LTREB Renewal: Climate driven acidification in lowland Neotropical streams: building on a 30-year study of groundwater-surface water interactions

North Carolina State University, Raleigh NC

Investigators

Abstract

Ongoing long-term studies in Costa Rica streams have found strong implications of drought on the relationship between the water chemistry and the adjacent forest soils. Prior long-term studies in environmental biology uncovered the occurrence of acidification events in these lowland streams. Acidification events are short periods, from hours to days, when the water becomes moderately acid (low pH values). Their occurrence is the result of interactions between the stream and the surrounding forest and result from changes in rainfall. During the dry season organic materials accumulate on the forest floor, when the rains start these materials breakdown, releasing carbon dioxide that moves with the water into streams, creating acid conditions in streams (or lowering the water pH). Acidification events become extreme during El Niño years, which in Costa Rica has drier and wetter seasons than normal. Changes in acidification affect how streams function and the associated stream biodiversity. Projected changes in rainfall associated with global climate change could exacerbate the effects of acidification events on stream ecosystems. Prior research in lowland streams in Costa Rica indicate that stream invertebrates and algae can withstand moderately acid conditions, but are affected when conditions are extreme or more frequent. This Long-term Research in Environmental Biology (LTREB) award will investigate acidification effects on stream ecosystems in lowland Costa Rica. Research will take place at La Selva Biological Station, on the Caribbean slope of Costa Rica, where streams have been under study for more than 30 years. The main objective of this award is to understand the mechanisms and consequences of changing rainfall patterns on the acidity and biotic response of tropical streams. This study would provide training opportunities for graduate students as well as outreach to the public. This award will explore the consequences of the episodic acidification events and the hypothesis that an influx of soil-derived CO2 via subsurface flow paths contributes to those pH declines, signaling a tight coupling among rainfall, terrestrial, and aquatic ecosystems. Climate-driven acidification events are stronger and more common in solute-poor, poorly-buffered, streams than in solute-rich, well-buffered, streams - another characteristic of the study landscape. The award will continue evaluating this hypothesis by addressing three main objectives: (1) to assess relations among daily, seasonal, annual, and decadal patterns in stream water chemistry with major climate events (e.g., El Niño and La Niña - ENSO), focusing on extreme events using high frequency sampling. (2) Assess biotic responses to climate-driven acidification, focusing on event duration under laboratory conditions. Finally, (3) to supplement the experimental buffering of a low-solute stream with laboratory experiments of individual taxa. An analysis of the long-term records of stream macroinvertebrates indicates an overall decline in insects and strong differences in assemblage composition between solute-poor, poorly-buffered, streams compared to solute-rich, well-buffered, streams. Climate trends and global circulation models predict changes in the frequency and intensity of weather extremes. For Central America, models predict greater seasonality, similar to patterns observed during ENSO years. In the Caribbean lowlands of Costa Rica, El Niño events result in abnormally low precipitation during the dry season, while La Niña results is high precipitation. Thus, the award will advance the understanding of the consequences of extreme climate events on tropical lowland stream ecosystems. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.