Rainfall-Driven Amplification of Seasonal Acidification in Poorly Buffered Tropical Streams

Abstract

Acidification in freshwater ecosystems has important ecological and biogeochemical effects. Temperate 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 season 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 Nino Southern Oscillation event in 1998 when pH values dropped below 4.0 in some streams. Dissolved CO2 accounts for the low baseline pH in the poorly buffered study streams, and we hypothesize that an influx of soil-derived CO2 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 acidification.