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The US Environmental Protection Agency's (USEPA's) current ambient water quality criteria (AWQC) for lead (Pb) in freshwater were developed in 1984. The criteria are adjusted for hardness, but more recent studies have demonstrated that other parameters, especially dissolved organic carbon (DOC) and pH, have a much stronger influence on Pb bioavailability. These recent studies have been used to support development of a biotic ligand model (BLM) for Pb in freshwater, such that acute and chronic Pb toxicity can be predicted over a wide range of water chemistry conditions. Following USEPA guidelines for AWQC development and using a methodology consistent with that used by the USEPA in developing its recommended BLM-based criteria for copper in 2007, we propose acute and chronic BLM-based AWQC for Pb in freshwater. In addition to the application of the BLM approach that can better account for site-specific Pb bioavailability, the toxicity data sets presented are much more robust than in 1984, and there are now sufficient chronic Pb toxicity data available that use of an acute-to-chronic ratio is no longer necessary. Over a range of North American surface waters with representative water chemistry conditions, proposed acute BLM-based Pb criteria ranged from approximately 20 to 1000 μg/L and chronic BLM-based Pb criteria ranged from approximately 0.3 to 40 μg/L. The lowest criteria were for water with low DOC (1.2 mg/L), pH (6.7), and hardness (4.3 mg/L as CaCO3), whereas the highest criteria were for water with high DOC (9.8 mg/L), pH (8.2), and hardness (288 mg/L as CaCO3). Environ Toxicol Chem 2017;36:2965-2973. © 2017 SETAC [ABSTRACT FROM AUTHOR]

We investigated the in vivo salinity-dependent behavior of transepithelial potential (TEP) in Fundulus heteroclitus (3–9 g) using indwelling coelomic catheters, a technique which was validated against blood catheter measurements in a larger species ( Opsanus beta; 35–70 g). In seawater (SW)-acclimated killifish, TEP was +23 mV (inside positive), but changed to −39 mV immediately after transfer to freshwater (FW). Acute transfer to dilute salinities produced a TEP profile, which rapidly attenuated as salinity increased (0, 2.5, 5 and 10% SW), with cross-over to positive values between 20 and 40% SW, and a linear increase thereafter (60, 80 and 100% SW). TEP response profiles were also recorded after acute transfer to comparable dilutions of 500 mmol L−1 NaCl, NaNO3, Na gluconate, choline chloride, N-methyl- d-glutamate (NMDG) chloride, or 1,100 mosmol kg−1 mannitol. These indicated high non-specific cation permeability and low non-specific anion permeability without influence of osmolality in SW-acclimated killifish. While there was a small electrogenic component in high salinity, a Na+ diffusion potential predominated at all salinities due to the low P Cl/ P Na (0.23) of the gills. The very negative TEP in FW was attenuated in a linear fashion by log elevations in [Ca2+] such that PCl/PNa increased to 0.73 at 10 mmol L−1. SW levels of [K+] or [Mg2+] also increased the TEP, but none of these cations alone restored the positive TEP of SW-acclimated killifish. The very negative TEP in FW attenuated over the first 12 h of exposure and by 24–30 h reached +3 mV, representative of long-term FW-acclimated animals; this reflected a progressive increase in P Cl/ P Na from 0.23 to 1.30, probably associated with closing of the paracellular shunt pathway. Thereafter, the TEP in FW-acclimated killifish was unresponsive to [Ca2+] (also to [K+], [Mg2+], or chloride salts of choline and NMDG), but became more positive at SW levels of [Na+]. Killifish live in a variable salinity environment and are incapable of gill Cl− uptake in FW. We conclude that the adaptive significance of the TEP patterns is that changeover to a very negative TEP in FW will immediately limit Na+ loss while not interfering with active Cl− uptake because there is none. Keeping the shunt permeability high for a few hours means that killifish can return to SW and instantaneously re-activate their NaCl excretion mechanism. [ABSTRACT FROM AUTHOR]