Collaborative LTREB: Acidification in lowland Neotropical streams: building on a 25-year study of groundwater-surface water interactions

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Long-term studies in tropical streams in Costa Rica have documented the occurrence of rainfall-driven acidification events in lowland streams. Acidification events are short periods of time, from a few days to weeks, when the stream water becomes moderately acidic. These events result from the breakdown of forest organic material releasing carbon dioxide that moves with the water into streams, creating acid conditions in streams. Changes in acidification can affect the way streams function and their biodiversity. Research in lowland streams in Costa Rica suggest that stream invertebrates and algae, can withstand moderately acid conditions, but are negatively affected when acid conditions are extreme. This project investigates how periodic acidification affect streams in lowland Costa Rica. The 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 25 years. The main objective of this work is to understand the mechanisms and consequences of changing precipitation patterns on the acidity, biology and ecosystem functions of tropical streams. Acidification is a common phenomenon in streams wordwide, thus it is important to understand it in order to protect and sustain stream ecosystems. The research (1) will contribute to understanding how changing patterns of rainfall affect stream biodiversity and ecosystem function, (2) provide research training for graduate and undergraduate students, and (3) provide environmental outreach through a citizen science project using the iNaturalist mobile device application. The study will further ongoing long-term research on the consequences of changing patterns of precipitation on lowland tropical streams in Costa Rica. Ongoing LTREB research described the occurrence of episodic acidification events. It was hypothesized 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. Precipitation-driven acidification events are stronger and more common in solute-poor, poorly-buffered, streams than in solute-rich, well-buffered, streams - a characteristic of the study landscape. This project will evaluate and further develop this hypothesis by (1) assessing the relation among daily, seasonal, annual, and decadal patterns in stream water chemistry with major climate events (e.g., El Niño and La Niña - ENSO), by (2) assessing biotic responses to climate-driven acidification, and by (3) experimentally buffering a low-solute stream against acidification related to ENSO events. 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 project will advance our understanding the consequences of extreme weather events on tropical lowland stream ecosystems.