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 NH 4 + , and a 91% increase in NO 3 -export. In contrast, in an adjacent stream where >⅓ 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 NO 3 -, NH 4 + , 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 -1 y -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.

Nutrient export and elemental stoichiometry in an urban tropical river

“…No other sites with such extraordinarily high values have been reported in the literature from either urban or wildland sites in the tropics, nor have they been observed at other sites with geothermal influence (Ganong et al. ).…”

“…It should be noted that the range of natural variation in phosphorus concentrations and flux is very high in the tropics. At an unusual undisturbed rain forest watershed in La Selva, Costa Rica (the Arboleda), geothermally altered groundwater enters via deep groundwater flowpaths and increases SRP to over 230 lg/L (Ganong et al 2015). No other sites with such extraordinarily high values have been reported in the literature from either urban or wildland sites in the tropics, nor have they been observed at other sites with geothermal influence (Ganong et al 2015).…”

Regional Groundwater and Storms Are Hydrologic Controls on the Quality and Export of Dissolved Organic Matter in Two Tropical Rainforest Streams, Costa Rica

“…Export fluxes of DOC were calculated by the flow-weighted mean method from Littlewood (1992) and Birgand et al (2010). This method has also been used in prior studies of DOC fluxes in the Arboleda and Taconazo watersheds (Ganong et al, 2015;Genereux et al, 2013). The flow-weighted mean is an averaging method in which the annual export flux is calculated as the product of the annual stream discharge volume and the flow-weighted mean concentration of the solute in the stream water (Littlewood, 1992):…”

“…Although storm flowpaths exert a major control on DOM exports in most streams, in the Arboleda RGW flow appears to modulate this balance by creating a continuous high export of water and solutes during baseflow and stormflow (Ganong et al, 2015). Trends in δ 13 C-DOC and SUVA 254 during storm events illustrate this dynamic by comparing a low precipitation storm on 13-14 June 2013 (total precipitation = 8.64 mm) to a high precipitation storm during 14-15 June 2013 (total precipitation = 46.48 mm) (Figure 3).…”

“…DOC and CDOM concentrations in tropical streams tend to increase during storm events and during wet seasons (Bass et al, 2011;Johnson et al, 2006;Newbold et al, 1995;Spencer et al, 2010). However, only a few studies have examined DOM quality in addition to quantity in tropical streams and rivers under contrasting flow regimes, and fewer still have considered the influence of groundwater on DOM quality (Ganong et al, 2015;Genereux et al, 2013;Spencer et al, 2010). Closing this knowledge gap is an essential task as hydrology, vegetation, and biological activity are expected to change with the increasing climate variability, particularly in the tropics, where rainfall is substantial and the frequency and duration of droughts and storms are changing (O'Gorman, 2012).…”