EPISODIC ACIDIFICATION OF SMALL STREAMS IN THE NORTHEASTERN UNITED-STATES - FISH MORTALITY IN-FIELD BIOASSAYS

In situ bioassays were performed as part of the Episodic Response Proj ect, to evaluate the effects of episodic stream acidification on morta lity of brook trout (Salvelinus fontinalis) and forage fish species. W e report the results of 122 bioassays in 13 streams of the three study regions: the Adirondack mountains of New York, the Catskill mountains of New York, and the Northern Appalachian Plateau of Pennsylvania. Bi oassays during acidic episodes had significantly higher mortality than did bioassays conducted under nonacidic conditions, but there was lit tle difference in mortality rates in bioassays experiencing acidic epi sodes and those experiencing acidic conditions throughout the test per iod. Multiple logistic regression models were used to relate bioassay mortality rates to summary statistics of time-varying stream chemistry (inorganic monomeric aluminum, calcium, pH, and dissolved organic car bon) estimated for the 20-d bioassay periods. The large suite of candi date regressors also included biological, regional, and seasonal facto rs, as well as several statistics summarizing various features of alum inum exposure duration and magnitude. Regressor variable selection and model assessment were complicated by multicollinearity and overdisper sion. For the target fish species, brook trout, bioassay mortality was most closely related to time-weighted median inorganic aluminum. Medi an Ca and minimum pH offered additional explanatory power, as did stre am-specific aluminum responses. Due to high multicollinearity, the rel ative importance of different aluminum exposure duration and magnitude variables was difficult to assess, but these variables taken together added no significant explanatory power to models already containing m edian aluminum. Between 59 and 79% of the variation in brook trout mor tality was explained by models employing between one and five regresso rs. Simpler models were developed for smaller sets of bioassays that t ested slimy and mottled sculpin (Cottus cognatus and C. bairdi) as wel l as blacknose dace (Rhinichthys atratulus). For these forage species a single inorganic aluminum exposure variable successfully accounted f or 86-98% of the observed mortality. Even though field bioassays showe d evidence of multiple toxicity factors, model results suggest that ad equate mortality predictions can be obtained from a single index of in organic Al concentrations during exposure periods.