Method development for conducting effluent tests with the mayfly Neocloeon triangulifer

Currently, USEPA relies on three freshwater species for short-term toxic-ity testing in a variety of programs and all tests are 8-d in duration or less. These methods are published in USEPA’s Short-Term Methods for Estimating the Chronic Toxicity of Effluents and Receiving Waters to Freshwater Organisms. The methods use a fish, a cladoceran, and an alga, but to date, there have been no EPA-approved methods to use sensitive invertebrates like mayflies, which are among the most sensitive species tested with metals and major ions, to support NPDES permit decision-making (i.e., via incorporation of whole effluent toxicity (WET) tests). Methods for conducting acute 4-day and full-life chronic (~25-30-d) tox-icity tests with the mayfly (Neocloeon triangulifer) have been published, but there is a need to extend the methodology so that it is applicable for testing effluents and receiving waters in a short-term exposure (e.g., 7 or 10 d). Our studies involved identifying an optimal starting age, test duration, and sub-lethal endpoint for WET testing. Others have compared sensitivity of this species at 0, 3, and 5 days-old, and we sought to further investigate this question with independent experiments comparing 0 and 7 day old organisms in 7 and 14-d tests. We also developed a length versus dry weight relationship for this species with the idea that while dry weight is a more sensitive endpoint than length, length is much easier to consistently and accurately measure with young instars of this species. The other objective of this study was to investigate and further refine various aspects of diatom culture technique on food quality and there-fore mayfly growth. Optimizing diet for these organisms may be critical for achieving consistently high growth rates with low intra-treatment variability. Results of the study will provide further data needed to guide the development of a whole effluent toxicity test method for mayflies.