Climate-driven acidification in lowland Neotropical streams: Insights from a 25-year dataset on ground water - surface water interactions

Background/Question/Methods

The current focus of our NSF LTREB Project is to examine the extent to which stream ecosystems are resistant and/or resilient to climate-driven acidification in a lowland Costa Rican rainforest.  Our overarching LTREB objective is to understand the ecological consequences of ground water - surface water interactions. We initially focused on identifying and validating stream solute patterns at multiple temporal/spatial scales and this has transitioned to understanding underlying mechanisms and ecological consequences for ecosystem functioning.  During the first decade of the project we examined ecological effects of high phosphorus (up to 400 ug SRP L-1) in streams that receive regional groundwater through natural inter-basin transfers.  We found that high phosphorus stimulates microbial activity and alters stoichiometric relationships throughout the food web.  Regional ground waters are also rich in bicarbonates and thus increase stream buffering capacity.  Our current focus has evolved over the past 15 years by responding to new data (pH declines) and making additional measurements of physical/ chemical parameters on both short (hourly) and longer (monthly) time scales.

Results/Conclusions

Climate forecasts for Central America predict increased variability in annual precipitation resulting in more severe dry seasons.  Our long-term (25 + yrs) study of stream chemistry has shown that pH declines occur in solute-poor streams at the onset of the wet season each year, with the magnitude of acidification inversely related to dry season rainfall totals.  A particularly severe dry season, associated with the 1998 ENSO event, resulted in pH declines from 6 to 4 for a period of several months.  In contrast, streams that receive bicarbonate-rich regional groundwater are buffered against pH declines, creating a spatial and temporal mosaic of stream chemistry.  The diatom community in poorly-buffered streams is characterized by acidophilic taxa, in contrast to euryhaline taxa in solute-rich streams.  Larval Chironomidae are dominant across all streams, with no seasonal or inter-annual variation related to in situ changes in stream pH, and larval growth rates are not altered in response to experimental reductions in pH (3-4) in laboratory studies.  Findings suggest that the biotic community in poorly-buffered streams is tolerant (or essentially pre-adapted) to seasonal pH declines, conferring some degree of resistance to the stream ecosystem.  Moreover, buffered streams (located in lower parts of the drainage) may serve as refugia during severe declines in pH.  Continued investigations will contribute to a more predictive understanding of ecosystem resistance and resilience in response to future environmental change.