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Episodic Acidification of Coastal Plain Streams: An Estimation of Risk to Fish

Gerritsen, Jeroen, Dietz, Jonathan M., Wilson, Harold T., Jr.
Ecological applications 1996 v.6 no.2 pp. 438-448
Alosa aestivalis, Alosa pseudoharengus, Perca flavescens, acidification, base flow, coastal plains, cumulative distribution, fish larvae, herring, juveniles, migratory behavior, models, mortality, pH, prediction, rain, regression analysis, risk, risk estimate, rivers, spring, streams, variance, watersheds, Chesapeake Bay
Episodic acidification is common in poorly buffered streams of the coastal plain in the Chesapeake Bay watershed, and may harm sensitive life stages of migratory fish that spawn in small streams of the region. We developed a model to predict the risk of episodic acidification in these streams that is likely to be harmful to juvenile fish. Risk predictions were based on a multiple‐regression model of acidification episodes, coupled with distribution of precipitation in the coastal plain. The regression model had four independent variables: mean baseflow, acid‐neutralizing capacity (ANC), event rainfall, a 14‐d precipitation index, and a 120‐d precipitation index; this model explained 74% of the variance in episodic pH. Bulk rainfall pH was not a significant predictor. A 30‐yr precipitation record was used to generate input data to estimate cumulative frequencies of low‐pH episodes, which also took into account the standard error of the regression. The cumulative distribution of minimum pH in streams of given ANC was in turn used to estimate the risk that fish larvae would be exposed to one or more episodes below a critical, harmful pH. Results for blueback herring and alewife larvae suggest that risk of exposure to harmful episodes (below critical pH) is ≈ 35%, for the larval stage during an average spring, in streams with baseflow ANC of 200 μ molc/L. The risk was estimated to be as high as 80% in the same streams during a wet spring. For yellow perch larvae, the estimated risk in the same streams (baseflow ANC = 200 μ molc/L) is negligible, because the critical pH for yellow perch larvae is lower than that for the herrings. The risk analysis suggests that larval mortality of river herrings due to acidification may be substantial during wet years, with > 50% of coastal plain streams affected. Management of these species should consider the risks of occasional large losses of precruitment larvae.