EFFECTS OF GEOTHERMAL GROUNDWATER ON NUTRIENT DYNAMICS OF A LOWLAND COSTA RICAN STREAM

Spatial and temporal variability of surface and groundwater nutrient chemistry was contrasted between two tributaries of the Salto River, which drain the terminus of Pleistocene lava flows in the Atlantic slope foothills of Barva Volcano, Costa Rica. Some riparian zones along the Salto are saturated by subsurface inflows of phosphorus-rich, geothermal groundwater of the sodium-chloride-bicarbonate type. The heterogeneous pattern of solute-rich groundwater inflows results in dramatic differences in soluble reactive phosphorus (SRP) concentration between stream waters of two closely adjacent tributaries. SRP levels also vary dramatically in the water table and in soils across stream riparian zones on a scale of metres. The Salto River (SRP 50-200-mu-g/L) receives steady phosphorus-rich groundwater inflows that become diluted with increasing precipitation. The Pantano lacks these inputs and retains a low background SRP concentration (typically < 10-mu-g/L). SRP levels in waters of the Salto were strongly negatively correlated with discharge (r = - 0.86), while in the Pantano there was no correlation. In situ nutrient bioassays of algal periphyton growth indicated that both nitrate and SRP were at growth-saturating levles in the Salto while, in the Pantano, algal growth was phosphorus limited. The Pantano had a high retention capacity for phosphorus as measured through a whole-stream tracer injection experiment. SRP concentration was reduced by 17 and 94% of injected phosphate over 40 and 800 m reaches, respectively. Results indicate the important linkage between geothermal activity and hydrogeochemical features of the volcanic landscape and their role in regulating ecological processes in these streams.