Seasonal variation in pH and alkalinity and recruitment of sunfish populations

 In a study of five ponds sensitive to acid precipitation, we document seasonal acidification profiles and assess the impact of short‐term acid pulses on the reproductive success of resident sunfish (Lepomis spp.).  Three years of water sampling at 2–3 week intervals showed substantial seasonal variation in pH and alkalinity consistent with a carbonate buffering system. Though all ponds shared a common seasonal pattern in pH, ponds with relatively low pH and alkalinity showed the greatest variation in these parameters. Spring minima may dispose some of the ponds towards episodes of extreme acidity during heavy spring rains.  Otolith analysis of young‐of‐the‐year sunfish revealed recruitment failures for eggs laid early in spring in ponds with relatively low alkalinity and pH, and, in the most extreme case, missing day classes at subsequent irregular intervals even though average pH and alkalinity were well above those demonstrated to affect centrarchid fishes.  Age‐class distributions of sunfishes revealed gaps in adult age distributions which could be traced through 3 years of the study, but there was no clear‐cut relationship between pond acidification and the age structure of adult fish.  Seasonal profiles of acidity may enable researchers to predict the time during which a pond or lake may be highly sensitive to acid inputs. Comparative otolith analysis of young‐of‐the‐year fishes and short‐term continuous monitoring of water chemistry may provide an early warning of biological effects of acidification in sensitive bodies of water.