Geothermally Modified Waters Surface at La Selva Biological Station, Costa Rica: Volcanic Processes Introduce Chemical Discontinuities into Lowland Tropical Streams

Abstract

Dilute geothermal springs of the sodium-chloride-bicarbonate type modify the chemistry of receiving streams draining La Selva Biological Reserve in the Caribbean lowlands of Costa Rica. Streams receiving geothermal waters are soluterich and co-occur with solute-poor streams that do not receive geothermal inputs. Geothermal waters contain significantly greater concentrations of Ca, Fe, Mg, Na, Si, Cl, SO4, soluble reactive phosphorus (SRP), and total phosphorus (TP) than waters unmodified by geothermal inputs. Streams that do not receive geothermal inputs are solute-poor, typically containing <10 percent relative concentration of each of the solutes mentioned above. Most geothermal waters that have been identified to date issue near the terminus of the youngest lava flow known for La Selva. The two major rivers of La Selva, the Salto and Sura, are both modified by inputs of geothermal waters and their main channels follow the lateral margins of the youngest lava flow, where it overlies older flows. Discharge points of geothermal seepages appear to be determined by the shape and hydrogeological properties of the lava flows and possible faulting. The lower watershed of the Salto River exhibits a large spatial variability in phosphorus concentration (5-250 ,ug L-' SRP) and conductivity (25-440 ,uS cm-') that reflects the heterogeneous location of geothermal seepages. Geothermal processes introduce natural chemical discontinuities into stream continua that can affect nutrient limitation of algal growth and potentially primary production. In addition, geothermal upwellings represent a source of variation in soil phosphorus levels that is relatively independent of the composition of parent material in the surficial catchment and may potentially influence plant growth and demography.