falling as society improves the management of phosphorus to deal with historical water pollution problems linked to wastewater treatment and agricultural runoff.14 The effect of low and/or falling food levels in lakes on the toxicity of road salt to aquatic herbivores has not been investigated, and needs to be evaluated. Received: Revised: Accepted: Published: 4673
December 17, 2014 March 6, 2015 March 9, 2015 March 9, 2015 DOI: 10.1021/es5061534 Environ. Sci. Technol. 2015, 49, 4673−4680
Article
Environmental Science & Technology In this paper, our purpose is to determine if food quantity influences the toxicity of Cl (provided as either a Ca or Na salt) to an aquatic herbivore, Daphnia, in soft-water lakes. We hypothesized that Cl toxicity to Daphnia would rise as food concentrations were lowered below levels needed to saturate growth. In addition, we hypothesized that there would be an interaction of food and Cl concentrations not just on the toxicity of road salt to Daphnia, but also on their development rates and reproduction, and thus on the intrinsic rate of population increase, a better ecotoxicological end point for extrapolation to population responses.15 Should our results support our hypotheses, there would be clear implications for the winter maintenance of roads traversing north temperate watersheds characterized by oligotrophic lakes, assuming that one criterion of such maintenance is lack of damage to the biota in receiving waters.
PK was cultured in a modified Bristol’s medium within a CONVIRON Cmp4030 controlled environment chamber (temperature 20 °C, 16:8-h light:dark cycles using fluorescent lighting (light intensity of 100 μmol m−2s−1 (7400 lx)), harvested in log phase, and stored prior to use at 4 °C in clear 50 mL polypropylene Falcon tubes (VWR Canada). The Bristol’s medium had an N concentration of 41.0 and a P concentration of 53.1 mg/L. To avoid the concentration of NaCl in this media affecting our results, most of the Bristol’s medium was siphoned from the settled stock solution and replaced with an equal amount of FLAMES medium, prior to use. We expressed algal food concentrations as particulate carbon (C) concentration per L of FLAMES medium. Levels of C were verified by particulate carbon analysis of samples taken from stock and working solutions of PK; C analyses were conducted by the Ontario Ministry of the Environment DESC water quality laboratory in Dorset, Ontario, Canada using a catalytic 630 °C combustion total organic carbon analyzer (Shimadzu Scientific Instruments, Inc., MD). On average, measured levels of C deviated from nominal concentrations by 4%. 14-Day Chloride (CaCl2 and NaCl) and Food Quantity Assay. We used
