J. N. Am. Benthol. Soc., 2000, 19(2):199–214 q 2000 by The North American Benthological Society
C and N dynamics in the riparian and hyporheic zones of a tropical stream, Luquillo Mountains, Puerto Rico TAMARA J. CHESTNUT1
WILLIAM H. MCDOWELL
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Department of Natural Resources, University of New Hampshire, 215 James Hall, Durham, New Hampshire 03824-3589 USA Abstract. Hydrologic and chemical characteristics were determined for both riparian and hyporheic subsurface flow along a 100-m reach of a sandy-bottom tributary of the Rio Icacos in the Luquillo Experimental Forest, Puerto Rico. Hydrologic data (vertical hydraulic gradient and hydraulic conductivity of streambed sediments) and the topographic and morphological features of the watershed indicated diffuse inputs of groundwater from the near-stream riparian zone along this site. Cumulative groundwater discharge, determined by tracer dilution techniques, was ;1.5 L/s or 10% of the total stream discharge. Spatial heterogeneity in hydrologic and chemical properties of riparian and hyporheic sediments was large. Hydraulic conductivity explained much of the variation in NH4-N and dissolved organic carbon (DOC) concentrations, with highest concentrations in sites having low conductivity. A mass-balance approach was used to examine the influence of the near-stream zone on nutrient transport and retention. Outwelling riparian groundwater had the potential to increase stream N concentrations by up to 84% and DOC concentrations by up to 38% along our 100-m reach. Because stream concentrations were constant downstream despite this input, we conclude that significant N and C retention or loss were occurring in the near-stream zone. Lotic ecosystems and their associated riparian groundwater can have a quantitatively significant impact on the nutrient budgets of tropical headwater catchments. Key words: tropical rainforest, nutrient cycling, nitrogen, DOC, riparian zone, hyporheic zone, hydrologic characteristics.
Riparian buffer zones have long been recognized for their ability to regulate N loading and maintain surface water quality following logging, tillage, or agricultural disturbance (Bormann and Likens 1979, Lowrance et al. 1984, Peterjohn and Correll 1984, Jacobs and Gilliam 1985b, Cooper 1990, Nelson et al. 1995). Many researchers have observed removal of NO3-N via denitrification as water passes through fine-textured, wet riparian soils (Davidson and Swank 1986, Groffman et al. 1992, Hanson et al. 1994). Redox conditions conducive to high denitrification rates may also lead to increased NH4-N levels because of a lack of nitrification of mineralized N (Mullholland 1992, McClain et al. 1994, Pinay et al. 1995) and increased mineralization through stimulated microbial growth (Pastor et al. 1984). Groundwater N inputs to the fluvial system that are relatively large, and concomitant stream N concentrations that are low, suggest that N retention or loss must be occurring within the near-stream zone. The hyporheic zone (where streamwater and subsurface water actively interact) is an area of rapid N transformation (Triska et al. 1989, 1990, 1
E-mail address:
[email protected]
Hendricks and White 1995, Cirmo and McDonnell 1997, Hill and Lymburner 1998). Various studies have shown that NO3-N concentrations can be elevated in the hyporheic zone as a result of high nitrification rates that occur when NH4-N–rich groundwater meets O2-rich water in the streambed (Triska et al. 1990, Grimm et al. 1991, Holmes et al. 1994, 1996, Jones et al. 1995). Denitrification can also occur in hyporheic sediments in zones of anoxia, which can be formed because of either physical constraints on O2 diffusion or O2 consumption by microbial respiration (Duff and Triska 1990, Triska et al. 1990). Coupling of nitrification and denitrification within the hyporheic zone lowers the N concentration of water entering the stream from riparian groundwater (Triska et al. 1990). Several morphological and topographic features dictate the amount of interaction occurring between surface waters and hyporheic sediments. Stream sinuosity or channel meandering can lead to subsurface flow as stream water flows through saturated sediments (Findlay 1995, Wroblicky et al. 1998). Similarly, geographic variation in bedform controls hyporheic subsurface flow because stream water often
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downwells into the streambed at upstream ends of riffles and subsequently upwells at downstream ends (Vaux 1968, White et al. 1987, Hendricks and White 1991, Harvey and Bencala 1993). The presence of secondary channels and their connectivity to the main stream affect the extent of the hyporheic zone (Stanford and Ward 1993, Wondzell and Swanson 1996a). Variation in hydraulic conductivity (K) and heterogeneity of alluvial sediments also impacts the extent of hyporheic exchange (Valett et al. 1996, 1997, Wroblicky et al. 1998). Other morphological features such as pressure variations over wavelike beds, gravel bars, large boulders, and woody-debris obstructions can also lead to downwelling and upwelling sequences (Vaux 1968, White et al. 1987, Harvey and Bencala 1993). Findlay (1995) and Valett et al. (1997) suggested that hydrology and geomorphology create a solid foundation with which to categorize streams and explain chemical trends in the hyporheic zone. Previous research on tropical streams in the Rio Icacos basin in Puerto Rico (McDowell et al. 1992) and the Barro Branco watershed in Central Amazonia (McClain et al. 1994, Brandes et al. 1996) indicates that distinctive patterns in N chemistry are found in the upslope forest and riparian zones, and suggests that N retention or loss near the streambed must be large. Neither study, however, examined hydrologic or chemical trends in the hyporheic zone or streambed. These linkages between hydrologic and chemical characteristics provide valuable information for understanding nutrient dynamics in the stream and near-stream zone (Findlay 1995, Valett et al. 1996, 1997), and are the focus of our study. We characterize chemically and hydrologically the riparian and hyporheic zones of a tropical stream in the highly weathered Rio Icacos basin, Puerto Rico. We applied a mass-balance approach and identified 3 distinct research tasks to quantify the influence of the near-stream zone on nutrient transport and retention: 1) calculation of groundwater discharge QGW, 2) description of spatial patterns and linkages between chemical constituents and K, and 3) calculation of nutrient flux and retention. Methods Study site The study site is located within the Luquillo Experimental Forest (LEF) in northeastern
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Puerto Rico. The Rio Icacos spans the colorado (Cyrilla racemiflora L.) forest zone (600–800 m asl) of the LEF and the study reach is located on a tributary within this life zone. Annual precipitation and average temperature are 3500 to 4500 mm (McDowell and Asbury 1994) and 218C, respectively. Parent material of the Icacos watershed is quartz diorite, which weathers very rapidly to create soils dominated by clays, clay loams, and sandy clay loams of the Utuado series (Boccheciamp 1977, White et al. 1998). Our study site is a 100-m reach of a tributary to the Rio Icacos with a channel width of 1.5 to 2 m and an average discharge of 15 L/s at baseflow. Watershed characteristics include steep slopes, flat, relatively narrow floodplains, and deep (1– 3 m) streambanks. Streambeds are characterized by highly weathered sandy sediments, low channel slopes (0.5%), and significant stream sinuosity (Fig. 1). Hydrology Br2 injection techniques.—Groundwater inputs were measured using dilution gaging with Br2 as the conservative tracer (Stream Solute Workshop 1990). A highly concentrated solution of NaBr was pumped at a steady known rate into a narrow portion of the stream reach using a portable battery-operated fluid metering pump (FMI, model QB1) with a multi-port injector to increase the rate of mixing. Background concentrations for Br2 along the stream reach were 0.03 mg/L (SD 5 0.003) with experimental plateau concentrations of 2.5 mg/L. Br2 tracer was continually pumped for 5 h and samples were taken 3 times during that period at 9 different sites (Fig. 1A). Replicate samples were taken across the stream at each site to measure proper mixing of tracer. Samples were filtered with 0.22mm Acrodisc filters and analyzed at the University of New Hampshire. Groundwater discharge was calculated following the technique of the Stream Solute Workshop (1990). The discharge at sites 1 and 9 was calculated using the equation: Qs 5 Qi ∗ (C1)/(C2 2 C0) where Qs is the stream discharge (L/s), Qi is the injection rate of the tracer (L/s), C1 is the concentration of the injectate, C2 is the concentration in stream at the measurement site, and C0 is the
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FIG. 1. A.—Topographic map of 100-m study reach along a tributary of the Rio Icacos. Scale 5 1:350, topographic contour interval 5 25 cm, elevation change from upstream to downstream 5 50 cm. Bold arrow indicates direction of stream flow. B.—Groundwater flownet for the study. Scale 5 1:350, water table contour interval 5 20 cm. Streamlines showing the direction of groundwater flow are represented by arrows.
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background concentration in the stream (Stream Solute Workshop 1990). Because there are no losing segments along this reach, the increase in discharge from upstream to downstream (sites 1 to 9) provides a measurement of QGW entering the stream (Wallis 1981, Zellweger 1994). We did 3 tracer injection experiments (12 March, 31 May, 29 July) in 1996 using the protocols described above. Each whole-reach enrichment experiment yielded similar results. We calculated groundwater inputs for each of the 3 experiments independently to generate a range of groundwater and stream discharges under baseflow conditions. Average Br2 concentrations were calculated from replicate samples at each site and means were compared statistically independently for each of the 3 whole-reach enrichments. ANOVA with sampling site as the treatment with 9 levels (i.e., 9 sampling sites) and pairwise comparisons (Tukey’s test) were used to determine if differences in Br2 concentration between sites were statistically significant. Groundwater and hyporheic wells.—Six well transects were installed along the 100-m stream reach. Floodplain wells were placed along transects on both sides of the stream at ;1 and 10 m from the stream, and are referred to as nearstream and far-stream wells, respectively (Fig. 1A). Floodplain wells were made of 5-cm PVC with 61 cm of slotted well screen that was covered with polyethylene geotextile fabric filter to reduce particle infiltration and clogging of screens. Well screens for riparian wells ranged from 1 to 3 m in depth depending on location. Hyporheic wells were installed at each transect as 3 sets of vertically nested wells located on the left, center, and right sides of the stream. Hyporheic wells were installed at depths of 10, 30, 50, and 80 cm below the ground surface with each nest oriented longitudinally along the stream. Hyporheic wells were made of 2.5-cm PVC pipe with 5-cm perforated well screens. Wells were capped on the bottom and were limited to 10 cm of pipe below the screen. Groundwater flow net.—All floodplain wells and each hyporheic well grouping were surveyed and mapped using standard techniques (Brinker 1969). Water table data were combined with ground elevations and site map information to generate a contour map of the water table (Fig. 1B); water table data were limited to dates
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without chemical sampling to avoid changes in head associated with bailing the wells prior to chemical sampling. Hydrologic characteristics.—Hydraulic conductivity was measured during baseflow conditions in June 1997 using a bail-mode test. All water was evacuated from the well, and the time required for the water level to return to pre-bail conditions was closely monitored and calculated following the Hvorslev (1951) method (Freeze and Cherry 1979). Hydraulic conductivity for floodplain wells was also calculated using an auger-hole method (Boersma 1965), which yielded similar results. This technique was designed specifically for circumstances where well screens are not fully submerged below the water table because of shallow placement and, therefore, was not required for most of our wells, which had sufficient depth below the water table. A shape factor was required for estimation of K for hyporheic wells because of the reduced length of the well screen (5 cm) used for vertical characterization (Cedergren 1989). The shape factor adjusts Hvorslev’s calculation to account for wells designed for point measurements. Hydraulic head for hyporheic and floodplain wells was measured during each of the 6 sampling periods and at 3 separate times under nobail conditions using a weighted tape measure (accuracy 5 2 mm). For hyporheic wells, the difference in stream and subsurface hydraulic head was measured as the difference between the water table level and the stream water level, which was measured along the outside of the hyporheic well to assure a consistent angle of head measurement (hyporheic wells shift slightly in the streambed over time). Vertical hydraulic gradient (VHG) was calculated as the hydraulic head divided by the depth of the well into the hyporheic sediments (Freeze and Cherry 1979). Following determination of hydrologic data, specific discharge (q) in m/s was calculated using the equation for Darcian velocity, which multiplies the hydraulic gradient (dh/dL) or VHG by K (cm/s). We also estimated the average VHG required to account for an average QGW of 1.5 L/s along our 100-m reach using Darcy’s equation (QGW 5 2KAdh/dL, where A is surface area of the stream, and dh/dL is VHG).
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Chemistry Chemical sampling.—Samples were taken from all wells and from stream water on 6 sampling dates from June 1996 to August 1996 under baseflow conditions. A stage height of 1.2 m or lower on the gage at a USGS weir (Station 50075000) located several hundred meters downstream of the tributary outlet on the mainstem of the Rio Icacos, along with stream height measurements along the tributary, indicated approximate baseflow conditions for sampling. All wells were bailed dry and allowed to refill 1 d prior to sample collection. Samples were filtered using pre-combusted Whatman GF/F 0.7-mm filters within 24 h of field collection. A 3-mL subsample for anion analysis was filtered with a 0.22-mm Acrodisc filter and refrigerated prior to analysis. All other samples were frozen in acid-washed polyethylene (HDPE) bottles. All samples were shipped from Puerto Rico to the University of New Hampshire for analysis. Dissolved O2 (DO) measurements were made with a YSI handheld O2 meter (Model 55). Chemical analysis.—NO3-N, Cl2, SO4-S, and Br2 were analyzed using high-performance liquid chromatography (HPLC) with a Dionex selfregenerating suppressor (4-mm), Ionpac AS4A (10–32) analytical column, Waters 431 conductivity detector, and a photo-diode array detector set to a wavelength of 214 nm for UV detection of NO3-N. NH4-N was analyzed by flow injection analysis colorimetry (Lachat) with the phenol hypochlorite method and sodium nitroprusside enhancement. A total organic C analyzer (Shimadzu TOC 5000) with acidification and sparging was used for measuring non-purgeable organic C (NPOC), which excludes highly volatile organic compounds from the dissolved organic C (DOC) measurement. NPOC is commonly referred to as DOC in both freshwater and marine ecosystems (Sharp et al. 1993) and we refer to NPOC as DOC hereafter. High-temperature catalytic oxidation (Shimadzu TOC 5000) with chemiluminescent N detection (Antek 720) was used to analyze total dissolved N (TDN, Merriam et al. 1996); dissolved organic N (DON) was estimated by the difference between TDN and NH4-N 1 NO3-N. US EPA certified reference standards were used for all analyses to ensure accuracy of working standards. Working standards were always within 10% of the certified standards, and
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typically within ,5%. Replicate samples and check standards were analyzed every 10 to 15 samples to check for accuracy throughout each individual run, and replicate samples were also carried over from previous runs to ensure consistency between runs. Statistical analysis.—Regression analysis and correlation were used to examine relationships between all chemical constituents analyzed, as well as relationships between chemical and hydrological characteristics (DOC, TDN, NH4-N, and K). One-way ANOVA was used to examine spatial and temporal patterns for chemical and hydrological characteristics. All data for K were log transformed prior to statistical analysis because of deviations from a normal distribution. All data reduction and statistical analysis were done using Microsoft Excel (Version 5.0, Microsoft Corporation, Seattle, Washington) and SYSTAT (Version 7.0, SPSS Inc., Chicago, Illinois). Associated with calculation of nutrient retention, we used 1-way ANOVA to determine if concentrations in stream water were significantly different from concentrations in near-stream riparian wells for all constituents analyzed. Nutrient flux and retention calculations To assess the biogeochemical activity of the near-stream zone, we calculated the possible nutrient flux from the hillslope to the stream channel and addressed its potential to impact nutrient concentrations in stream water. We calculated the potential groundwater input for all chemical constituents that showed a significant difference between stream and groundwater concentrations (TDN, DOC, NH4-N, and NO3N). We calculated these potential groundwater inputs into the stream using the concentrations of wells in the riparian zone. Following Wallis (1981), we calculated the net nutrient flux (mg/ s) from groundwater as the difference between average near-stream riparian well concentrations and ambient stream concentrations multiplied by QGW. Nutrient concentrations were represented by grand means, whereas QGW was calculated independently from each tracer injection experiment. We also obtained a 2nd estimate of nutrient flux based on concentrations weighted by the q for each well. Specific discharge for each riparian well was log transformed and subsequently used to rank each well based on its potential contribution to groundwater input.
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TABLE 1. Hydrologic data for the 100-m study reach along a tributary of the Rio Icacos. Discharge (Q) data are reported as a range for 3 whole-reach tracer experiments. QGW 5 groundwater discharge. Hydraulic conductivity (K) is reported as a geometric mean based on data from hyporheic wells (n 5 35). Vertical hydraulic gradient (VHG) is reported as a mean (61 SD) based on data from hyporheic wells (n 5 70). Variable
Value
QGW (L/s) Qstream (L/s) % QGW contributing to Qstream Stream gradient (%) K of streambed (cm/s) VHG of streambed (cm/cm)
1.1–1.9 12.8–15.0 8–15 0.50 1.0 3 1022 0.003 (0.02)
The well with the highest q was given a contribution factor of 100%, and contribution factors for other wells were calculated as the % of this value. The % contribution factor was then used to weight TDN, DOC, NH4-N, and NO3-N concentrations in individual wells to obtain a flowweighted mean concentration for all wells. These adjusted concentrations were then used to calculate a 2nd and lower estimate of nutrient flux. Last, we calculated a nutrient load (gm22 d21) for these constituents by dividing the nutrient flux estimates from groundwater by the surface area of the stream. Stream surface area was estimated based on an average stream width of 1.5 m (SD 5 0.2, n 5 8).
FIG. 2. Br2 concentration (mean 61 SD) with distance downstream during a whole-reach enrichment on 29 July 1996. Letters indicate significant differences between sampling sites. All whole-reach enrichments showed similar results.
the hyporheic zone was slight throughout the stream reach (Table 1). Among individual hyporheic wells, the range of average VHG (averaged for ;6 sampling periods) was 20.06 to 0.07 cm/cm. The average VHG necessary to ac-
Results Hydrologic characterization Whole-reach tracer experiments indicated groundwater inputs to the stream reach under baseflow conditions of ;1.5 L/s or 10% of total stream discharge (Table 1). Br2 concentrations differed significantly among study sites (ANOVA, Tukey’s test, p , 0.05) with the differentiation occurring gradually along the 100-m reach (Fig. 2). Groundwater inputs during each of 3 whole-reach enrichments remained similar in both absolute measures (QGW) and as a % of stream flow (Table 1). Ks of hyporheic sediments reflected a predominance of highly porous sediments (Table 1), as opposed to the variable sediment types found in the riparian zone (Fig. 3). The VHG in
FIG. 3. Frequency distribution of hydraulic conductivity (K) for riparian and hyporheic sediments based on bail-mode measurements of wells.
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count for QGW into the stream was small (0.02 6 0.006 cm/cm) and within the range of VHG measured for this study. Chemical characterization DO data were available for most wells (;50% of hyporheic wells, 100% of floodplain wells) and did not show any marked trends or consistent relationships with depth. DO levels in riparian wells were generally below 10% saturation, whereas stream DO levels were 5.9 mg/L or 71% saturation. Riparian wells with high NH4-N and DOC had DO concentrations below 0.5 mg/L or 6% saturation, and the few hyporheic wells with elevated levels of NO3-N had DO concentrations .1 mg/L (12–22% saturation). TDN, DOC, and NH4-N concentrations were strongly related to K (Fig. 4). Wells with high K had low concentrations of these constituents and, conversely, wells in areas with low K had high concentrations in both floodplain and hyporheic wells (Fig. 4). Cl2 concentrations were not significantly related to K, indicating that evaporation is not driving the observed relationships between concentration and K (Fig. 4). Spatial comparison of chemical characteristics revealed significant differences among the stream, hyporheic zone, 1-m riparian zone, and 10-m riparian zone for many of the constituents analyzed (Table 2). NH4-N and TDN both increased as water moved from 10 m in the riparian zone to the near-stream riparian zone. Concentrations subsequently decreased significantly within the hyporheic zone with a further decrease in the stream. Concentrations of NO3N were ,0.05 mg/L in all riparian wells regardless of site or distance from the stream and were significantly lower than hyporheic and stream water concentrations (Table 2). DON concentrations were higher in the riparian zone (10m and near-stream locations) than in the hyporheic zone or stream. Similarly, DOC showed a consistent decrease across the riparian zone to the stream (Table 2). In contrast, concentrations of SO4-S were low throughout the system and varied only slightly among subsystems. No significant differences in Cl2 concentration occurred among groups. NH4-N and DOC were significantly related with a positive linear relationship (r2 5 0.43, p , 0.01). NH4-H and NO3-N were also related; however, the relationship was negative and ex-
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ponential (r2 5 0.30; p , 0.01), showing high NH4-N only at low NO3-N concentrations and little to no NH4-N under conditions of high NO3-N. DOC and DON were also significantly related (r2 5 0.26; p , 0.01), but in general DON concentrations were low in all wells sampled (Table 2). Temporal variability (% CV) across the 6 sampling periods was ,30% for all chemical constituents measured and was typically ,15%. Nutrient flux and retention Water table mapping (Fig. 1B) indicated groundwater flow toward the stream. SDs for stream concentrations of all constituents analyzed were low (Table 2) and did not vary significantly from upstream to downstream along our study reach (p . 0.1). Predicted stream concentrations from groundwater TDN inputs (in the absence of any TDN retention or loss in the near-stream riparian and hyporheic sediments) were up to 84% higher than observed stream concentrations (Table 3). DOC showed a similar pattern with concentrations potentially increasing by as much as 38% without retention (Table 3). Mass balance calculations for NH4-N and NO3-N indicated that, without retention, groundwater influences on stream NH4-N may cause as much as an order of magnitude increase in concentration. In contrast, conservative transport of NO3-N from the riparian zone to the stream would have resulted in stream concentrations ;10% lower than those actually observed (Table 3). Discussion Hydrologic characterization The streambed sediments of the Icacos are highly weathered and sandy with a relatively high K (1.0 3 1022 cm/s) typical of sandy sediments (Freeze and Cherry 1979). Watershed topography is steep and groundwater streamlines indicate direct flow of groundwater toward the stream. Although the Rio Icacos has a large degree of channel curvature, the consistent flow of groundwater perpendicular to the stream suggests that hyporheic flow across channel meanders (sensu Boulton 1993, Harvey and Bencala 1993, Wroblicky et al. 1998) is limited in this system. Distinct riffle and pool sequences,
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FIG. 4. Relationships between total dissolved N (TDN), NH4-N, dissolved organic C (DOC), Cl2, dissolved organic N (DON), and NO3-N (all concentrations in mg/L) and natural log (Ln) of saturated hydraulic conductivity (K).
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TABLE 2. Spatial comparison of means (61 SD) for all constituents (mg/L) using 1-way ANOVA with letters indicating significant differences between means. DON 5 dissolved organic N, TDN 5 total dissolved N, and DOC 5 dissolved organic C. Distance of riparian zone from the stream is shown. Riparian (10 m) NO3-N NH4-N DON TDN DOC SO4-S Cl2
0.03 0.23 0.14 0.40 5.1 0.62 7.4
(0.01) (0.23)a (0.20)a (0.18)a (2.5)a (0.23)a (1.0)a a
Riparian (1 m) 0.02 0.96 0.12 1.09 2.6 0.29 6.8
which create hyporheic downwelling and upwelling zones (Hendricks and White 1991, 1995, Harvey and Bencala 1993), are also absent at our study site and are typically uncommon along the Rio Icacos (W. H. McDowell, unpublished data). Stream channel slope along our 100-m reach was gradual (0.5%) and generally constant reducing the potential for gradient-driven downwelling and upwelling (Harvey and Bencala 1993). VHG values for all hyporheic wells were very small and did not indicate any distinct upwelling or downwelling zones within this system. Other sites with distinct upwelling and downwelling zones were characterized by VHG that exceeded 0.20 cm/cm (Valett et al. 1994, Wondzell and Swanson 1996a) or were consistently negative or positive (Hendricks and White 1995). As indicated by groundwater streamlines, the steep hydraulic gradient between the riparian zone and the stream results from the steep watershed topography of our study area and generates a constant, diffuse outwelling of groundwater into the stream at baseflow. Quantitative significance of nutrient retention in the Rio Icacos If all N in riparian groundwater entered the stream without transformation or retention in the near-stream zone, overall levels of TDN in streamwater would increase by 21 to 84% after travelling the length of the study reach. Although our study only examined a 100-m reach of stream, estimates from USGS topographic maps and ground surveys indicate that this tributary has morphologic and hydrologic characteristics similar to our 100-m section for
(0.01) (1.11)b (0.17)a (1.20)b (2.4)b (0.19)b (1.2)b a
Hyporheic 0.06 0.26 0.06 0.39 1.4 0.27 6.3
(0.09) (0.56)a (0.10)b (0.61)a (1.7)c (0.15)b (1.0)b b
Stream 0.11 0.01 0.07 0.18 0.8 0.33 6.3
(0.01)c (0.01)a (0.02)ab (0.03)a (0.3)c (0.04)b (1.0)b
.1000 m in length. Similar riparian input throughout the drainage basin would have the potential to increase the flux of N in stream water by as much as 8 times. This result suggests that processing in the near-stream zone is a quantitatively significant mechanism for removing N from groundwater prior to entering the stream. As with N, without some form of retention or transformation, outwelling riparian groundwater would have a significant impact on DOC concentrations in the stream. DOC flux from groundwater has the potential to increase overall DOC concentrations in the stream by 10 to 38% for our 100-m reach (Table 3). DOC concentrations in streamwater were more variable than TDN (Table 2), but showed no evidence of increasing from upstream to downstream. In other streams, both groundwater discharge of DOC (Fiebig 1995) and streamwater DOC levels (Findlay et al. 1993, Findlay and Sobczak 1996) are significantly lowered (,50%) as a result of retention in hyporheic sediments. Fiebig (1995) found that hyporheic sediments were capable of immobilizing 71% of groundwater DOC prior to entering a 1st-order stream. Although there is strong evidence for C retention by the hyporheic zone, groundwater discharge of DOC into streams may be a significant source of C for stream benthic organisms in areas where ambient streamwater DOC concentrations are not exceedingly high (Cronan 1990, Fiebig and Lock 1991, Fiebig 1992). Other studies of undisturbed forested watersheds in tropical environments (McClain et al. 1994) and the Rio Icacos in particular (McDowell et al. 1992) have found a dramatic decrease in N concentrations between the riparian zone
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TABLE 3. Mass balance results for constituents with a statistically significant difference between riparian groundwater and stream water. Hydrologic data are specific to each stream addition date; groundwater and stream nutrient concentrations are averaged over all sampling sites and dates. Minimum values were generated using groundwater concentrations weighted by specific discharge (q). QGW 5 groundwater discharge and Qstream 5 stream discharge. GW 5 groundwater. TDN 5 total dissolved N and DOC 5 dissolved organic C. 12 Mar 96 min. QGW (L/s) Qstream (L/s) GW [TDN] (mg/L) Ambient stream [TDN] (mg/L) Predicted stream [TDN] from GW input (mg/L)a GW TDN flux (mg/s)b Ambient stream TDN flux (mg/s)b GW TDN load (g m22d21)c Potential increase in TDN from GW input (%)d GW [DOC] (mg/L) Ambient stream [DOC] (mg/L) Predicted stream [DOC] from GW input (mg/L)a GW DOC flux (mg/s)b Ambient stream DOC flux (mg/s)b GW DOC load (g m22d21)c Potential increase in DOC from GW input (%)d GW [NH4-N] (mg/L) Ambient stream [NH4-N] (mg/L) Predicted stream [NH4-N] from GW input (mg/L)a GW NH4-N flux (mg/s)b Ambient stream NH4-N flux (mg/s)b GW NH4-N load (g m22d21)c Potential increase in NH4-N from GW input (%)d GW [NO3-N] (mg/L) Ambient stream [NO3-N] (mg/L) Predicted stream [NO3-N] from GW input (mg/L)a GW NO3-N flux (mg/s)b Ambient stream NO3-N flux (mg/s)b GW NO3-N load (g m22d21)c Potential increase in NO3-N from GW input (%)d
1.9 12.8 0.6 0.18 0.07 0.9 2.3 0.5
b
c
max.
1.2
0.15 1.9 1.0
min. 1.7 15.0 0.6 0.18 0.05 0.8 2.7 0.4
29 July 96
max.
1.2
0.12 1.7 0.9
min. 1.1 13.1 0.6 0.18 0.04 0.5 2.4 0.3
max.
1.2
0.09 1.1 0.6
37 1.6 0.8
84 2.8
28 1.6 0.8
64 2.8
21 1.6 0.8
48 2.8
0.1 1.6 10.0 0.9
0.3 3.8
0.1 1.4 11.7 0.8
0.3 3.4
0.1 0.9 10.2 0.5
0.2 2.2
16 0.53 0.01
38 1.02
14 0.53 0.01
34 1.02
9 0.53 0.01
23 1.02
0.08 1.0 0.1 0.5
0.15 1.9
0.07 0.9 0.2 0.5
0.13 1.7
0.04 0.6 0.1 0.3
0.09 1.1
772 0.01 0.11
1499 0.02
691 0.01 0.11
1341 0.02
447 0.01 0.11
868 0.02
20.01 20.2 1.4 20.1
20.01 20.2
20.01 20.2 1.7 20.1
20.01 20.2
20.01 20.1 1.4 20.1
20.01 20.1
28
27
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Predicted [Stream] 5 GW flux/Qstream Flux 5 ([GW] 2 [Stream]) ∗ QGW Load 5 flux/160 m2 stream surface area d Potential increase 5 GW flux/stream flux ∗ 100 a
31 May 96
2.1
1.0
20.1 212
212
1.8
0.9
20.1 211
1.2
0.6
20.1
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and streamwater, suggesting N removal or retention by the near-stream zone or hyporheic zone. Brandes et al. (1996) further examined the Barro Branco catchment using 15N isotopic studies and found that groundwater dissolved inorganic N (DIN) and streamwater DIN were substantially decoupled; DIN in streamwater was primarily driven by remineralization of organic N within the stream. Coupled with our results, this finding suggests that virtually all groundwater DIN in these types of systems may be removed or retained by the streambank or hyporheic sediments prior to entering the stream. Chemical and hydrological linkages Previous research on N retention and removal by riparian buffer zones has generally been done using plot-level studies of subsurface flow from upslope areas through riparian land (Peterjohn and Correll 1984, Jacobs and Gilliam 1985a, 1985b, Cooper 1990, Lowrance 1992, Simmons et al. 1992, Nelson et al. 1995, Groffman et al. 1996, Hill 1996, Verchot et al. 1997). Well or piezometer nests were used to identify changes in organic and inorganic N concentrations, and to identify mechanisms of removal (denitrification, plant uptake, and other microbial processes). However, little research has closely examined the hydrologic network on a whole-reach or watershed scale to determine if the N or DOC transformations observed in plotlevel studies are significant on a watershed basis. Results from our study reach indicate that near-stream and hyporheic zones play an important role in immobilizing both N and DOC prior to entering the stream. However, these concentrations of N and DOC in groundwater are also spatially heterogeneous. Therefore, studies that do not examine the spatial variation in hydrologic flow paths and groundwater chemical content may be under- or over-estimating the effects of the riparian zone on stream water quality. Although there was considerable spatial heterogeneity in soil and sediment hydrologic and chemical properties along our 100-m stream reach, relationships between these 2 characteristics can aid in explaining some of this variability. The strong relationship we observed between hydrology (specifically K) and concentrations of DOC, TDN, and NH4-N (Fig. 4) may be
209
the result of 2 different scenarios. First, low K because of the grain size and texture of the soil might increase hydraulic residence time and allow for the accumulation of metabolic end-products (DOC, NH4-N). An alternative explanation for the relationship between K and concentrations is that in regions with high organic matter accumulation (e.g., leaf litter buried in alluvial deposits) an actively growing biofilm (Freeman and Lock 1995) might clog sediment or soil pore spaces and thereby reduce hydraulic conductivity. Pinay et al. (1995) also found a strong relationship between soil texture and/or water retention times and rates of mineralization and denitrification in the floodplain of 2 streams in southwestern France. They determined that riparian zones containing fine-textured soils with high water-holding capacity had higher rates of mineralization and denitrification than sandy, more porous soils. These findings are consistent with our data and support the conceptual model that areas with low K have increased waterholding capacity leading to losses of NO3-N through denitrification and build-up of NH4-N from increased mineralization and reduced rates of nitrification. Although we found a strong relationship between various chemical constituents and sediment K, the spatial variability associated with K itself was large and difficult to interpret. We found distinct differences in the hydrologic characteristics of riparian and streambed sediments at spatial scales as small as 1 m. Other studies have also reported considerable variability in hydraulic conductivity and N processing within the riparian zone (Cooper 1990, Mullholland 1992, Nelson et al. 1995, Pinay et al. 1995) and the hyporheic zone (Triska et al. 1989, 1993, Duff and Triska 1990, Harvey and Bencala 1993). However, because of the sampling design used in this study, we were able to quantify K and provide N and DOC fluxes that include within-reach variability and thus improve our ability to extrapolate to the watershed level. Characterization of the terrestrial-aquatic interface: undisturbed streams of wet tropical forests Previous research on the Rio Icacos (McDowell et al. 1992) and the Barro Branco watershed in the central Amazon (McClain et al. 1994)
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showed strong similarities in topography, geomorphology, and chemistry for the upslope forest, the riparian zone, and the stream at these 2 tropical sites. Groundwater is dominated by NO3-N in the upslope forest, and concentrations decline across the riparian floodplain at both locations. In contrast, NH4-N concentrations dramatically increase from the upslope forest to the near-stream riparian zone. TDN concentrations also decline significantly across the stream channel interface with a further shift to NO3-N dominance in stream water. Although our study did not examine patterns in the upslope forest, we found similar trends in the riparian zone and stream water. Topography, riparian soil, and streambed characteristics of the central Amazonian site are strikingly similar to those of the Rio Icacos (McClain et al. 1994, 1997). Thus we hypothesize that diffuse outwelling of groundwater from the riparian zone into the stream and large changes in N concentrations in floodplain groundwater may be typical of highly weathered, wet tropical systems. Many of the studies that have examined the effects of riparian zones as a potential source or sink for groundwater N have focused on agricultural or disturbed ecosystems (Peterjohn and Correll 1984, Cooper 1990, Groffman et al. 1992, Lowrance 1992, Simmons et al. 1992, Groffman et al. 1996, Hill 1996, Verchot et al. 1997, Hedin et al. 1998). Riparian groundwater NO3-N concentrations from these disturbed ecosystems are as high as 44 mg/L with typical concentrations of ;8 mg/L, considerably higher than concentrations reported for undisturbed ecosystems (Table 4). N retention for many of these agricultural and disturbed systems is also high (;90%) with few exceptions, but comparisons with more pristine streams are problematic because of the relatively undisturbed nature of the stream and surrounding catchment. Studies of other relatively undisturbed ecosystems (Triska et al. 1989, 1994, Valett et al. 1990, Mullholland 1992, McClain et al. 1994, 1997, Fiebig 1995, Valett et al. 1996, Wondzell and Swanson 1996b) have examined the impacts of riparian and hyporheic processing on N and C dynamics in stream ecosystems. Most of these studies have been carried out in northern temperate systems. To provide a comparative argument for the function of the
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terrestrial–aquatic interface in streams of wet tropical forests, we have summarized key chemical parameters characteristic of a number of systems in varying biomes (Table 4). DIN (NO3-N and NH4-N) concentrations in both stream water and riparian groundwater for several undisturbed ecosystems were typically lower than those found in our study (Table 4). Average NH4-N concentrations in the riparian zone at the Rio Icacos were considerably higher than at other undisturbed sites, but were very similar to those of the Barro Branco in central Amazonia (McClain et al. 1994, Table 4). DON has not been measured at many of these sites, but our site is within the range of values reported. The decrease we observed in DOC concentrations from the riparian zone to the stream was much larger than those reported for other systems with the exception of Breitenbach (Fiebig 1995, Table 4). Stream concentrations of NH4-N and NO3-N were within the range of those found for other undisturbed sites and were quite low overall (Table 4). Concentrations of DOC in stream water were highly variable for these undisturbed sites, ranging from 0.7 to 8.4 mg/L with the Rio Icacos yielding concentrations at the low end of the range. Overall, the Rio Icacos showed marked similarities in DOC and N concentrations in both riparian groundwater and stream water with the Barro Branco site in Amazonia (McClain et al. 1994, 1997), and within the range of values reported for other undisturbed sites. Understanding the hydrologic characteristics of watersheds may be an effective way to explain spatial variation in nutrient concentrations within the riparian and hyporheic zones, and to assess the quantitative significance of hydrologic linkages between watershed and stream chemistry. Our results suggest that in wet tropical forest these hydrologic linkages are particularly important in regulating losses of C and N from the terrestrial landscape. Maintenance of these hydrologic linkages and their associated biogeochemical functions is likely to be critically important in maintaining water quality as tropical landscapes undergo the changes in land use and atmospheric deposition of N associated with increased population levels (Matson et al. 1999).
Breitenbach, Germanyj
Rio Calaveras, New Mexicoi
Aspen Creek, New Mexicoi
McRae Creek, Oregonh
Maple River, Michigang
Wet Tropical Forest Wet Tropical Forest Wet Tropical Forest Wet Tropical Forest Desert Northern Coniferous Forest Eastern Deciduous Forest Northern Hardwood Forest Old Growth Coniferous High Elevation Coniferous High Elevation Coniferous Meadow –
0.15
0.04
0.01
–
0.02
0.00
0.01
0.21
0.04
0.03 0.03
0.03 0.01 0.08 0.02 0.24 0.45
Riparian
0.11 0.08 0.09 0.06 0.04 0.08
Stream
–
0.04
0.03
0.00
0.08
0.03
0.01 0.02 0.01 0.01 0.03 0.01
Stream
–
0.12
0.06
0.01
0.18
0.04
0.60 0.76 0.04 0.29 0.06 0.03
Riparian
–
–
–
0.03
–
0.04
0.07 0.05 0.05 0.20 – –
Stream
–
–
–
0.03
–
0.04
0.13 0.22 0.25 0.28 – –
Riparian
1.4
2.1
2.7
–
8.4
0.7
0.8 – – 4.5 – 1.2
Stream
5.5
1.3
3.6
–
5.9
1.2
3.9 – – 3.5 – 0.7
Riparian
DOC
N
Rio Icacos, Puerto Rico Rio Icacos, Puerto Ricob Bisley, Puerto Ricob Barro Branco, Amazoniac Sycamore Creek, Arizonad Little Lost Man Creek, Californiae Walker Branch, Tennesseef
Biome
DON
AND
a
NH4-N
C
NO3-N
TABLE 4. Comparison of riparian and stream N and C concentrations (mg/L) at various undisturbed sites with a wide range of hydrologic, geomorphologic, and climatic conditions. DON 5 dissolved organic N and DOC 5 dissolved organic C. – 5 no data. aThis study; bMcDowell et al. 1992; cMcClain et al. 1994, 1997; dValett et al. 1990; eTriska et al. 1989, 1994; fMullholland 1992; gHendricks and White 1995; hWondzell and Swanson 1996b; iValett et al. 1996; jFiebig 1995.
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Acknowledgements We thank Steve Wondzell, Claire McSwiney, J. Matthew Davis, Dan Zarin, Jeff Merriam, and Adam Chestnut for assistance in writing, laboratory analysis, and field work. The research was funded by grant DEB-9411973 from the National Science Foundation to the Terrestrial Ecology Division, University of Puerto Rico, and the International Institute of Tropical Forestry as part of the Long-term Ecological Research Program in the Luquillo Experimental Forest. Additional support was provided by the USDA Forest Service and the University of Puerto Rico. Literature Cited BOCCHECIAMP, R. A. 1977. Soil survey of Humacao area of eastern Puerto Rico. United States Department of Agriculture Soil Conservation Service, San Juan, Puerto Rico. US Government Printing Office, Washington, DC. (Available from: Natural Resources Conservation Service, 14th and Independence Ave., Washington, DC 20250 USA.) BOERSMA, L. 1965. Field measurements of hydraulic conductivity below a water table. Pages 222–233 in C. A. Black (editor). Methods of soil analysis Part 1—Physical and mineralogical methods. 1st edition. American Society of Agronomy—Soil Science Society of America, Madison, Wisconsin. BORMANN, F. H., AND G. E. LIKENS. 1979. Pattern and process in a forested ecosystem. Springer-Verlag, New York. BOULTON, A. J. 1993. Stream ecology and surface-hyporheic hydrologic exchange: implications, techniques, and limitations. Australian Journal of Marine and Freshwater Research 44:553–564. BRANDES, J. A., M. E. MCCLAIN, AND T. P. PIMENTEL. 1996. 15N evidence for the origin and cycling of inorganic nitrogen in a small Amazonian catchment. Biogeochemistry 34:45–56. BRINKER, R. C. 1969. Elementary surveying. 5th edition. International Textbook Company, Scranton, Pennsylvania. CEDERGREN, H. R. 1989. Seepage, drainage, and flow nets. 3rd edition. John Wiley and Sons, New York. CIRMO, C. P., AND J. J. MCDONNELL. 1997. Linking the hydrologic and biogeochemical controls of nitrogen transport in the near-stream zones of temperate-forested catchments: a review. Journal of Hydrology 199:88–120. COOPER, A. B. 1990. Nitrate depletion in the riparian zone and stream channel of a small headwater catchment. Hydrobiologia 202:13–26. CRONAN, C. S. 1990. Patterns of organic acid transport form forested watersheds to aquatic systems. Pag-
W. H. MCDOWELL
[Volume 19
es 245–260 in E. M. Perdue and E. T. Gjessing (editors). Organic acids in aquatic ecosystems. John Wiley and Sons, Chichester, UK. DAVIDSON, E. A., AND W. T. SWANK. 1986. Environmental parameters regulating gaseous nitrogen losses from two forested ecosystems via nitrification and denitrification. Applied and Environmental Microbiology 52:1287–1292. DUFF, J. H., AND F. J. TRISKA 1990. Denitrification in sediments from the hyporheic zone adjacent to a small forested stream. Canadian Journal of Fisheries and Aquatic Sciences 47:1140–1147. FIEBIG, D. M. 1992. Fates of dissolved free amino acids in groundwater discharged through streambed sediments. Hydrobiologia 235/236:311–319. FIEBIG, D. M. 1995. Groundwater discharge and its contribution of dissolved organic carbon to an upland stream. Archiv fu¨r Hydrobiologie 134: 129–155. FIEBIG, D. M., AND M. A. LOCK. 1991. Immobilization of dissolved organic matter from groundwater discharging through the stream bed. Freshwater Biology 26:45–55. FINDLAY, S. 1995. Importance of surface-subsurface exchange in stream ecosystems: the hyporheic zone. Limnology and Oceanography 40:159–164. FINDLAY, S., AND W. V. SOBCZAK. 1996. Variability in removal of dissolved organic carbon in hyporheic sediments. Journal of the North American Benthological Society 15:35–41. FINDLAY, S., D. STRAYER, C. GOUMBALA, AND K. GOULD. 1993. Metabolism of streamwater dissolved organic carbon in the shallow hyporheic. Limnology and Oceanography 38:1493–1499. FREEMAN, C., AND M. A. LOCK. 1995. The biofilm polysaccharide matrix: a buffer against changing organic substrate supply? Limnology and Oceanography 40:273–278. FREEZE, R. A., AND J. A. CHERRY. 1979. Groundwater. Prentice-Hall, Englewood Cliffs, New Jersey. GRIMM, N. B., H. M. VALETT, E. H. STANLEY, AND S. G. FISHER. 1991. Contribution of the hyporheic zone to stability of an arid-land stream. Verhandlungen der Internationalen Vereinigung fu¨r theoretische und angewandte Limnologie 24: 1595–1599. GROFFMAN, P. M., A. J. GOLD, R. C. SIMMONS. 1992. Nitrate dynamics in riparian forests: microbial studies. Journal of Environmental Quality 21:666– 671. GROFFMAN, P. M., G. HOWARD, A. J. GOLD, AND W. M. NELSON. 1996. Microbial nitrate processing in shallow groundwater in a riparian forest. Journal of Environmental Quality 25:1309–1316. HANSON, G. C., P. M. GROFFMAN, AND A. J. GOLD. 1994. Denitrification in a riparian wetland receiving high and low groundwater nitrate inputs. Journal of Environmental Quality 23: 917–922.
2000]
C
AND
N
DYNAMICS IN A TROPICAL STREAM
HARVEY, J. W., AND K. E. BENCALA. 1993. The effect of streambed topography on surface-subsurface water exchange in mountain catchments. Water Resources Research 29:89–98. HEDIN, L. O., J. C. VON FISCHER, N. E. OSTROM, B. P. KENNEDY, M. G. BROWN, AND G. P. ROBERTSON. 1998. Thermodynamic constraints on nitrogen transformations and other biogeochemical processes at soil-stream interfaces. Ecology 79:684– 703. HENDRICKS, S. P., AND D. S. WHITE. 1991. Physicochemical patterns within a hyporheic zone of a northern Michigan river, with comments on surface water patterns. Canadian Journal of Fisheries and Aquatic Sciences 48:1645–1654. HENDRICKS, S. P., AND D. S. WHITE. 1995. Seasonal biogeochemical patterns in surface water, subsurface hyporheic, and riparian groundwater in a temperate stream ecosystem. Archiv fu¨r Hydrobiologie 134:459–490. HILL, A. R. 1996. Nitrate removal in stream riparian zones. Journal of Environmental Quality 25: 743– 755. HILL, A. R., AND D. J. LYMBURNER. 1998. Hyporheic zone chemistry and stream-subsurface exchange in two groundwater-fed streams. Canadian Journal of Fisheries and Aquatic Sciences 55:495–506. HOLMES, R. M., S. G. FISHER, AND N. B. GRIMM. 1994. Parafluvial nitrogen dynamics in a desert stream ecosystem. Journal of the North American Benthological Society 13:468–478. HOLMES, R. M., J. B. JONES, S. G. FISHER, AND N. B. GRIMM. 1996. Denitrification in a nitrogen-limited stream ecosystem. Biogeochemistry 33:125–146. HVORSLEV, M. J. 1951. Time lag and soil permeability in groundwater observations. US Army Corps of Engineers, Waterways Experimental Station Bulletin 36. US Army Corps of Engineers, Vicksburg, Mississippi. JACOBS, T. C., AND J. W. GILLIAM. 1985a. Headwater stream losses of nitrogen from two coastal plain watersheds. Journal of Environmental Quality 14: 467–472. JACOBS, T. C., AND J. W. GILLIAM. 1985b. Riparian losses of NO3 from agricultural drainage water. Journal of Environmental Quality 14:472–478. JONES, J. B., S. G. FISHER, AND N. B. GRIMM. 1995. Nitrification in the hyporheic zone of a desert stream ecosystem. Journal of the North American Benthological Society 14:249–258. LOWRANCE, R. 1992. Groundwater nitrate and denitrification in a coastal plain riparian forest. Journal of Environmental Quality 21:401–405. LOWRANCE, R., R. TODD, J. J. FAIL, O. J. HENDRICKSON, R. LEONARD, AND L. ASMUSSEN. 1984. Riparian forests as nutrient filters in agricultural watersheds. BioScience 34:374–377. MATSON, P. A., W. H. MCDOWELL, A. R. TOWNSEND,
213
P. M. VITOUSEK. 1999. The globalization of N deposition: ecosystem consequences in tropical environments. Biogeochemistry 46:67–83. MCCLAIN, M. E., J. E. RICHEY, J. A. BRANDES, AND T. P. PIMENTEL. 1997. Dissolved organic matter and terrestrial-lotic linkages in the central Amazon basin of Brazil. Global Biogeochemical Cycles 11: 295–311. MCCLAIN, M. E., J. E. RICHEY, AND T. P. PIMENTEL. 1994. Groundwater nitrogen dynamics at the terrestrial-lotic interface of a small catchment in the Central Amazon Basin. Biogeochemistry 27:113– 127. MCDOWELL, W. H., AND C. E. ASBURY. 1994. Export of carbon, nitrogen, and major ions from three tropical montane watersheds. Limnology and Oceanography 39:111–125. MCDOWELL, W. H., W. B. BOWDEN, AND C. E. ASBURY. 1992. Riparian nitrogen dynamics in two geomorphologically distinct tropical rain forest watersheds: subsurface solute patterns. Biogeochemistry 18:53–75. MERRIAM J., W. H. MCDOWELL, AND W. S. CURRIE. 1996. A high-temperature catalytic oxidation technique for determining total dissolved nitrogen. Soil Science Society of America Journal 60:1050– 1055. MULHOLLAND, P. J. 1992. Regulation of nutrient concentrations in a temperate forest stream: roles of upland, riparian, and in-stream processes. Limnology and Oceanography 37:1512–1526. NELSON,W. M., A. J. GOLD, AND P. M. GROFFMAN. 1995. Spatial and temporal variation in groundwater nitrate removal in a riparian forest. Journal of Environmental Quality 24: 691–699. PASTOR, J., J. D. ABER, C. A. MCCLAUGHERTY, AND J. M. MELLILO. 1984. Aboveground production and N and P cycling along a nitrogen mineralization gradient on Blackhawk Island, Wisconsin. Ecology 65:256–268. PETERJOHN, W. T., AND D. L. CORRELL. 1984. Nutrient dynamics in an agricultural watershed: observations on the role of a riparian forest. Ecology 65: 1466–1475. PINAY, G., C. RUFFINONI, AND C. ARLES. 1995. Nitrogen cycling in two riparian forest soils under different geomorphic conditions. Biogeochemistry 30:9–29. SHARP, J. H., R. BENNER, L. BENNETT, C. A. CARLSON, R. DOW, AND S. E. FITZWATER. 1993. Re-evaluation of high temperature combustion and chemical oxidation measurements of dissolved organic carbon in seawater. Limnology and Oceanography 38:1774–1782. SIMMONS, R. C., A. J. GOLD, AND P. M. GROFFMAN. 1992. Nitrate dynamics in riparian forests: groundwater studies. Journal of Environmental Quality 21:659–665. AND
214
T. J. CHESTNUT
AND
STANFORD, J. A., AND J. V. WARD. 1993. An ecosystem perspective of alluvial rivers: connectivity and the hyporheic corridor. Journal of the North American Benthological Society 12:48–60. STREAM SOLUTE WORKSHOP. 1990. Concepts and methods for assessing solute dynamics in stream ecosystems. Journal of the North American Benthological Society 9:95–119. TRISKA, F. J., J. H. DUFF, AND R. J. AVANZINO. 1990. Influence of exchange flow between the channel and hyporheic zone on nitrate production in a small mountain stream. Canadian Journal of Fisheries and Aquatic Sciences 47:2099–2111. TRISKA, F. J., J. H. DUFF, AND R. J. AVANZINO. 1993. Patterns of hydrologic exchange and nutrient transformation in the hyporheic zone of a gravel bottom stream: examining terrestrial-aquatic linkages. Freshwater Biology 29:259–274. TRISKA, F. J., A. P. JACKMAN, J. H. DUFF, AND R. J. AVAZINO. 1994. Ammonium sorption to channel and riparian sediments: a transient storage pool for dissolved inorganic nitrogen. Biogeochemistry 26: 67–84. TRISKA, F. J., V. C. KENNEDY, R. J. AVANZINO, G. W. ZELLWEGER, AND K. E. BENCALA. 1989. Retention and transport of nutrients in a third-order stream in Northwestern California: hyporheic processes. Ecology 70:1893–1905. VALETT, H. M., C. N. DAHM, M. E. CAMPANA, J. A. MORRICE, M. A. BAKER, AND C. S. FELLOWS. 1997. Hydrologic influences on groundwater-surface water ecotones: heterogeneity in nutrient composition and retention. Journal of the North American Benthological Society 16:239–247. VALETT, H. M., S. G. FISHER, N. B. GRIMM, AND P. CAMILL. 1994. Vertical hydrologic exchange and ecological stability of a desert stream ecosystem. Ecology 75:548–560. VALETT, H. M., S. G. FISHER, AND E. H. STANLEY. 1990. Physical and chemical characteristics of the hyporheic zone of a Sonoran desert stream. Journal of the North American Benthological Society 9: 201–215. VALETT, H. M., J. A. MORRICE, C. N. DAHM, AND M. E. CAMPANA. 1996. Parent lithology, surfacegroundwater exchange, and nitrate retention in
W. H. MCDOWELL
[Volume 19
headwater streams. Limnology and Oceanography 41:333–345. VAUX, W. G. 1968. Intergravel flow and interchange of water in a stream bed. US National Marine Fisheries Service Fishery Bulletin 66:479–489. VERCHOT, L. V., E. C. FRANKLIN, AND J. W. GILLIAM. 1997. Nitrogen cycling in piedmont vegetated filter zones: II. Subsurface nitrate removal. Journal of Environmental Quality 26:337–347. WALLIS, P. M. 1981. The uptake of dissolved organic matter in groundwater by stream sediments—a case study. Pages 97–111 in M. A. Lock and D. D. Williams (editors). Perspectives in running water ecology. Plenum Press, New York. WHITE, A. F., A. E. BLUM, M. S. SCHULZ, D. V. VIVIT, D. A. STONESTROM, M. LARSEN, S. F. MURPHY, AND D. EBERL. 1998. Chemical weathering in a tropical watershed, Luquillo Mountains, Puerto Rico: I. Long-term versus short-term weathering fluxes. Geochimica et Cosmochimica Acta 62:209– 226. WHITE, C. H., C. H. ELZINGA, AND S. P. HENDRICKS. 1987. Temperature patterns within the hyporheic zone of a northern Michigan river. Journal of the North American Benthological Society 6:85–91. WONDZELL, S. M., AND F. J. SWANSON. 1996a. Seasonal and storm dynamics of the hyporheic zone of a 4th-order mountain stream. I. Hydrologic processes. Journal of the North American Benthological Society 15:3–19. WONDZELL, S. M., AND F. J. SWANSON. 1996b. Seasonal and storm dynamics of the hyporheic zone of a 4th-order mountain stream. II. Nitrogen cycling. Journal of the North American Benthological Society 15:20–34. WROBLICKY, G. J., M. E. CAMPANA, H. M. VALETT, AND C. N. DAHM. 1998. Seasonal variation in surfacesubsurface water exchange and lateral hyporheic area of two stream-aquifer systems. Water Resources Research 34:317–328. ZELLWEGER, G. W. 1994. Testing and comparison of four ionic tracers to measure stream flow loss by multiple tracer injection. Hydrological Processes 8:155–165. Received: 17 June 1998 Accepted: 16 February 2000
