Background/Question/Methods   Acidification of freshwater ecosystems can have harmful biological effects, and the causes and effects of anthropogenic acidification have been well-documented. Effects of seasonality and climate change on natural acidification events have received less attention. In this study, we analyzed a twelve-year dataset of monthly pH measurements collected from streams at La Selva Biological Station, Costa Rica, that are solute-rich (well-buffered) or solute-poor (poorly-buffered) depending on whether they receive inputs of geothermally-derived groundwater.

Results/Conclusions   These data indicate a seasonal pattern in which pH declines by 0.5 units following the onset of the wet season (>250 mm rainfall per month); this pattern was further supported by a 2.5 year dataset of hourly pH measurements in both a high- and low-solute stream. Dry season rainfall explains 60% of the inter-annual variation in wet season pH in low-solute streams. Measurements of dissolved inorganic carbon (DIC) and degassing experiments show that DIC is an important contributor to low baseline pH and seasonal declines in pH. During the wet season of 1998, which followed an historically large El Niño-Southern Oscillation event, pH in six poorly-buffered streams dropped from 5.5 to 4.0 for seven months. Evidence supports our hypothesis that this prolonged acidification event was an amplification of normal seasonal trends, largely due to increases in dissolved carbon dioxide. We hypothesize that low precipitation totals during the 1998 dry season resulted in an increased supply of labile organic carbon in the soil, which resulted in increased soil respiration when the wet season commenced. Based on predicted changes in inter- and intra-annual variation in rainfall for the tropics, we predict increased magnitude of seasonal acidification in unbuffered, solute-poor Neotropical streams, with potentially important biological implications.