Carbon dioxide as a contributor to seasonal pH shifts in Neotropical rainforest streams

Background/Question/Methods

Altered precipitation regimes caused by global climate change may have dramatic repercussions in aquatic as well as terrestrial ecosystems.  In Central American rainforests, increasingly long dry seasons (as predicted by climate models) could have profound effects on stream ecosystems, since dry-season precipitation is negatively correlated with wet-season pH decreases in streams.  A potential explanation for this correlation is that CO₂ (from microbial respiration) accumulates in soil during dry season and then is transported into streams via groundwater during wet season.  The goal of this study was to examine seasonal relationships between CO₂ and pH at a landscape scale to test (1) whether increased CO₂ concentrations in groundwater correspond with pH decreases, and (2) if groundwater pH is sufficiently low to impact streams.  Seep interfaces were used to estimate the chemistry of groundwater emerging into streams.  Naturally well-buffered (regional bicarbonate-rich groundwater) and poorly-buffered (local groundwater) seeps and streams at La Selva Biological Station in lowland Costa Rica were sampled in both dry and wet seasons for CO₂, pH, conductivity (to determine the extent of regional groundwater input), and other physicochemical parameters.  Dissolved CO₂ concentrations were determined via headspace equilibrium analysis.

Results/Conclusions

In both dry and wet seasons, CO₂ levels were higher in seeps (2.8-59.7 mg CO₂-C/L) than in streams (0.3-20.9 mg CO₂-C/L) (P < 0.0001), while pH in seeps was lower than in streams (P < 0.0001).  Poorly-buffered streams and seeps exhibited a significant negative correlation between pH and CO₂ (P < 0.005, R² = 0.15).  Although well-buffered sites had higher CO₂ levels than poorly-buffered sites, there was no significant relationship between pH and CO₂ across all sites or in well-buffered sites, presumably because well-buffered sites have naturally high DIC concentrations (due to regional groundwater inputs).  These results (1) support the hypothesis that elevated CO₂ concentrations in groundwater play a role in determining the pH of poorly-buffered rainforest streams, and (2) suggest that well-buffered streams (with more constant pH) could potentially provide refugia for aquatic fauna affected by pH shifts.