EPISODIC FLOW-DURATION ANALYSIS - A METHOD OF ASSESSING TOXIC EXPOSURE OF BROOK TROUT (SALVELINUS-FONTINALIS) TO EPISODIC INCREASES IN ALUMINUM

Episodic flow-duration curves were developed that summarize stream flo w data by considering the duration of sustained flows at or above spec ified levels. These curves and relationships between dissolved aluminu m concentrations and stream flow were combined for five Appalachian (P ennsylvania) streams to determine the frequency of occurrence of acute ly toxic aluminum conditions for brook trout (Salvelinus fontinalis). Concentrations of total dissolved aluminum (Al-TD) increased with stre am flow in all streams, but reached a plateau or decreased during extr eme high flows in some streams. Al-TD was dominated by inorganic monom eric species in all streams except Benner Run and Baldwin Creek. Episo dic Al-TD concentrations and durations were compared with a toxicity c urve developed using caged wild and hatchery-reared brook trout. Resul ts predicted that lethal episodes occurred on Linn, Stone, and Roberts runs, but not on Benner Run and Baldwin Creek. With the exception of Stone Run, brook trout populations were largest in streams without pre dicted lethal episodes (Benner Run and Baldwin Creek) and were smalles t in streams with predicted lethal episodes (Linn and Roberts runs). T his analysis could be adapted for prediction of toxic conditions in st reams for other pollutants where toxicity varies with both concentrati on and duration of exposure.