As used in this chapter:
    (1) “”Acute toxicity”” shall mean a concentration greater than one-third (1/3) of the amount lethal to 50% of the test organisms in 96 hours (96 hr LC50) for a species protective of the indigenous aquatic community for a substance not identified in Fl. Admin. Code R. 62-302.500(1)(c), or for mixtures of substances, including effluents.
    (2) “”Annual average flow”” is the long-term harmonic mean flow of the receiving water, or an equivalent flow based on generally accepted scientific procedures in waters for which such a mean cannot be calculated. For waters for which flow records have been kept for at least the last three years, “”long-term”” shall mean the period of record. For all other waters, “”long-term”” shall mean three years (unless the Department finds the data from that period not representative of present flow conditions, based on evidence of land use or other changes affecting the flow) or the period of records sufficient to show a variation of flow of at least three orders of magnitude, whichever period is less. For nontidal portions of rivers and streams, the harmonic mean (Qhm) shall be calculated as