ECOHYDROLOGYEcohydrol. 3, 373–377 (2010)Published online 14 July 2010 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/eco.140

Ecohyrdology Bearings—Invited Commentary

What do we still need to know about the ecohydrologyof riparian zones?

Tim Burt,1* Gilles Pinay2 and Sergi Sabater3

1 Department of Geography, Durham University, Durham DH1 3LE, UK2 School of Geography, Earth & Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

3 Catalan Institute for Water Research, University of Girona, 17071 Girona, Spain

ABSTRACT

Riparian zones have become increasingly important elements of river basin ecohydrology given their relevance of nutrientdynamics to water quality management. Most attention has been given to nitrogen cycling, denitrification in particular, butnew processes have emerged as potentially important and need to be evaluated. Riparian zones are sensitive locations, so it isimportant to understand the impact of changes that can fundamentally alter their ecosystem processes and responses. Betterunderstanding of how riparian buffering varies in time and space provides the basis for a catchment-scale approach, but itseems likely that low-order basins will remain the focus for research and management, given that low-order basins constitutethe majority of basin area. Finally, it is argued that new ecohydrological indicators are needed that combine innovativebiogeochemical parameters and landscape connectivity measures. Copyright 2010 John Wiley & Sons, Ltd.

KEY WORDS riparian zone; nitrate; nitrogen cycling

Received 23 March 2010; Accepted 6 May 2010

INTRODUCTION

Riparian zones are important regulators within catchmentsystems because they can function either as a conduitor as a barrier for energy and material moving downslope (Burt and Pinay, 2005). Processes operating in near-stream areas often control stream water chemistry so that,for some solutes at least, there is no similarity between‘hillslope’ water and ‘stream’ water. Due to their situationas an interface between hillslopes and channels, riparianzones provide a classic case for which an ecohydrolog-ical approach is necessary to understand their dynamicsand functioning. Indeed, interactions between hydrolog-ical and biological processes are constantly shaping theroles that riparian zones can play, such as flood attenu-ation, sedimentation, low-streamflow maintenance, habi-tat for biodiversity, energy and matter supply to streambiota and diffuse nitrogen buffering capacity (Naimanand Decamps, 1997). Despite research beginning in the1940s, the study of riparian zones is a relatively youngbranch of science and there is still a lot to learn, espe-cially regarding their role as nitrogen buffers (Burt et al.,2010). Here, we suggest some priorities for research onthe ecohydrology of riparian zones.

* Correspondence to: Tim Burt, Department of Geography, DurhamUniversity, Durham DHI 3LE, UK. E-mail: t.p.burt@durham.ac.uk

CHALLENGING THE TRADITIONAL PATHWAYSFOR NITROGEN CYCLING IN RIPARIAN ZONES

It may be that too much attention has been given to den-itrification (Burgin and Hamilton, 2007). Indeed, severalnew pathways within the nitrogen cycle have been dis-covered in recent years, thanks to new molecular andisotopic techniques. For example, anaerobic ammoniumoxidation (anammox) is a process by which ammonia,combined with nitrite under anaerobic conditions, pro-duces N2 (Jetten et al., 1998). This is known to beimportant in deep sea (Engstrom et al., 2009) and estuar-ine (Nicholls and Trimmer, 2009) sediments, but almostnothing is known about the occurrence or importance ofthis process in riparian zones. Most likely, the process issuppressed by simple organic compounds normally foundin large quantity in soils and freshwater sediments, butthis assumption needs to be confirmed.

Dissimilatory nitrate reduction to ammonium (DNRA),a microbial process reducing nitrate to ammonium, isnot a new pathway (Tiedje et al., 1982). However, littleattention has been given so far to DNRA in riparianzones because DNRA does not remove nitrogen fromsoils; instead, nitrogen accumulates as ammonium, theend product of DNRA. However, some recent studiessuggest that DNRA could be a significant process in someplaces (Matheson et al., 2002; Davis et al., 2008). Indeed,DNRA appears to be important in humid tropical soilsas a potential N-conserving mechanism when coupled to

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374 T. BURT, G. PINAY AND S. SABATER

plant uptake (Templer et al., 2008). So far, accumulationof ammonium in wetlands and riparian zones has beenmostly attributed to the lack of nitrification in anaerobiczones. To what extent is DNRA an important process forretaining and recycling nitrogen in riparian zones?

Nutrient cycles are usually studied in isolation, whereasthere is in fact tight coupling between different elementalcycles. For instance, the DNRA pathway can be linkedto sulphur oxidation (Dannenberg et al., 1992); somesulphur-oxidizing bacteria can use nitrate to oxidize sul-phide (H2S) to sulphate (SO4

2�). Nitrate can then bereduced to N2 or ammonium. This pathway has been usedas a method of water treatment, but the occurrence of thispathway in the riparian zone has not yet been confirmed.To date, only two field studies have measured potentialactivity by injecting nitrate into the sulphur-rich reducedzone (Whitmire and Hamilton, 2005; Burgin and Hamil-ton, 2008). The challenge now is to determine whethernitrate and sulphur can coexist in situ to evaluate thereal importance of this dissimilation pathway in riparianzones. Another example of coupled cycles is providedby Clement et al. (2005) who hypothesized coupling ofthe ‘ferrous wheel’ and anaerobic nitrification within theriparian zone. The possible oxidation of ammonium tonitrite by ferric ions in reduced zones could bypass theaerobic nitrification pathway, allowing nitrite to be deni-trified. Again, this pathway needs further in situ study todetermine its ability to remove ammonium from riparianwetlands. Moreover, such studies need to be carried outunder different climatic conditions to establish the realcontribution of riparian zones to the residence time ofnitrogen in drainage basins.

DETERMINING THE EFFECTS OF GLOBAL(AND CLIMATE) CHANGE IN NITROGEN

DYNAMICS IN RIPARIAN ZONES

It is well known that the active riparian zone expandsand contracts with changes in water storage; riparianzones are larger during high flows but contract during dryperiods (Beven and Kirkby, 1979). Extreme examples ofthese variations may be found in tropical, semiarid andarid rivers on one side, as well as in large floodplains onthe other side. How far these dynamics may be reinforcedin these regions of high variability (or expanded to lessobviously sensitive areas) because of climate and human-driven change is unknown.

In arid and semi-arid regions, aerobic and anaerobicconditions in soils alternate through shifts from dry tosaturated conditions (Baldwin and Mitchell, 2000), andthe microbial processes related to N dynamics fluctuateaccordingly. The reinvigorated biogeochemical activityfrom water returning to a previously dry stream bed isassociated with a restart of microbial processes (McClainet al., 2003). Under chronic drought, this process may beeven more dramatic. Groundwater extraction and overex-ploitation (which affects large portions of human-alteredlandscapes) may dramatically accelerate this process. An

extended depression of the water table in the riparianzone also may subsequently impair riparian forest vege-tation and favour replacement by shrubs and herbs. Soilmoisture retention also may be affected, because rootsmay be the soil’s hydraulic conductivity (Wickel et al.,2008).

The extent to which changes in hydrological connec-tivity and water flow paths produced in response toland use change (e.g. shifting from forest to pasture,transformation to agricultural landscapes) affect nitro-gen dynamics in tropical riparian zones remains to bedetermined. Although beneficial effects of retaining ripar-ian vegetation on fauna (invertebrates and fishes) havebeen observed (Lorion and Kennedy, 2009a,b), large-scale analysis of hydrological alterations and their effectson nitrogen fluxes are still in their infancy (Chaves et al.,2009).

In large floodplains, hydrological connections are oftenvery complex. Flooding is the most important event alonglarge alluvial rivers and triggers a variety of responses.Floods not only increase aquatic surface areas but alsoreshape aquatic and terrestrial habitats and maintain habi-tat complexity. Bertoldi et al. (2009) identified varioushydrological thresholds for a floodplain of the Taglia-mento River (NE Italy). Removal of annual flood eventsmay lead to an increase in vegetated areas, alteringthe flow-inundation regime of the entire river–floodplaincomplex. Understanding the fine-scale hydrology of theselarge-scale systems remains a challenge, both in terms ofmodelling and field observation, but it is essential if theeffects on nitrogen processing at the local scale are to bedetermined.

So far, only minor progress has been made in studyingthe effect of large-scale disturbance (both natural and dueto climate or global change) on hydrology and nutrientcycles. Fire may frequently affect riparian areas in aridand semi-arid zones, but fire also may occur in someseasonally dry subtropical and tropical regions (Pettit andNaiman, 2007). Fire not only affects vegetation but alsoaffects soils, significantly influencing soil hydrology andthe soil’s ability to process nitrogen. Flooding eventsoccurring after fire may be highly destructive of soiland river banks, reducing water infiltration and causingerosion (Shakesby and Doerr, 2006). In dry climates, salt-water intrusion of groundwater can be a complicatingfactor in the ecohydrology of riparian zones (Lamontagneet al., 2006). The extent and relevance of these processesdriven by global change may become significant atthe landscape scale, and need to be considered sothat ecosystem services provided by the riparian zone(Basnyat et al., 2000; Relu, 2010) are accounted for in areliable manner.

RESOLVING THE CONSEQUENCESOF CONTINUOUS VERSUS DISCRETE PATTERNS

OF N DELIVERY

Bishop et al. (2004) argued that a particularly impor-tant function of the riparian zone is to set stream water

Copyright 2010 John Wiley & Sons, Ltd. Ecohydrol. 3, 373–377 (2010)DOI: 10.1002/eco

ECOHYDROLOGY OF RIPARIAN ZONES 375

chemistry, because this is the last soil in contact withthe water before this water becomes runoff. The extentto which the setting of stream water chemistry happenswithin the riparian zone depends upon the residence timeof water there which, in turn, depends on the dominantflow paths operating in the area (Dahm et al., 1998). Thislinks neatly to the concept of ‘hot spots’ where hydrologi-cal flow paths converge with essential substrates, bringingtogether solutes with soil microbial activity or other flowpaths containing complementary or missing reactants.These hot spots exhibit disproportionately high-reactionrates relative to the surrounding matrix (McClain et al.,2003). Indeed, diffuse pollution is often more focussedthan generally realized (Burt and Arkell, 1987; Laneet al., 2008) because certain parts of catchments gener-ate disproportionate levels of pollution risk (Heathwaiteet al., 2000). In effect, some fields generate greater pol-lution risk than others, and this spatial variability mustbe captured in modelling at the catchment scale. How-ever, hydrological disconnection along flow paths meansthat, even where risks are generated, they do not nec-essarily connect with the drainage network (Lane et al.,2006). Even if flow paths do connect, the characteris-tics of the media that they connect through can have amajor impact upon their behaviour. This is reflected inthe kind of interventions often adopted for diffuse pollu-tion control, such as buffer strips (Haycock et al., 1997).This again points to the need to capture localized sinkand transformation processes, as well as the dynamics ofthese processes across space and over time.

Another aspect of the discrete pattern of nitrogen deliv-ery to riparian zones is timing. Indeed, nitrate is oftennot delivered continuously, and redox conditions control-ling the nitrogen cycle in riparian zones are not constant,especially under dry or temperate climate. This requiresconsideration that biogeochemistry in riparian zones canalso be ‘hot’ in the temporal dimension: ‘hot moments’ orperiods during which rates of biogeochemical processesare enhanced (McClain et al., 2003). The consequencesof a temporally discrete pattern of nitrogen delivery andcycling have not been yet fully addressed although tech-nology for the continuous measurement of these solutesis becoming available. For instance, in situations wheredenitrification is important, pulsed inputs can stronglyaffect the signal of the commonly used natural υ15N toestimate denitrification rates, one of the main processesremoving nitrogen in riparian zones (Bedard-Haughnet al., 2003; Clement et al., 2003; Lefebvre et al., 2007).Indeed, if nitrate is not delivered continuously, or at aslower rate than the denitrification process (Sebilo et al.,2003), then the 15N signal disappears because the heavynitrate will ultimately be used as well. Consequently,landscape assessment of denitrification hot spots mightbe underestimated. The inherently variable character ofriparian zones has to be better quantified to evaluatetheir real nitrogen buffering capacity. Indeed, riparianzones appear to be the last landscape feature able to mit-igate upslope inputs because recent findings have shownthat in-stream losses of nitrogen in small streams remain

static as nitrogen concentration increases (Lefebvre et al.,2005; Mulholland et al., 2008; Brookshire et al., 2009).

BROADENING THE APPROACH TO MODELLINGN BUFFERING CAPACITY AT THE CATCHMENT

SCALE

It may be trivial to state that topography and sedimentaryarchitecture determine which flow paths are operatingin the riparian zone. Although this is well known intheory, it is difficult to forecast in practice (Alexanderet al., 2009). Therefore, we require both better three-dimensional physically based modelling of flow pathswithin the riparian zone and improved field survey tech-niques to support the modelling. As far as modelling isconcerned, what is needed is a spatially explicit, multi-scale framework, able to capture sources, transport, sinksand transformation of individual nutrient species. Thisframework should contain the minimum level of processrepresentation necessary to incorporate local variation indiffuse pollution input and the key elements required tosimulate relevant biogeochemical processes, while alsobeing capable of upscaling to address river basin man-agement. We also need to couple new-generation diffusepollution models to the riparian zone models. For exam-ple, landscape connectivity models (Lane et al., 2009)will allow spatially varying inputs to the riparian zone tobe estimated while physically based, coupled hillslope-channel hydrological models (Reaney et al., 2007) canprovide the necessary linkage between terrestrial andaquatic systems.

The high local variability of hydro-geomorphologicaland land use contexts in small sub-catchments (i.e. low-stream orders) means that nutrient fluxes vary markedlyin time and space. It is clear from case studies in smallcatchments that the degree of functional connectivitybetween different landscape units is a key element inthe linkage between hydrology and biogeochemistry andis often crucial in determining the location of hot spotswithin the landscape. As much less variability is observedat the large river basin scale, we hypothesize that largevariability of fluxes reflects the variability of land-coverarrangements at the small-catchment scale; whereas atlarger scale, water quality reflects the average percentageof the different land covers (Burt and Pinay 2005).In other words, the signal to noise ratio is low inlarge basins (i.e. subtle changes in land managementpractices cannot be detected at the basin outlet) buthigh in smaller tributaries (Strayer et al., 2003; Bishopet al., 2008). Therefore, another way of addressing theevaluation of nitrogen buffering capacities might be tofocus on the likelihood of hot spots and hot momentsexisting in small drainage basins, riparian zones beingjust one type of landscape component. In a recent study,Houlton and Bai (2009) estimated that about one thirdof nitrogen input in small pristine catchments was lostby denitrification. A worldwide study published earliercame to a similar conclusion (Seitzinger et al., 2006).The challenge now is to predict and quantify the existence

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376 T. BURT, G. PINAY AND S. SABATER

of these biogeochemical hot spots in small catchments;these are especially significant areas as they constitute asurprisingly large fraction of any large river basin (over50% of perennial stream length is composed of streamorder 1: Brinson, 1993). Several methods are available.Stable isotopes can reveal information about the source,pathway and residence time of water in a single measure(McGuire et al., 2005). Nitrogen sources and processessuch as nitrification and denitrification can be determinedusing isotopic natural abundance of 15N and 18O (Burnset al., 2009; Billy et al., 2010). Potential flow throughanaerobic organic-rich zones can be assessed using thecerium anomaly within the rare earth elements (REE)series (Worrall and Pearson, 2001; Duncan and Shaw,2003; Gruau et al., 2004). These proxies combined withlong-term measurements of nitrogen fluxes at the outletof several small drainage basins could allow landscapepatterns, water residence times and nitrogen retentioncapacity to be related. This offers the most effectivemeans of intervening in human activity to positivelyinfluence overall water quantity, quality and biodiversitygoals.

THE NEED FOR NEW ECOHYDROLOGICALINDICATORS

New ecohydrological indicators are needed that com-bine innovative biogeochemical parameters and land-scape connectivity measures. As noted above, tracerssuch as natural isotopes or REE seem to offer great poten-tial to generalize beyond the individual case study andallow us to map riparian zone function across catchments.We anticipate fundamental research in this area over thenext decade that will allow results from experimentalstudies of riparian zones to the wider catchment area;projects that adopt identical experimental design acrosseco-regions would seem particularly beneficial. ‘Hydro-logically intelligent’ measures of landscape connectivityhave already been included in catchment-scale diffusepollution models (Lane et al., 2006, 2009). The use ofagent-based models to follow nutrients moving throughthe catchment system seems also to merit careful atten-tion (Reaney et al., 2007; Reaney, 2008). These sorts ofmodels, together with more statistical approaches, willprovide a focus for the ongoing study of riparian zoneecohydrology.

Riparian zones need to be seen within the context ofa basin-wide approach to catchment management (Burtand Pinay, 2005). Given their location at the inter-face between hillslopes and channels, an ecohydrologi-cal approach is necessary to understand their dynamicsand functioning. Riparian zones are sensitive locationswithin the catchment system and their ecohydrologi-cal processes are vulnerable to transformative ecosys-tem change (Wilcox, 2010). Hillslope hydrology pro-vides the underlying linkage, controlling biogeochemicalpathways. Given growing interest in ecosystem services,the variety of services provided in the riparian zone

needs to be appreciated, including pollution mitigation,flood water retention, habitat conservation, landscape andamenity. An integrated, basin-wide approach is requiredto ensure that catchment managers can take full advantageof the ecosystem services supplied by the riparian zone;diffuse pollution modelling needs to be conducted at thecatchment scale, therefore, to ensure that the results ofprocess-based field research can be upscaled to an appro-priate level to support planning decisions.

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