Insights from a 25+ year dataset in lowland Costa Rica: Effects of hydrologic connectivity from the mountains to the sea on stream ecosystems of an inland protected area

Background/Question/Methods

We examine how regional hydrologic connections affect the response of stream ecosystems in a biological reserve to environmental variability.  For the past 25 years we have been conducting long-term research on groundwater-surface water interactions at La Selva Biological Station, a 1600ha reserve that is owned and operated by the Organization for Tropical Studies.  La Selva provides an invaluable scientific infrastructure to support long-term ecological research and is located at the base of the Central Mountain range on Costa Rica’s Caribbean slope. While most of the lowland streams draining the reserve are solute-poor, some receive regional groundwater through natural inter-basin transfers from high elevations in the volcanic mountains. This regional groundwater is rich in solutes, including phosphorus (>400 µg SRP L^-1) and bicarbonate.  During the first 15 years of our project, we focused on effects of high phosphorus in regional groundwater on stream food web dynamics and nutrient cycling.  After observing dramatic pH declines (from 6 to <4), in some poorly-buffered streams in response to the 1998 ENSO event, we expanded our studies to examine mechanisms behind pH declines and ecological consequences.

Results/Conclusions

Stream ecosystems of La Selva are influenced by hydrologic connections at a regional scale between the mountains and the sea.  Phosphorus in regional groundwater stimulates microbial activity and alters stoichiometric relationships in food webs, while bicarbonates enhance the buffering capacity of receiving streams. Poorly-buffered streams have acidophilic algae and exhibit seasonal declines in pH, with the magnitude of declines being greatest following the driest dry seasons (e.g., 1998 ENSO).  A 15-yr data set indicates that macroinvertebrates do not respond strongly to phosphorus nor variations in pH, but are related with discharge. Laboratory studies show that insects appear to be well-adapted to seasonal pH declines, but they are vulnerable to declines <4.  At the regional scale, shrimp and some fish have migratory life cycles and are vulnerable to disruptions in riverine connectivity with the sea.  Overall, our project highlights the importance of hydrologic connections outside of the local watershed in determining patterns of stream ecosystem structure and function. These findings are particularly relevant, given that climate forecasts for Central America predict increased variability in annual precipitation and severe dry seasons. Continued investigations will enhance our understanding of how hydroclimatic variability interacts with other changes, such as dams and land use change, to affect stream ecosystems and contribute to a more predictive understanding of ecosystem resilience to environmental change.