Interbasin flow of Geothermally modified ground water stabilizes stream exports of biologically important solutes against variation in precipitation

Abstract

Geothermally modified ground water (GMG) in tectonically active areas can be an important source of stream nutrients, and the relative importance of GMG inflows is likely to change with shifts in precipitation that are predicted to occur in response to climate change. However, few studies have quantified the influence of GMG inflows on export of biologically important solutes from watersheds across years differing in precipitation. We quantified N, soluble reactive P (SRP), and dissolved organic C (DOC) export during a year with high precipitation (6550 mm rain) and a year with average precipitation (4033 mm rain) in 2 gauged tropical streams at La Selva Biological Station in lowland Costa Rica. One stream receives extensive inputs of regional GMG, whereas the other is fed entirely by local runoff. In the stream fed only by local runoff, a 62% increase in precipitation from the dry year to the wet year led to a 68% increase in stream discharge, a 67% increase in export of SRP, DOC, dissolved organic N (DON), and NH4+, and a 91% increase in NO3- export. In contrast, in an adjacent stream where >1/3 of discharge consists of GMG, the same increase in precipitation from dry year to wet year led to a 14% increase in discharge, a 14 to 31% increase in export of NO3-, NH4+, DON, and DOC, and only a 2% increase in SRP export. We are unaware of an SRP export rate from a natural system that is higher than the export from the stream receiving interbasin flow of GMG (19 kg ha-1y-1). Our results illustrate that regional ground water, geothermally modified or not, can stabilize stream export of biologically relevant solutes and water across a varying precipitation regime.