FresliuKiter Biology (1993) 30, 409-417 Seasonal variation in pH and alkalinity and recruitment of sunfish populations J.N. JARRETT,* M.B. CUTLER, J.P. EBERSOLE AND W.G. HAGAR University of Massachusetts, Boston Harbor Campus, Boston, MA 02125, U.S.A. ^Present address: Tufts University, Medford, MA 02155, U.S.A. SUMMARY 1. In a study of five ponds sensitive to add precipitation, we document seasonal , acidification profiles and assess the impact of short-term acid pulses on the reproductive success of resident sunfish (Lqyomis spp.). 2. 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. 3. 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. 4. 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 tbe age structure of adult fish. 5. Seasonal profiles of acidity may enable researchers to predict the time during which a pond or lake may be highly sensitive to add 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 addification in sensitive bodies of water. Introduction Evidence of the impact of add deposition on water chemistry comes from studies of spring snowmelt and corresponding changes in alkalinity (Galloway ct al., 1987; Semkin & Jeffries, 1988; Molot, Dillon & Lazarte, 1989), controlled lake experiments (Schindler et aL, 1985; Watras & Frost, 1989; Webster et al., 1992), historical (long-term) and recent (short-tenn) measurements of water chemistry (Battarbee et aL, 1988), and palaeolimnologica! studies of diatom (Dixit, Dixit & Smol, 1989) and chrysophyte remains (Smol, 1986). Natural variation in pH and alkalinity, however, makes it difficult to determine both present conditions and long-term changes in conditions of surface waters. Susceptibility of surface water to acidification varies in time because pH and alkalinity are subject to natural seasonal fluctuations. In particular, acid rainfall and acid snowmelt may have a greater impact in spring when alkalinity and pH are often low (Kratz et aL, 1987). Aquatic biota may be severely affected by transient acid pulses during seasonal periods of depressed pH and alkalinity which may be missed by spot-check measurements. Diel and seasonal fluctuations in pH and alkalinity due to natural lake processes must be identified and their magnitude detennined if accurate assessments of addification are to be made. Recruitment failure seems to be the most import- ant cause of declines in fish populations (Beamish et aL, 1975; Mills et aL, 1987). Although addification may affect reproductive physiology and behaviour adversely (Beamish et aL, 1975; Schofield, 1976; McCormick, Stokes & Hermanutz, 1989; Hesthagen et aL, 1992), vulnerability of early life stages (eggs