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Development of short-term toxicity test methods to estimate chronic toxicity using the freshwater parthenogenic mayfly, neocloeon triangulifer
Citation:
Soucek, D., A. Dickinson, AND T. Norberg-King. Development of short-term toxicity test methods to estimate chronic toxicity using the freshwater parthenogenic mayfly, neocloeon triangulifer. SETAC Europe 2019, Helsinki, N/A, FINLAND, May 26 - 30, 2019.
Impact/Purpose:
Clean Water Act (CWA) implementation starts with the development of water quality criteria to protect uses, such as aquatic life. While aquatic life is exposed to innumerable chemical mixtures in the nation’s waters, EPA has developed just 45 numeric aquatic life criteria for use in National Pollution Discharge Elimination System (NPDES) permits and assessment of the nation’s waters. There is an urgent need to better protect aquatic life (including species federally-listed per the Endangered Species Act, or ESA) by establishing additional test methods for sensitive aquatic life. Historically, the choice of invertebrate species used in most current water and sediment toxicity test protocols represent a combination of practical and ecological considerations. This has led to the development of widely applied toxicity test protocols that incorporate reliable, though not necessarily highly sensitive, test species. Early exploration of candidate species for toxicity testing included evaluation of EPT (Ephemeroptera, Plecoptera, Trichoptera) taxa. However, many of these were found to be difficult to culture (e.g., many EPT taxa require flowing water), biological requirements were not well known, most would not reproduce under laboratory conditions, and many were sensitive to the laboratory environment (often failing to meet control test acceptability criteria during tests). In recognition of recent advances in culturing and application in aqueous and sediment-based toxicity testing of EPT species, a project was initiated to report on the state of the science for the application of mayflies in ecotoxicity testing. Key goals of the project include the identification and prioritization of knowledge gaps that can serve as areas for future research, the assessment of the prospect for routine incorporation of mayflies in toxicity testing, and the creation of standardized toxicity test protocols followed by a mayfly workshop in September. When we have additional methods available, they can be used by EPA, state, and tribal NPDES permitting programs and can support the completion of ecological risk assessments conducted in accordance with the CWA, ESA and other laws.
Description:
For decades, toxicity tests with aquatic invertebrates have been conducted and yet a small number of model organisms are routinely used. Test organisms are usually easily cultured in the laboratory, have rapid life-cycles, exhibit sensitivity to a variety of pollutants with reproducible results, and are generally available year-round. The US EPA effluent testing program uses short-term chronic freshwater tests (4d to 8d) with cladocerans (Cladocera, Ceriodaphnia dubia), green algae (ýSphaeropleales, Raphidocelis subcapitata) and fish (Cypriniformes, Pimephales promelas). These species have been used extensively in acute and short-term toxicity tests using EPA standardized methods to assess the hazard of chemicals and effluents in freshwater environments. EPA has standardized Hyalella azteca (Amphipoda) and Chironomus dilutus (Dipteran) test methods for sediments; yet EPAs effluent and ambient testing manuals don’t provide acute or short-term test methods for H. azteca, C. dilutus or mayflies (Ephemeroptera,Hexagenia, Neocloeon).To add a sensitive insect, we have focused on using the mayfly, N. triangulifer, as it is parthenogenetic, has a short life cycle (~30d at 25C), and is sensitive to various toxicants. While methods for conducting acute 4d and chronic (~25-30d) toxicity tests with this mayfly have been published, a need exists to extend and standardize the methodology for applicable methods for testing in short-term exposures (e.g., 7d or 10d). Studies began with identifying an optimal starting age, test duration, and optimal sublethal endpoint for whole effluent toxicity testing. While others have compared the sensitivity of this species at 0d, 3d, & 5d old, we sought to further investigate this question with independent experiments comparing 0d and 7d old organisms in 7d and 14d tests. We developed a length versus dry weight relationship for this mayfly; dry weight is a more sensitive endpoint than length, but length is easier to measure more consistently and accurately with young instars. Efforts to refine the various aspects of diatom culture technique on food quality and therefore mayfly growth are underway and optimizing the diet for these organisms may be critical for achieving consistently high growth rates with low intra-treatment variability. Results of the study should provide data needed to guide the development of a toxicity test method to support NPDES permit decision-making.