Soluble Reactive Phosphorus Transport and Retention in Tropical, Rainforest Streams Draining a Volcanic and Geothermally Active Landscape in Costa Rica
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Biogeochemistry (2006) 81:131–143 DOI 10.1007/s10533-006-9026-6 ORIGINAL PAPER Soluble reactive phosphorus transport and retention in tropical, rainforest streams draining a volcanic and geothermally active landscape in Costa Rica. : Long-term concentration patterns, pore water environment and response to ENSO events Frank J. Triska Æ Catherine M. Pringle Æ John H. Duff Æ Ronald J. Avanzino Æ Alonso Ramirez Æ Marcelo Ardon Æ Alan P. Jackman Received: 21 March 2005 / Accepted: 3 April 2006 / Published online: 3 June 2006 Ó Springer Science+Business Media B.V. 2006 Abstract Soluble reactive phosphorus (SRP) events on SRP-enriched reaches. Mean interan- transport/retention was determined at four sites in nual SRP concentrations (– standard deviation) three rainforest streams draining La Selva Bio- were 89 – 53lg/l in the Salto (1988–1996), logical Station, Costa Rica. La Selva is located at 21 – 39lg/l in the Pantano (1988–1998), and the base of the last remaining intact rainforest 26 – 35lg/l in the Sabalo (1988–1996). After transect from ~30 m above sea level to 3000 m January, 1997 the separate upland-lowland along the entire Caribbean slope of Central contributions to discharge and SRP load were America. Steam SRP levels can be naturally high determined monthly in the Salto. SRP in Upper there due to regional, geothermal groundwater Salto was low (19 – 8lg/l, 1997–2002) until discharged at ambient temperature. Monitoring enriched at the upland-lowland transition by since 1988 has revealed distinctive long-term dif- regional groundwater. Mean SRP concentration in ferences in background SRP and total P (TP) for Lower Salto (108 – 104lg/l) was typically highest three streams in close proximity, and identified the February–April, the driest months, and lowest impact of ENSO (El Nino Southern Oscillation) July–September, the wettest. SRP concentration was positively correlated to the inverse of dis- charge in Lower Salto when ENSO data were omitted (1992 and 1998–1999), but not in the F. J. Triska (&) Æ J. H. Duff Æ R. J. Avanzino Upper Salto, Pantano, or Sabalo. TP was positively U.S. Geological Survey, 345 Middlefield Rd., MS 439, correlated to the inverse of discharge in all three Menlo Park, CA 94025, USA streams when ENSO data were omitted. High SRP e-mail: [email protected] springs and seeps along the Lower Salto contrib- C. M. Pringle Æ M. Ardon uted ~36% of discharge but ~85% of SRP export Institute of Ecology, University of Georgia, Athens, 1997–2001. Annual SRP flux from the total Salto GA 30602, USA watershed (1997–2001) averaged 2.9 kg/ha year, but only 0.6 kg/ha year from the Upper Salto. A A. P. Jackman Department of Chemical Engineering, University of dye tracer injection showed that pore water envi- California, Davis, CA 95616, USA ronments were distinctly different between Upper and Lower Salto. Upper Salto had high surface A. Ramirez water–pore water exchange, high dissolved Institute for Tropical System Studies, University of Puerto Rico, San Juan, PR 00936, USA oxygen, low SRP, and low conductivity similar to 123 132 Biogeochemistry (2006) 81:131–143 surface water, and Lower Salto had low surface that P uptake length decreased with decreasing water–pore water exchange, low dissolved oxygen, concentration. Mulholland et al. (1997) reported high SRP, and high conductivity reflecting geo- greater SRP retention in Hugh White Creek, NC, thermal groundwater influence. SRP export from USA with larger hyporheic volume compared to the Salto was controlled by regional groundwater Walker Branch TN, USA with smaller hyporheic transfer, which in similar volcanic settings could be volume. In contrast Hall et al. (2002) found P a significant P source. However, ENSO events uptake unrelated to hyporheic volume at Hub- modified the SRP concentration in the Salto sug- bard Brook, NH, USA, because P uptake there gesting that long-term monitoring is required to was dominated by abiotic sorption Within years, understand underlying SRP dynamics and P flux to storm flows can induce high-P pulses to an downstream communities. otherwise P-limited biotic community. Between years, P retained in a dry year can be transported Keywords Phosphorus Æ Retention Æ Sorption Æ in a subsequent wet year (Meyer and Likens SRP Æ Streams Æ Tropical Æ TP Æ Transport Æ 1979). Volcanic Æ ENSO Shifts in SRP concentration, exacerbated by land use, are increasingly common especially in large watersheds with combined urban and agri- Introduction cultural use. Land use modifies temporal concen- tration patterns by shifting SRP sources under Phosphorus cycling in fluvial environments variable hydrologic conditions (Devito et al. 1989; involves numerous interacting biotic and abiotic Dorioz et al. 1998). For example, SRP retention reactions that regulate its uptake and transport. decreased following logging, becoming more dis- Physical factors affecting SRP include pH (Hol- charge related than before (Butturini and Sabater tan et al. 1988; Kleeberg and Schlungbaum 1993), 1998). Although a downstream increase in SRP temperature (Mulholland et al. 1985; Bothwell concentration is usually associated with land use 1988; Holtan et al. 1988; D’Angelo et al. 1991; (e.g., McClain et al. 1998), in some drainages SRP Meals et al. 1999), sorption/desorption (Meyer concentration is naturally elevated. 1979; Hill 1982), interstitial redox conditions The Salto River, drains dense tropical rainfor- (Enell and Lofgren 1988; Holtan et al. 1988; est at La Selva Biological Station, Costa Rica. Gonsiorczyk and Koschel 1997), and discharge High, natural SRP input, attributed to regional, (Meyer 1979). All control P levels in surface wa- geothermally modified groundwater, is intro- ter and depending on concentration, can limit duced at the ambient surface water temperature biotic production and decomposition (Elwood of ~24 °C (Pringle et al. 1990). In this study we et al. 1981; Newbold et al. 1981; Rosemond et al. compared natural input, output and concentration 2002; Ramirez et al. 2003). P occurs in numerous among upland and lowland reaches of the Salto dissolved and particulate forms in fluvial envi- River and two nearby streams, the Pantano and ronments. Dissolved P occurs as either SRP or Sabalo. All three streams had similar geologic and unreactive P. SRP is the most biologically avail- climatic setting, annual day length, and nearly able and thus the focus of most fluvial studies. constant annual temperature. The three streams Differing SRP results reported in published field were compared using both short- and long-term studies are likely due to differences in back- approaches. The study had three objectives: (1) to ground concentration, geologic setting, magni- determine long-term trends of SRP concentration tude of experimental amendment, channel from three streams with differing SRP levels; (2) geomorphology, or study duration. For example, to examine SRP concentration and retention rel- Munn and Meyer (1990) reported rapid SRP ative to the pore water environment above and uptake in an eastern USA stream draining deeply below sites of high SRP input; and (3) to examine weathered granitic bedrock, but less uptake in a short-term SRP retention by in situ and labora- stream draining volcanic bedrock. Within a simi- tory studies. This report examines the first two lar geologic setting, Hart et al. (1992) reported objectives. 123 Biogeochemistry (2006) 81:131–143 133 Study area watershed is located in La Selva Biological Reserve and the upper two-thirds lies in Braulio All study sites were within or adjacent to La Selva Carrillo National Park. The area of the upper Biological Station, a 3300 ha reserve located at watershed above the sampling site Salto 60, as the transition between the Caribbean lowland indicated on Fig. 1,is~726 ha, while the lower plains and the steep foothills of Costa Rica’s watershed between stations Salto 60 and Salto 30, central mountain range (Fig. 1). La Selva receives is ~394 ha. More than 95% of the watershed is average annual rainfall of ~4 m. The wettest roadless, primary, tropical rainforest (Pringle months are July–September and the driest are et al. 1990; Pringle et al. 1993). February–April, although some years lack a def- Upper Salto drains a basalt formation believed inite dry season. Our focus was the Salto River, to be of Quaternary origin. The stream gradient is an ungaged third order stream that drops from steep (0.10) and its bed is composed of large 300 to ~36 m above mean sea level where it joins boulders with sand interstices. Lower Salto drains the Puerto Viejo River. The base of this 1120 ha swamp-forest composed primarily of terraces of Fig. 1 Map of Salto River basin at La Selva Biological Station indicating its location in Costa Rica and long-term sampling locations in the Salto, Pantano, and Sabalo Rivers 123 134 Biogeochemistry (2006) 81:131–143 alluvial sands and gravels of volcanic origin pling using a Marsh-McBirney current meter. The (Bourgeois et al. 1972). The Salto mainstem flows long-term SRP and TP concentrations were fit with along the eastern boundary of the younger of two a linear regression through the inverse of discharge lava flows at La Selva (Pringle et al. 1990). P-rich since dilution was considered the most likely con- springs emerge near the edge of this formation as trol of SRP concentration. Before determining if the channel crosses the gradient break and enters the slope of the regression line was significantly lowland swamp-forest (Pringle and Triska, 1991). different than zero, residuals were examined for Swamp-forest soils are poorly drained, blue-grey normality using a probability plot correlation and blackish-brown silts and clays. Soils are coefficient (PPCC) test (Looney and Gulledge slightly acidic (pH 4.0–5.2), contain up to 25% 1985). Where residuals were nearly normally dis- organic content, and have a fragile structure tributed, data points corresponding to the extreme (Bourgeois et al.