The effects of an environmental flow release on water quality in the hyporheic zone of the Hunter River, Australia

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1 Hydrobiologia (25) 552:75 85 Ó Springer 25 D. Ryder, A. Boulton & P. De Deckker (eds), Conservation and Management of Australia s Water Resources: 2/2 Vision or Blind Faith A Tribute to the late Bill Williams DOI 1.17/s The effects of an environmental flow release on water quality in the hyporheic zone of the Hunter River, Australia Peter J. Hancock* & Andrew J. Boulton Ecosystem Management, University of New England, Armidale 235, NSW, Australia (*Author for correspondence: phancoc2@une.edu.au) Key words: groundwater, nitrate, environmental flows, hyporheic zone, river regulation Abstract Environmental flow releases have been advocated as a useful rehabilitation strategy for improving river condition but assessments of their success have typically focused on surface water quality and biota. In this study, we investigated the impacts of an environmental flow release on water temperature, conductivity, dissolved oxygen, and nitrate concentrations in surface and subsurface (hyporheic) water at upwelling and downwelling zones in three sites along the Hunter River, New South Wales, Australia. We hypothesised that the flow pulse would flush the sediments with oxygenated water, stimulating hyporheic microbial activity and nitrification, enhancing nitrate concentrations over time. Surface and subsurface samples were collected before, 7 days after, and 49 days after an environmental flow release of 5 Ml for a period of 3 days. No lasting effects on dissolved oxygen or conductivity were evident at most sites although dissolved oxygen declined over time at the downwelling site at Bowmans Crossing. At the downwelling zones at all sites, hyporheic nitrate concentrations declined initially following the release, but then rose or leveled off by Day 49. This initial drop in concentration was attributed to flushing of nitrate from the sediments. At two sites, nitrate concentrations had increased by Day 49 in the upwelling zones while at the third site, it fell significantly, associated with very low dissolved oxygen and likely reductive loss of nitrate. Electrical conductivity data indicate that potential inputs of agriculturally enriched groundwater may contribute to the nitrogen dynamics of the Hunter River. This study highlights the spatial heterogeneity that occurs in the hyporheic zone within and among sites of a regulated river, and emphasises the need for multiple-site surveys and an understanding of groundwater dynamics to assess physicochemical responses of the hyporheic zone to environmental flow releases. Introduction Gravel-bed rivers can be viewed as three-dimensional mosaics of patches where surface and subsurface water exchanges over multiple scales (Malard et al., 22). In streams that have a porous connection with the underlying aquifer, surface water downwells into the riverbed and mixes with groundwater in the hyporheic zone (Fiebig & Lock, 1991). This creates a dynamic ecotone (Gibert et al., 199; Vervier et al., 1992) between these systems whose boundary fluctuates in response to discharge and the volume of water exchanged with the surface stream (White, 1993). During low river flows, limited surface water enters the groundwater/surface water ecotone and the main contribution comes from deeper groundwater or lateral aquifers. Conversely, at high river flows or during spates, surface river water predominantly downwells into the hyporheic zone, altering subsurface water chemistry and residence time (Stanley & Boulton, 1995).

2 76 This view of gravel-bed rivers perfusing the landscape and connecting the groundwater to surface waters is now commonly accepted by lotic ecologists (Jones & Mulholland, 2) and hydrologists (Harvey & Wagner, 2). Aquatic ecologists and fisheries biologists have been aware of the ecological and functional significance of groundwater/surface water ecotones for several decades (reviews in Findlay, 1995; Brunke & Gonser, 1997; Boulton et al., 1998; Dent et al., 2). Not only can they harbour rich invertebrate biodiversity (Marmonier et al., 1993) and support the reproduction of several commercially important fish species (e.g., salmonids, Power et al., 1999), but they also make substantial contributions to overall stream respiration through microbially aided processing of organic matter and nutrient transformations (Marmonier et al., 1995). The chemistry of water in the groundwater/ surface water ecotone changes dramatically depending on the length of time that the water is in contact with the microbial biofilms coating the sediments (Valett et al., 1997; Dahm et al., 1998). Many of the nutrients transformed or generated in the hyporheic zone promote hotspots at upwelling zones in streams where productivity would otherwise be limited (Boulton, 1993; Dent et al., 2). Similarly, lateral extensions of the hyporheic zone can be important interceptors of agriculturally polluted groundwater, especially beneath riparian sediments rich in organic matter, preventing excessive nitrogen loading in streams at risk of eutrophication (Cle ment et al., 23). Hyporheic zones are essential components of several stream ecological processes such as nutrient spiraling and organic matter processing, yet the inclusion of hyporheic zones in river management and rehabilitation strategies in Australia has been slow or nonexistent (Boulton, 2) so that they are seldom explicitly mentioned in river management or rehabilitation plans. In all rivers, flow regulation by dams and weirs disrupts the natural sediment and flow regimes, with a myriad of repercussions on the riverine biota and ecological processes (reviews in Walker, 1985; Kingsford, 2; Naiman et al., 22). In an attempt to reduce the ecological effects of flow regulation, a common practice is to release environmental flows as a way of restoring natural river processes through partially simulating the pre-regulation regime (Ladson & Finlayson, 22). These releases seek to restore natural variability in flow, especially event-based responses to spates and floods, and ensure that key chemical, ecological, and geomorphologic processes occur in the adjacent dependent ecosystems (Gippel, 21). To date, virtually all studies of the ecological effects of environmental flow releases have focused on surface riverine biota and processes (e.g., Gore et al., 21; Arthington & Pusey, 23) although some authors have acknowledged the significance of flushing flows for the sediments of river beds (e.g., Wilcock et al., 1996). The potential for river regulation to affect hyporheic zones stems mainly from the weakening fluxes between the sediment and river due to interstitial sedimentation (colmation) and decreased hydraulic exchange (Hancock, 22). Weakened exchange between the stream and hyporheic zone can starve the interstitial environment of oxygen and promote reducing conditions, altering the nutrient dynamics of the river ecosystem (Brunke & Gonser, 1997). In this paper, we report on the effects of an environmental flow release on hyporheic water quality in downwelling and upwelling zones of three sites along the Hunter River, a large coastal river in New South Wales. We hypothesised that increased hydraulic exchange during the release would flush the sediments with oxygenated water, stimulating aerobic bacterial processes such as nitrification. This would increase nitrate concentrations in the sediments and at upwelling zones, and renew sediment microbial activity, as reported in many other gravel-bed streams (Duff & Triska, 2). Further, we hypothesised that the effect of the environmental flow would differ among sites with differing substrate size, and would decrease with longitudinal distance downstream as the magnitude of the pulse decreased. Study area The Hunter River is a sand and cobble-bed river that drains to the eastern coast of New South Wales (Fig. 1). Rising in the Barrington Tops National Park and flowing almost 3 km, the river s catchment of approximately 22 km 2 is dominated by Permian sediments in its lower

3 77 Pages R. NEW SOUTH WALES. Aberdeen Glenbawn Dam Hunter R. MUSWELLBROOK Goulburn R. Bowmans Crossing Sampling sites Town N Moses Crossing.Hunter R. Merriwa R Wollombi Brook SINGLETON km 4 NEWCASTLE Figure 1. Hunter River showing the locations of the three sites (triangles) at Aberdeen, Bowmans Crossing, and Moses Crossing. section, Triassic sediments to the south, and Devonian and Carboniferous rocks to the northeast (Chessman et al., 1997). These sediments have eroded over millennia to generate rich alluvial plains underlain by shallow aquifers closely associated with the river. Streams flowing from the north of the catchment supply gravel and cobble sediments to the Hunter River, while bedload from the southern and western tributaries generally consists of sand (Raine, 2). Near its headwaters, the Hunter River is impounded by Glenbawn Dam (87 Ml, Fig. 1), which provides water for irrigation, industry (mostly coal mining and power generation), and domestic purposes. The dam also plays a major role in flood mitigation in the valley. The main effect of Glenbawn Dam on the Hunter River has been a reduction in the size and frequency of small to medium spates, and the maintenance of steady low flows when the river might otherwise be dry (Chessman et al., 1997). We sampled the hyporheic zones of downwelling heads and upwelling tails of gravel riffles at three sites along the Hunter River Aberdeen, Bowmans Crossing, and Moses Crossing (Fig. 1). Aberdeen was the most upstream site, occurring 17 km downstream of Glenbawn Dam. The site has a riffle that drops 5 cm over its 17 m length, ranges between 25 and 3 m wide, and has a thalweg of 45 1 cm deep at baseflow. Mean substrate size in the riffle is 46 mm (obtained from 36 frozen sediment cores to a depth of 5 cm, P. Hancock unpublished data), and the bed is dominated by cobbles with sand/ silt filling the gaps between particles. Bowmans Crossing is 112 km downstream of Glenbawn Dam and has a substantially higher proportion of sand than Aberdeen (mean particle size of 34 mm, P. Hancock unpublished data). At this site, the riffle is 25 m wide and 3 m long, with thalweg depths ranging from 22 to 5 cm at baseflow. Located a further 12 river-km

4 78 downstream, the Moses Crossing riffle is 5 m long and 2 m wide. This site s substrate is a similar mix of sand and cobble to that of Bowmans Crossing (mean particle size of 33 mm, P. Hancock unpublished data), and the riffle s thalweg ranges in depth from 23 to 39 cm. Methods Sampling schedule and the environmental flow release Samples of surface and hyporheic water before the release (before-samples) were collected at the three sites from 9 to 11 September, 21. A 3-day environmental flow allocation of 5 Ml per day was then released from Glenbawn Dam on 12 to 14 September. The flow pulse had passed Aberdeen by 19 September and Bowmans Crossing and Moses Crossing by 2 September, allowing river stage to return to approximate pre-release levels (Fig. 2). From paired rising-stage samplers placed at each site, the increase in water level during the release was 38 ± 2.5 cm at Aberdeen, 32 ± 2.5 cm at Bowmans Crossing, and 27 ± 2.5 cm at Moses Crossing. Subsequent sampling was then conducted on September and 7 9 November, 7 and 49 days respectively after the flow pulse ceased. In between the second and third sampling occasions, river flow declined substantially (Fig. 2). Prior to any sampling, a small spate occurred, peaking at the Aberdeen gauge on 28 August and at the Jerrys Plains gauge (between Bowmans Crossing and Moses Crossing) on 29 August but flow had returned to pre-spate levels by 7 September (Fig. 2) Field sampling Samples were collected from the hyporheic zones of the study riffle at each site using a handoperated bilge-pump (Boulton, 1993) to extract water from PVC wells (16 mm internal diameter) hammered into the sediments with a metal driving rod to a depth of 4 cm. The location of each sampling point was carefully mapped and marked with a metal picket driven into the opposite bank so that each point could be re-sampled on subsequent occasions. A constant pumping speed was used to minimise variability among sites, times, and habitats (Hunt & Stanley, 2). Samples of Mean daily discharge /8 19/8 26/8 2/9 9/9 16/9 23/9 Figure 2. Hydrograph of mean daily flows (Ml) from 12 August to 18 November, 21 at Aberdeen (broken line) and Jerrys Plains (solid line). Jerrys Plains is located between Bowmans Crossing and Moses Crossing. Sampling times are marked on the graph for Aberdeen (triangles), Bowmans Crossing (squares) and Moses Crossing (circles). 3/9 Date 7/1 14/1 21/1 28/1 4/11 11/11 18/11

5 79 4 l were removed from triplicate wells beneath the head (downwelling zone DW) and tail (upwelling zone UW) of each riffle. Electrical conductivity (TPS MC81 Meter), dissolved oxygen (% saturation), and temperature (TPS WP-824 Dissolved Oxygen Meter with a YSI 5739 probe) were measured in each sample before filtering for dissolved nutrient analyses. Nutrient samples were filtered through Whatman GF/C papers into acid-washed polyethylene bottles that were rinsed thrice in the filtered sample water. Samples were frozen for transport back to the laboratory where they were analysed for nitrate- and nitrite-nitrogen (collectively referred to as NO x ) using the cadmium copper reduction method (Wood et al., 1967). Analyses were conducted in the laboratory using a Varian DMS8 spectrophotometer. After collecting water quality samples, vertical hydraulic head (HH) in each well was measured with a probe consisting of a light-emitting diode connected to charged (9 V) wires running down a graduated length of rigid tubing. This was lowered slowly into the well until the internal water level closed the circuit, causing the diode to illuminate. HH was determined as the difference between water levels in the well and the adjacent river (Dahm & Valett, 1996). Data analyses Three-way crossed ANOVAs were used to test the null hypotheses that mean water temperature, conductivity (EC), nitrate-nitrite concentration (NO x ), and percent oxygen saturation (DO) did not vary among Sites, Times, and Habitats. Prior to analysis, all variables were tested for normality using a Wilk Shapiro test in Statistix (Version 7, Analytical Software, Tallahassee, Florida) while homogeneity of variance was assessed by analysis of residuals. All data, except HH, needed to be log (x + 1) transformed to comply with the assumptions of the ANOVAs, which were computed using SYSTAT for Windows (Version 9.1, SPSS Incorporated, Evanston, Illinois). The level of significance was re-calculated using a Bonferroni adjustment to minimise Type I errors (Quinn & Keough, 22). To assess correlations among the environmental variables, Pearson correlation coefficients were determined on transformed data. High inter-variable correlations indicated that principal components analysis (PCA) would be useful to summarise the redundancy and illustrate trends of temporal change in environmental conditions at downwelling and upwelling zones at each site associated with the flow release. PCAs were done on log (x + 1)- transformed data using the PRIMER statistical package (Version 5.2.9, Plymouth Marine Laboratories, Plymouth, UK). Results Environmental variables Mean percentage saturation of dissolved oxygen (DO) in the hyporheic zone of the Hunter River ranged from 2.83 ±.81 to ± 2.4% throughout the study whereas in the surface stream, DO saturation was typically high (93.5 ± 1.1 to ±.55%, Table 1). Predictably, DO saturations in downwelling zones at all sites were significantly higher than in the corresponding upwelling zones (F 1,36 = , p <.1, Fig. 3a). This pattern was associated with the strong negative hydraulic heads in the downwelling zones (Fig. 3b) transporting stream water into the hyporheic zone, and there was a significant negative correlation between DO and HH (r = ).65, p <.1). Despite the strong association with water exchange, trends in hyporheic DO in response to the environmental flow were not consistent across sites (Time Habitat interaction: F 2,36 = 6.148, p =.5). At Aberdeen and Moses Crossing, DO saturations in the downwelling zones remained unchanged before and after the environmental release whereas at Bowmans Crossing, DO declined over time (Fig. 3a). DO saturations were consistently low in the upwelling zones at all sites, and declined over time at the sites with finer sediments (Bowmans Crossing and Moses Crossing, Fig. 3a). Vertical hydraulic head differed significantly between the downwelling and upwelling zones (Habitat: F 1,36 = , p <.1) at all sites. In the downwelling zones, the heads were much more pronounced ()6 to )134 mm) than at the upwelling zones (+2 to +26 mm). HH was greatest at Moses Crossing in both downwelling and

6 8 Table 1. Mean surface measurements (± SE) from the Hunter River at the three sampling times Before Day 7 Day 49 Aberdeen DO (% saturation) ± ± ± 1.1 Temperature ( C) ± ± ±.15 Electrical conductivity (ms cm )1 ).34 ±.1.27 ±.1.34 ±.1 Nitrate nitrite (mg l )1 ).4 ±.1.2 ±.1.2 ±.3 Bowmans Crossing DO (% saturation) ± ± ±.75 Temperature ( C) ± ± ±.5 Electrical conductivity (ms cm )1 ).46 ±.1.41 ±.1.59 ±.1 Nitrate nitrite (mg l )1 ).13 ±.2.4 ±.1.9 ±.1 Moses Crossing DO (% saturation) ± ± ±.55 Temperature ( C) 18.7 ±.1 2. ± ±.1 Electrical conductivity (ms cm )1 ).56 ±.1.42 ±.1.63 ±.3 Nitrate nitrite (mg l )1 ).1 ±.1.3 ±.1.3 ±.1 upwelling habitats (Site: F 2,36 = 11.22, p <.1, Fig. 3b). Overall, the environmental flow release had no detectable effect on HH (F 2,36 = 3.2, p =.53) across the sites combined but there were some site-specific patterns. Although there was no significant change in HH in upwelling or downwelling zones over time at Aberdeen, the magnitude of exchange at Bowmans Crossing increased with each consecutive sampling occasion (Time Site interaction: F 4,36 = 12.71, p <.1, Fig. 3b). Similarly, at Moses Crossing, there was a net rise in HH at Day 49 brought about by the marked increase in downwelling (Fig. 3b). Surface water temperatures ranged between 13 and 23 C during the study (Table 1). Differences in water temperature between the downwelling zones and the surface were not significant but water in the upwelling zone was typically cooler at all sites. As the study progressed into spring, surface and hyporheic temperatures increased over time (F 2,36 = 9.641, p <.1, Fig. 3c). On average, hyporheic water temperature among the sites increased downstream (F 2,36 = 1.383, p <.1, Fig. 3c). The electrical conductivity (EC) of hyporheic water was higher at Bowmans Crossing and Moses Crossing than at Aberdeen (F 2,36 = , p <.1, Fig. 3d). During the study, hyporheic EC ranged from.35 to.62 ±.1 ms cm )1 (Fig. 3d) while the EC of surface water spanned ±.4 ms cm )1 (Table 1). At the upwelling and downwelling zones of all sites, hyporheic EC fell significantly on Day 7 (F 2,36 = 81.68, p <.1), perhaps in response to lower surface water EC. By Day 49, EC had returned to pre-release concentrations at Aberdeen and exceeded them at Bowmans Crossing and Moses Crossing in both surface and hyporheic samples (Fig. 3d). Average NO x concentrations during the study ranged between.26 and.2 mg l )1 in the hyporheic zone (Fig. 3e) compared to.2.13 mg l )1 in the surface stream (Table 1). The significant three-way interaction for NO x in the Hunter River hyporheic zone (Time x Site x Habitat: F 4,36 = 5.446, p <.2) reflected the decline in hyporheic NO x on Day 7 in all habitats except upwelling at Bowmans Crossing (Fig. 3e). NO x concentrations then rose by Day 49 at Aberdeen and the upwelling zone at Moses Crossing, but remained constant or declined at Bowmans Crossing (Fig. 3e). Upwelling hyporheic water consistently contained higher NO x concentrations than in the downwelling zones at Aberdeen and Moses Crossing (F 2,36 = , p <.1) whereas the pattern was reversed at Bowmans Crossing (Fig. 3e). A PCA plot of the environmental variables separated samples from all the downwelling zones

7 81 (a) Dissolved Oxygen (% saturation) ABER BOWM MOSE (b) Hydraulic Head (mm) (c) -15 Temperature ( C) (d).6 Conductivity (mscm -1 ).4.2 (e).25 Nitrate-nitrite (mgl -1 ) DW UW DW UW DW UW Figure 3. Temporal changes in hyporheic dissolved oxygen (a), hydraulic head (b), water temperature (c), conductivity (d) and nitratenitrite concentrations (e) in downwelling (DW) and upwelling (UW) zones at Aberdeen (ABER), Bowmans Crossing (BOWM), and Moses Crossing (MOSE) before the environmental release (fully shaded), 7 days later (gray) and 49 days later (unshaded). from upwelling zone samples along the second principal component (PC2, 36.2% of variance, Fig. 4), reflecting differences in DO (r = ).648) and HH (r =.695). PC1, accounting for 38.4 % of the total variance, separated Aberdeen hyporheic samples from those of the other two sites (Fig. 4). This component was negatively, significantly loaded by EC (r = ).649) and temperature (r = ).62). Plotting the temporal changes in ordination space of the collective environmental variables illustrated consistent trends in the upwelling zones at all three sites (Fig. 4) with

8 82 an small increase from Before to Day 7 along PC2 (strengthening HH, declining DO) before a strong decline along PC1 associated with rising water temperature and EC. Conversely, trends in downwelling zones at Aberdeen and Moses Crossing ran in the opposite way with a small decrease along PC2 up to Day 7 whereas at Bowmans Crossing, the pattern resembled those of the upwelling zones. After Day 7 in all downwelling zones, trajectories mirrored those in the upwelling zones, declining along PC1 as water temperature and EC rose (Fig. 4). Thus, with the exception of Aberdeen and Moses Crossing downwelling zones, trends in the collective environmental variables over time were consistent across sites and zones. Discussion This study is the first to explore the influence of an environmental flow release on the hyporheic zone of a large regulated coastal river in Australia, and some transient changes in water quality were observed associated with the 3-day flow pulse. Hyporheic nitrate-nitrite concentrations initially declined after the environmental release in the Hunter River, implying that interstitial nitrate nitrite was flushed from the sediments by the increased flow. This corroborates the findings by Martı et al. (2) who observed similar patterns in hyporheic nitrate concentrations following a flash flood in a sand-bed Sonoran Desert stream, Sycamore Creek. In addition to flushing, declines in interstitial nitrate concentrations could have resulted from enhanced microbially facilitated denitrification. Following a flow event that increased the water level of the Garonne River in France by 35 cm, Baker & Vervier (24) observed accelerated rates of denitrification that they attributed partly to an increase in low molecular weight organic acids. Whatever the mechanism, the environmental flow caused an initial decline in the concentration of nitrate in the Hunter River. 2 DO Strength of HH 1 EC, Temperature PC PC2 Figure 4. Principal components analysis of environmental data from downwelling (open symbols) and upwelling (crossed symbols) zones at Aberdeen (triangles), Bowmans Crossing (squares), and Moses Crossing (circles) before the environmental release (fully shaded), 7 days later (gray) and 49 days later (unshaded). Dashed arrows indicate temporal changes within each zone at each site. Solid arrows near the two axes indicate their main associations with individual environmental variables. -3

9 83 As predicted, DO was lower in upwelling zones than downwelling zones in the Hunter River, and this has been commonly observed in other rivers (e.g., Sycamore Creek, Arizona, Valett et al., 199; Maple River, Michigan, Hendricks & White, 1995). These longitudinal declines in hyporheic DO signify active interstitial bacterial communities, and further evidence of this can be seen in aerobic sediments where increases in the concentrations of nitrate occur between connected downwelling and upwelling zones (Duff & Triska, 2). Such gradients have been reported in Little Lost Man Creek, California (Triska et al., 199) and Sycamore Creek (Jones et al., 1995), and were consistently evident at two of the sites in our study. The decline in nitrate concentrations at the third site (Bowmans Crossing) is harder to explain but may reflect the effects of the much lower DO concentrations (<2%) consistently found in the upwelling zones, leading to reductive conditions and denitrification. Another explanation for these patterns in hyporheic water quality must be considered in the Hunter River where the river has strong hydrological links with groundwater underlying a largely agricultural catchment. Although groundwater samples were not collected during this study, water beneath agricultural land can contain high concentrations of nitrates and low DO (Spruill, 2). For example, water from an aquifer beneath grassed agricultural land near German Branch, Maryland, contained 2 3 mg l )1 of nitrate, and despite significant denitrification in the riparian zone, concentrations were still high (12 18 mg l )1 ) when the water entered the stream (Correll et al., 1997). In another case, east of Lyon, France, groundwater contained 5 mg l )1 of nitrate below agricultural land, and 2 mg l )1 nitrate beneath suburban areas (Mauclaire et al., 2). Each of our study sites in the Hunter River is bounded by agricultural land. The presence of abundant groundwater-dwelling invertebrates in the hyporheic zones of these sites (P. Hancock, unpublished data) indicates a hydrological connection to the groundwater aquifer (cf. Bruno & Perry, 24). Therefore, changes in nitrate and DO concentrations may have been partially due to alterations in the strength of groundwater inputs rather than solely a result of interstitial microbial activity. The strong contribution of groundwater to the hyporheic flow in the Hunter River is further supported by correlations between EC and nitrate, as conductivity is often useful for indicating groundwater influx to streams (Atekwana & Krishnamurthy, 24). If continual groundwater inputs are supplying nitrate to the Hunter River, what hydrological influence might a pulsed environmental release have? Extensive irrigation of agricultural land in the Hunter River catchment relies on water pumped from the river, and there is likely to be a net flow from irrigated lands to the channel with perhaps lagged and weak pulses in response to the application of water to fields. When a flow pulse passes down the Hunter River, the rise in river level and increasing hydraulic pressure is expected to temporarily impede groundwater input and interstitial water flowpaths may even reverse so that the shallow aquifers are recharged from the channel (Gordon et al., 1992). At the same time, shallow hyporheic flowpaths are likely to be flushed and re-aerated by the increased area and strength of downwelling zones. In our study, a natural spate occurred just before the environmental flow release, potentially flushing accumulated nutrients including interstitial nitrate from the hyporheic sediment. Thus, the concentrations recorded before the flow release were probably lower than normal, effectively providing a flushed interstitial environment from which our experiment was able to commence. This may have prevented detection of any short-term flushing of nutrients by the environmental flow release, and is supported by nitrate concentrations that were higher in the upwelling zones of two sites by Day 49 than they were before the release. Low DO in the sediments at Bowmans Crossing probably meant that denitrification was transforming hyporheic nitrate in this upwelling zone. In contrast to our hypothesis, while the environmental flow release caused an increase in the DO of the hyporheic zone at two sites, this was not complemented by subsequent increases in nitrate concentrations as would be expected if there was stimulated nitrification. However, the environmental release did lead to a temporary decline in interstitial nitrate concentrations. While this study did not conclusively isolate the mechanism (or mechanisms) of nitrate removal, it emphasises the potential significance of environmental flow releases in flushing nitrates from

10 84 storage zones in nutrient-rich streams. Further, this study indicates that longitudinal responses to environmental flows between downwelling and upwelling can differ between sites of similar substrate composition (Moses Crossing and Bowmans Crossing), yet can be similar between sites that have very different substrate size and are further separated along the river continuum (Moses Crossing and Aberdeen). Further studies aiming to determine the effects of flow pulses on the hyporheic zone should assess how environmental releases impact on ecosystem processes such as nitrification. However, such studies need also to consider the potential impact of groundwater influx, whose nutrient concentrations and physico-chemical properties may also contribute to the dynamics of the hyporheic zone. The thornier issue in studies such as this is finding control sites (sensu Downes et al., 22) in equivalent rivers where environmental flows are not released. Ecologists will need to become more ingenious to tackle these problems at the broader scale (e.g., Chessman & Royal, 24), because human modification in river systems is now wide-spread. Acknowledgements We are grateful to the Department of Land and Water Conservation (now Department of Infrastucture, Planning and Natural Resources) for funding, and to Allan Raine and Sandra Mitchell (DIPNR) who arranged the environmental flow release. Kathrine Flink, Debbie Burgis, and Dean Olsen provided assistance in the field, and Marion Costigan assisted with the chemical analyses. Two anonymous referees and Dr Darren Ryder provided useful comments that improved an earlier draft of the manuscript. References Arthington, A. H. & B. J. Pusey, 23. Flow restoration and protection in Australian rivers. River Research and Applications 19: Atekwana, E. A. & R. V. Krishnamurthy, 24. Investigating landfill-impacted groundwater seepage into headwater streams using stable carbon isotopes. Hydrological Processes 18: Baker, M. A. & P. Vervier, 24. Hydrological variability, organic matter supply and denitrification in the Garonne River ecosystem. Freshwater Biology 49: Boulton, A. J., Stream ecology and surface-hyporheic exchange: implications, techniques and limitations. Australian Journal of Marine and Freshwater Research 44: Boulton, A. J., 2. River ecosystem health down under: assessing ecological condition in riverine groundwater zones in Australia. Ecosystem Health 6: Boulton, A. J., S. Findlay, P. Marmonier, E. H. Stanley & H. M. Valett, The functional significance of the hyporheic zone in streams and rivers. Annual Review of Ecology and Systematics 29: Brunke, M. & T. Gonser, The ecological significance of exchange processes between rivers and groundwater. Freshwater Biology 37: Bruno, M. C. & S. A. Perry, 24. Exchanges of copepod fauna between surface- and ground-water in the Rocky Glades of Everglades National Park (Florida, U.S.A.). Archiv fu r Hydrobiologie 159: Chessman, B. C., J. E. Growns & A. R. Kotlash, Objective derivation of macroinvertebrate family sensitivity grade numbers for the SIGNAL biotic index: application to the Hunter River system, New South Wales. Marine and Freshwater Research 48: Chessman, B. C. & M. J. Royal, 24. Bioassessment without reference sites: use of environmental filters to predict natural assemblages of river macroinvertebrates. Journal of the North American Benthological Society 23: Clément, J.-C., R. M. Holmes, B. J. Peterson & G. Pinay, 23. Isotopic investigation of denitrification in a riparian ecosystem in western France. Journal of Applied Ecology 4: Correll, D. L., T. E. Jordan & D. E. Weller, Failure of agricultural riparian buffers to protect surface waters from groundwater nitrate contamination. In Gibert, J., J. Mathieu & F. Fournier (eds), Groundwater/Surface Water Ecotones Biological and Hydrological Interactions and Management Options. Cambridge University Press, Cambridge: Dahm, C. N. & H. M. Valett, Hyporheic zones. In Hauer, F. R. & G. A. Lamberti (eds), Methods in Stream Ecology. Academic Press, San Diego: Dahm, C. N., N. B. Grimm, P. Marmonier., H. M. Valett & P. Vervier, Nutrient dynamics at the interface between surface waters and groundwaters. Freshwater Biology 4: Dent, C. L., J. J. Schade, N. B. Grimm & S. G. Fisher, 2. Subsurface influences on surface biology. In Jones, J. B. & P. J. Mulholland (eds), Streams and Ground Waters. Academic Press, San Diego: Downes, B. J., L. A. Barmuta, P. G. Fairweather, D. P. Faith, M. J. Keogh, P. S. Lake, B. D. Mapstone & G. P. Quinn, 22. Monitoring Ecological Impacts: Concepts and Practice in Flowing Waters. Cambridge University Press, Cambridge, UK. Duff, J. H. & F. J. Triska, 2. Nitrogen biogeochemistry and surface subsurface exchange in streams. In Jones, J. B. & P. J. Mulholland (eds), Streams and Ground Waters. Academic Press, San Diego:

11 85 Fiebig, D. M. & M. A. Lock, Immobilization of dissolved organic matter from groundwater discharging through the stream bed. Freshwater Biology 26: Findlay, S., Importance of surface subsurface exchange in stream ecosystems: the hyporheic zone. Limnology and Oceanography 4: Gibert, J., M.-J. Dole-Olivier, P. Marmonier & P. Vervier, 199. Surface water-groundwater ecotones. In Naiman, R. J. & H. Décamps (eds), UNESCO, Paris and Parthenon Publishers, Carnforth: Gippel, C. J., 21. Geomorphic issues associated with environmental flow assessment in alluvial non-tidal rivers. Australian Journal of Water Resources 5: Gordon, N. D., T. A. McMahon & B. L. Finlayson, Stream Hydrology - An Introduction for Stream Ecologists. John Wiley & Sons, Chichester. Gore, J. A., J. B. Layzer & J. Mead, 21. Macroinvertebrate instream flow studies after 2 years: a role in stream management and restoration. Regulated Rivers Research and Management 17: Hancock, P. J., 22. Human impacts on the stream groundwater exchange zone. Environmental Management 29: Harvey, J. W. & B. J. Wagner, 2. Quantifying hydrologic interactions between streams and their subsurface hyporheic zones. In Jones, J. B. & P. J. Mulholland (eds), Streams and Ground Waters. Academic Press, San Diego: Hendricks, S. P. & D. S. White, Seasonal biogeochemical patterns in surface water, subsurface hyporheic, and riparian ground water in a temperate stream ecosystem. Archiv fu r Hydrobiologie 134: Hunt, G. W. & E. H. Stanley, 2. An evaluation of alternative processes using the Bou-Rouch method for sampling hyporheic invertebrates. Canadian Journal of Fisheries and Aquatic Sciences 57: Jones, J. B., S. G. Fisher & N. B. Grimm, Nitrification in the hyporheic zone of a desert stream ecosystem. Journal of the North American Benthological Society 14: Jones, J. B. & P. J. Mulholland, 2. Streams and Ground Waters. Academic Press, San Diego. Kingsford, R. T., 2. Ecological impacts of dams, water diversions and river management on floodplain wetlands in Australia. Austral Ecology 25: Ladson, A. & B. Finlayson, 22. Rhetoric and reality in the allocation of water to the environment: a case study of the Goulburn River, Victoria, Australia. River Research and Applications 18: Malard, F., K. Tockner, M.-J. Dole-Olivier & J. V. Ward, 22. A landscape perspective of surface-subsurface hydrological exchanges in river corridors. Freshwater Biology 47: Marmonier, P., D. Fontvieille, J. Gibert & V. Vanek, Distribution of dissolved organic carbon and bacteria at the interface between the Rhoˆ ne River and its alluvial aquifer. Journal of the North American Benthological Society 14: Marmonier, P., P. Vervier, J. Gibert & M.-J. Dole-Olivier, Biodiversity in ground waters. Trends in Ecology and Evolution 8: Martí, E., S. G. Fisher, J. J. Schade & N. B. Grimm, 2. Flood frequency and stream-riparian linkages in arid lands. In Jones, J. B. & P. J. Mulholland (eds), Streams and Ground Waters. Academic Press, San Diego: Mauclaire, L., J. Gibert & C. Claret, 2. Do bacteria and nutrients control faunal assemblages in alluvial aquifers? Archiv fu r Hydrobiologie 148: Naiman, R. J., S. E. Bunn, C. Nilsson, G. E. Petts, G. Pinay & L. C. Thompson, 22. Legitimising fluvial ecosystems as users of water: an overview. Environmental Management 3: Power, G., R. S. Brown & J. G. Imhof, Groundwater and fish insights from northern North America. Hydrological Processes 13: Quinn G. P. & M. J. Keough, 22. Experimental Design and Data Analysis for Biologists. Cambridge University Press, Cambridge. Raine A., 2. State of the Hunter River. In Hunter Catchment Management Trust (ed.), Water (2) Forum: Our River, Our Future. Wyndham Estate, Branxton, New South Wales: Spruill, T. B., 2. Statistical evaluation of effects of riparian buffers on nitrate and ground water quality. Journal of Environmental Quality 29: Stanley, E. H. & A. J. Boulton, Hyporheic processes during flooding and drying in a Sonoran Desert stream I. Hydrologic and chemical dynamics. Archiv fu r Hydrobiologie 134: Triska, F. J., V. C. Kennedy, R. J. Avanzino, G. W. Zellweger & K. E. Bencala, 199. In situ retention-transport response to nitrate loading and storm discharge in a third-order stream. Journal of the North American Benthological Society 9: Valett, H. M., C. N. Dahm, M. E. Campana, J. A. Morrice, M. A. Baker & C. S. Fellows, Hydrologic influences on groundwater surface water ecotones: heterogeneity in nutrient composition and retention. Journal of the North American Benthological Society 16: Valett, H. M., S. G. Fisher & E. H. Stanley, 199. Physical and chemical characteristics of the hyporheic zone of a Sonoran Desert stream. Journal of the North American Benthological Society 9: Vervier, P., J. Gibert, P. Marmonier & M.-J. Dole-Olivier, A perspective on the permeability of the surface freshwater groundwater ecotone. Journal of the North American Benthological Society 11: Walker, K. F., A review of the ecological effects of river regulation in Australia. Hydrobiologia 125: White, D. S., Perspectives on defining and delineating hyporheic zones. Journal of the North American Benthological Society 12: Wilcock, P. R., G. M. Kondolf, V. G. W. Matthews & A. F. Barta, Specification of sediment maintenance flows for a large gravel-bed river. Water Resources Research 32: Wood, E. D., F. A. J. Armstrong & F. A. Richards, Determination of nitrate in seawater by cadmium copper reduction to nitrite. Journal of the Marine Biology Association 47: