Influence of diltiazem on fathead minnows across dissolved oxygen gradients


Water resources in many arid to semi‐arid regions are stressed by population growth and drought. Growing populations and climatic changes are influencing contaminant and water chemistry dynamics in urban inland waters where flows can be dominated by, or even dependent on, wastewater effluent discharge. In these watersheds, interacting stressors such as dissolved oxygen (DO) and environmental contaminants (e.g., pharmaceuticals) have the potential to affect fish physiology and populations. Recent field observations from our group identified the calcium channel blocker diltiazem in fish plasma exceeding human therapeutic doses (e.g., Cmin) in aquatic systems impaired due to nonattainment of DO water quality standards (WQS) and criteria. Thus, our study objectives examined: 1) standard acute and chronic effects of DO and diltiazem to fish, 2) influences of DO, at criteria levels deemed protective of aquatic life, on diltiazem toxicity to fish, and 3) whether sublethal effects occur at diltiazem water concentrations predicted to cause a human therapeutic level in fish plasma (therapeutic hazard value, THV). DO x diltiazem co‐exposures significantly decreased survival at typical stream, lake, and reservoir WQS of 5.0 and 3.0 mg DO/L. DO and diltiazem growth effects were observed at 2x and 10x their LC50 values (1.7 and 28.2 mg/L, respectively). Larval fathead minnow swimming behavior following all DO and diltiazem exposures generally decreased and significantly reduced light:dark bursting distance traveled, number of movements, and duration at concentrations as low as the THV. Individual and population level consequences of such responses are not yet understood; however, these observations suggest that assessments with pharmaceuticals and other contaminants may underestimate the effects in fish across DO levels considered protective of aquatic life.