In situ Bioassays of Brook Trout (<i>Salvelinus fontinalis</i>) and Blacknose Dace (<i>Rhinichthys atratulus</i>) in Adirondack Streams Affected by Episodic Acidification

In situ bioassays were conducted using native Adirondack brook trout (Salvelinus fontinalis) and blacknose dace (Rhinichthys atratulus) in four headwater streams. Conductivity, pH, temperature, and stage height were monitored continuously, and water samples for laboratory analysis were collected during hydrologic episodes. Fish survived well during baseflow conditions, but during periods of spring snowmelt or large precipitation events, survival was poor. Blacknose dace were more sensitive than brook trout, and mortality was best correlated with the log of median inorganic monomeric aluminum (Al im ) concentration. Brook trout mortality was best correlated with a two-variable model that included dissolved organic carbon (DOC) and a concentration–duration variable (median Al im during the episode times the duration of the episode). Brook trout mortality was inversely correlated with DOC. Bioassay fish that had been in the streams 15–24 d survived episodes better than fish that had either not become acclimatized or recovered from handling. Duration of exposure to acidic episodes was critical. Extended periods of poor water quality resulted in fish mortality and may be more important to native populations than short acidic episodes.