Rainfall-Driven Amplification of Seasonal Acidification in Poorly Buffered Tropical Streams Gaston E. Small, 1 * Marcelo Ardo ´n, 2 Alan P. Jackman, 3 John H. Duff, 3 Frank J. Triska, 3 Alonso Ramı ´rez, 4 Marcı´aSnyder, 5 and Catherine M. Pringle 5 1 Department of Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, Minnesota 55108, USA; 2 Department of Biology, East Carolina University, Greenville, North Carolina 27858, USA; 3 Water Resources Division, U.S. Geological Survey, Menlo Park, California 94025, USA; 4 Institute for Tropical Ecosystem Studies, University of Puerto Rico, San Juan, Puerto Rico 00931, USA; 5 Odum School of Ecology, University of Georgia, Athens, Georgia 30602, USA ABSTRACT Acidification in freshwater ecosystems has impor- tant ecological and biogeochemical effects. Tem- perate streams affected by anthropogenic acidification have been extensively studied, but our understanding of natural acidification in tropical streams has been constrained by the lack of long- term datasets. Here, we analyze 14 years of monthly observations from 13 sampling stations in eight tropical streams in lowland Costa Rica. Stream pH increased during the 4-month dry sea- son and declined throughout the wet season. The magnitude of the seasonal pH decline was greatest following the driest dry seasons, including the historically large El Nin ˜o Southern Oscillation event in 1998 when pH values dropped below 4.0 in some streams. Dissolved CO 2 accounts for the low baseline pH in the poorly buffered study streams, and we hypothesize that an influx of soil- derived CO 2 via subsurface flow paths contributes to the observed seasonal pH declines. Our results show tight coupling between rainfall, terrestrial, and aquatic ecosystems in the tropics. Predicted decreases in dry season rainfall for the tropics may lead to an increased magnitude of seasonal acidifi- cation. Key words: acidification; carbonic acid; climate; CO 2 ; El Nin ˜ o; stream; tropical. INTRODUCTION The causes and consequences of acidification in aquatic ecosystems have been the focus of exten- sive study in recent decades (for example, Likens and others 1972; Doney and others 2009). Many aquatic species are highly sensitive to declines in pH, and acidification can lead to decreased species richness and simplified food webs (Sutcliffe and Carrick 1973; Hall and others 1980; Fabry and others 2008). Besides its direct effect on organisms, pH is also a master variable for biogeochemical reactions, potentially altering ecosystem nutrient dynamics by controlling interactions between dis- solved nutrients and mineral surfaces (Stumm and Morgan 1996). Potential drivers of acidification in aquatic ecosystems include dilution of acid neu- tralizing capacity (ANC), inputs of mineral acids (as in acid precipitation), oxidation–reduction (redox) Received 22 November 2011; accepted 2 May 2012; published online 22 June 2012 Electronic supplementary material: The online version of this article (doi:10.1007/s10021-012-9559-6) contains supplementary material, which is available to authorized users. Author Contributions: C. M. P. and F. J. T. designed the study, and all authors contributed to performing the research. G. E. S. led the data analysis and writing with contributions from all authors. *Corresponding author; e-mail: gesmall@umn.edu Ecosystems (2012) 15: 974–985 DOI: 10.1007/s10021-012-9559-6 Ó 2012 Springer Science+Business Media, LLC 974