EPISODIC ACIDIFICATION OF SMALL STREAMS IN THE NORTHEASTERN UNITED-STATES - EFFECTS ON FISH POPULATIONS

As part of the Episodic Response Project (ERP), we studied the effects of episodic acidification on fish in 13 small streams in the northeas tern United States: four streams in the Adirondack region of New York, four streams in the Catskills, New York, and five streams in the nort hern Appalachian Plateau, Pennsylvania. In situ bioassays with brook t rout (Salvelinus fontinalis) and a forage fish species (blacknose dace (Rhinichthys atratulus), mottled sculpin (Cottus bairdi), or slimy sc ulpin (Cottus cognatus), depending on the region) measured direct toxi city. Movements of individual brook trout, in relation to stream chemi stry, were monitored using radiotelemetry. Electrofishing surveys asse ssed fish community status and the density and biomass of brook trout in each stream. During low flow, all streams except one had chemical c onditions considered suitable for the survival and reproduction of mos t fish species (median pH 6.0-7.2 during low flow; inorganic Al < 60 m u g/L). ERP streams with suitable conditions during low flow, but mode rate-to-severe episodic acidification during high flow, had higher fis h mortality in bioassays, net downstream movement of brook trout durin g events, and lower brook trout density and biomass compared to nonaci dic streams, and lacked acid-sensitive fish species (blacknose dace an d sculpin). Movement of trout into refugia (areas with higher pH and l ower Al) during episodes partially mitigated the adverse effects of ep isodes. Recolonization from alkaline tributaries or microhabitats can maintain low densities of fish in streams that experience severe acidi c episodes, but it is not sufficient to sustain fish densities and com munity composition at levels expected in the absence of these episodes . Fish responses to acid-base chemistry were fairly consistent across regions. In general, trout abundance was reduced and acid-sensitive fi sh species were absent from ERP streams with median pH < 5.0-5.2 durin g high flow and inorganic Al > 100-200 mu g/L. We conclude that episod ic acidification can have long-term effects on fish communities in sma ll streams.