1203 Effects of hydromorphological integrity on biodiversity and functioning of river ecosystems

GLOBAL CHANGE AND RIVER ECOSYSTEMSEffects of hydromorphological integrity on biodiversity and functioning of river ecosystemsArturo Elosegi •Joserra Dı´ez •Michael Mutz Received:5August 2009/Accepted:29December 2009/Published online:22January 2010ÓSpringer Science+Business Media B.V.2010Abstract River channels tend to a dynamic equilib-rium driven by the dynamics of water and sediment discharge.The resulting fluctuating pattern of channel form is affected by the slope,the substrate erodibility,and the vegetation in the river corridor and in the catchment.Geomorphology is basic to river biodiver-sity and ecosystem functioning since the channel pattern provides habitat for the biota and physical framework for ecosystem processes.Human activities increasingly change the natural drivers of channel morphology on a global scale (e.g.urbanization increases hydrological extremes,and clearing of forests for agriculture increases sediment yield).In addition,human actions common along world rivers impact channel dynamics directly,e.g.river regulation simplifies and fossilizes channel form.River conser-vation and restoration must incorporate mechanisms ofchannel formation and ecological consequences of channel form and dynamics.This article (1)summa-rizes the role of channel form on biodiversity and functioning of river ecosystems,(2)describes spatial complexity,connectivity and dynamism as three key hydromorphological attributes,(3)identifies prevalent human activities that impact these key components and (4)analyzes gaps in current knowledge and identifies future research topics.Keywords River ecosystem ÁHydromorphology ÁBiodiversity ÁFunctioningIntroductionLotic ecosystems are integral elements of landscapes,shaped by the transport of water and materials from their drainage basins (Hynes,1975).Because the unidirectional transport occurs in a dendritic network and is highly episodic,river channels are spatially complex and temporally variable (Rosgen,1996).As river ecologists discovered the relevance of transport,flood dynamics,channel complexity,parafluvial and floodplain areas,and other key characteristics of river ecosystems,different concepts dominated the field of river ecology.Pioneering works (Hawkes,1975)described rivers as composed of discrete biological zones set downstream in a predictable order.The River Continuum Concept (RCC,Vannote et al.,1980),on the other hand,stressed the fact that zonesGuest editors:R.J.Stevenson,S.Sabater /Global Change and River Ecosystems –Implications for Structure,Function and Ecosystem ServicesA.Elosegi (&)ÁJ.Dı´ez Faculty of Science and Technology,University of the Basque Country,PO Box 644,48080Bilbao,Spain e-mail:arturo.elosegi@ehu.esM.MutzDepartment of Freshwater Conservation,Brandenburg University of Technology,Seestr.45,15526Bad Saarow,Germany123Hydrobiologia (2010)657:199–215DOI 10.1007/s10750-009-0083-4are not discrete,but change in a rather continuous way along the river,driven primarily by changes in channel morphology.The RCC was criticized by some authors(Winterbourn et al.,1981;Statzner& Higler,1985),and concepts were advanced regarding morphological discontinuity.For instance,the Serial Discontinuity Concept(Ward&Stanford,1983) made predictions on the effect of dams,largely based on the RCC,and the network dynamics hypothesis (Benda et al.,2004)analyzed the effect of tributary confluences.On the other hand,the Flood Pulse Concept(Junk et al.,1989;Junk&Wantzen,2003) stressed the role of theflood regime on the ecology of large rivers,and the Riverine Productivity Model (Thorp&Delong,1994)challenged some of the tenets of the RCC regarding the source of organic matter.More recently,the Riverine Ecosystem Syn-thesis(Thorp et al.,2006)depicted rivers as an array of large‘hydrogeomorphic patches’(equivalent to constrained,braided or meandering sections),and stressed the fact that there is no simple way to predict the position of these patches along a river,but they are associated to distinct functional zones.Thus, biological communities are shaped by a hierarchy of environmental factors,from ones affecting regional-scale distribution of organisms,down to factors affecting communities at the reach-scale,and even at the scale of individual riffles(Parsons&Thoms, 2007).Although scientists have long explored the role of channel morphology on river ecology,managers only recently recognized hydromorphology as an impor-tant element of streams and rivers,often focusing mainly onfish and benthic invertebrate habitat, neglecting other aspects of biodiversity and ecosys-tem functioning.This probably occurred because river morphology is highly variable,depending on a hierarchy of controlling factors in the catchment, including upstream and,to a lesser extent,down-stream parts of the river network.Natural constraints such as climate and geology and human activities such as land use andflow regulation,determine the main drivers—hydrological regime,sediment regime and riparian vegetation—that shape the local morphology.Because channel form and hydraulics provide a structural template that shapes ecological processes, many authors stressed their importance for river biodiversity and functioning.Different terms have been coined to stress this interplay:hydromorphology (EU,2000),eco-geomorphology(Thoms&Parsons, 2002),functional ecomorphology(Fisher et al.,2007) and others.Despite this recognition there is still a lack of understanding of complex responses of biological processes to hydromorphology,which often shows shifting patterns rather than static balances(Lenders et al.,1998).Even when acknowl-edged,hydromorphology is mostly seen as static(e.g. riffle-pool sequences),neglecting the dynamic aspect of channel form,which is central to hydromorpho-logical quality.Newson&Large(2006)defined natural river channels as those whose geometry and features represent the full interplay of unmanaged water and sedimentfluxes with local boundary conditions.Such channels are free to adjust by aggradation,degradation or by lateral interaction with thefloodplain or valley floor in response to unmanagedflows and sediment supplies(short term)or in response to long-term changes in system or local drivers.They are not wilderness channels but may inspire a holistic percep-tion of being‘intact’,a popular human perception of reference conditions deriving mainly from landscape aesthetics.‘Natural’channels require minimum man-agement intervention to offer resilience and a diversity of physical habitat,though neither of these‘natural services’is universal or perpetual.Here we analyze the importance of hydromorpho-logical integrity(i.e.of natural river channels),on biodiversity and functioning of river ecosystems, focusing in three key attributes for stream ecosystem functioning:spatial complexity,connectivity and dynamism.We also discuss the effects of human activities on the hydromorphological integrity of rivers and identify current gaps in knowledge,and topics for future research.Hydromorphological attributes of importanceto stream ecologyThe term hydromorphology,which was recently made popular by the European Water Framework Directive(EU,2000),reflects the inseparable asso-ciation of channel form andflow.Depending on river type,hydromorphology is expressed by a different array of morphological elements and hydrodynamic features.For instance,oxbow lakes are important200Hydrobiologia(2010)657:199–215 123constituents of lowland rivers meandering in wide valleys,but not in v-shaped constrained ones.It is beyond the objectives of the present article to give a comprehensive list of all elements constituting channel form and dynamics and to specify their characteristics for different rivers.We instead give ecologically significant examples that illustrate three attributes of key importance for river ecosystems—spatial com-plexity,connectivity and dynamism(Table1).Spatial complexity is created and maintained by a series of processes acting from microhabitat to catchment scale and driven by periodic and stochastic events in time, such as discharge and sediment input.Sequences of complex reaches along a river network further increase dynamics and spatial complexity.Hence channel complexity,connectivity,and dynamism are closely linked.Spatial complexity of channel and river corridor Channel complexity results from processes occurring at a wide variety of mutually dependent scales (Frisell et al.,1986;Thorp et al.,2006).At the microhabitat scale,smaller than the width of the stream,sediments are usually sorted by grain size, resulting in patches that provide habitat for different organisms.Sorting processes depend on the local flow pattern that is generated at the reach scale,and largely modified by localflow obstructions such as boulders,macrophytes or large wood(Fig.1).In the vertical and lateral dimension,variability of sedi-ments and hydraulic conductivity often reflect a series of succeeding historic channel forms,former flood events and sediment deposition.Legacies of historical channel processes increase the complexity of current morphological pattern at all scales(Frisell et al.,1986;Gregory et al.1991).At the reach scale (at least1–2magnitudes of the stream width), physical complexity is expressed as changes in slope, cross-section and plan form.Stream ecologists often categorize distinct visible morphological elements like riffles,various forms of pools,bars,banks and so on.These elements differ in width,depth,water velocity and grain size,sorting and packing of the sediments,and hence control the microhabitat com-plexity.At the catchment scale,channel form differs along the drainage network,according to constraints such as geology and lithology,increasing discharge and changes in sediment load or slope and valley form.On this large scale,there is a general sequence of straight to braided to meandering morphology from the source to the mouth of a river,and this sequence affects complexity at both the reach and microhabitat scales.Another general trend along the course of rivers is the increasing width of the river corridor and increasing significance of thefloodplain, which itself can be expressed in variousfloodplain-specific water bodies.However,these general patterns are often broken by stream confluences, lakes,regional geology and changes of valley form (Ward&Stanford,1983).ConnectivityThe concept of connectivity wasfirst applied to river systems by Amoros&Roux(1988).Although mainly recognized as an important ecosystem aspect,con-nectivity in rivers is primarily a hydromorphological attribute.Pringle(2001)defined hydrologic connec-tivity as water-mediated transfer of matter,energy and organisms within or between elements of the hydrologic cycle.In rivers,connectivity works in three dimensions(Kondolf et al.,2006).Longitudinal connectivity controls the downstreamflux of water and sediments along the river network,and thus the basic processes shaping channel form.To a limited degree there is even downstream to upstream con-nectivity,which can be seen in case of backward erosion or sediment aggradation caused by shifts in downstream erosion base level.Additionally,water and sediments can be transported in the lateral and vertical teral connectivity between the channel and thefloodplain is also important;sedi-ments deposited duringfloods form thefloodplains and can return to the main channel when the channel migrates laterally(Junk et al.,1989).Hydraulic connectivity between the stream channel and shallow groundwater aquifers can extend for considerable distances laterally into the banks and belowflood-plains(Standford&Ward,1988),though the major exchange in most streams and rivers is in vertical dimension across the stream bed.Vertical connectivity is created by the exchange of water between the water column and the hyporheic zone as well as the vertical accretion of sediment deposits.Waterflux across the bed and banks is driven by hydraulic pressure vari-ability and depends on the porosity of the sediments. Vertical connectivity is hence closely related toHydrobiologia(2010)657:199–215201123T a b l e 1A t t r i b u t e s o f h y d r o m o r p h o l o g y o f p a r t i c u l a r r e l e v a n c e t o s t r e a m a n d r i v e r e c o l o g y ,s c a l e s a t w h i c h t h e y a r e i m p o r t a n t ,e x a m p l e s o f v a r i a b l e s a f f e c t e d ,e c o l o g i c a l s i g n i fic a n c e a n d m a i n h u m a n i m p a c t sA t t r i b u t e S c a l eF e a t u r e s r e l a t e dS i g n i fic a n c e f o r b i o d i v e r s i t y S i g n i fic a n c e f o r f u n c t i o n i n g M a i n i m p a c t sC o m p l e x i t y M i c r o h a b i t a tS e d i m e n t g r a i n s i z e ,s o r t i n g a n d p a c k i n g ;w o o d ,C P O M F u n c t i o n a l h a b i t a t d i v e r s i t y ;r e f u g e ;f o o d a v a i l a b i l i t y ;s p a w n i n g a r e a sD i v e r s i t y /f u n c t i o n l i n k s ;h y d r a u l i c ,n u t r i e n t a n d O M r e t e n t i o n ;r e d o x g r a d i e n t s ,m e t a b o l i s m S i l t a t i o n ;flo w d e t r a c t i o n a n d r e g u l a t i o n ;s n a g g i n g ;c a n a l i z a t i o nR e a c hC h a n n e l s l o p e ,c r o s s -s e c t i o n &p l a n f o r m ;r i f fle /p o o l s s e q u e n c e s ;b a r s &u n d e r c u t b a n k sM a c r o h a b i t a t d i v e r s i t y ;r e s o u r c e p a r t i t i o n i n g f o r o r g a n i s m s F u n c t i o n a l z o n e s ;m e t a b o l i s mG r a v e l m i n i n g ,b a n k a n d b e d r e v e t m e n t s ;v e g e t a t i o n r e m o v a lC a t c h m e n tC h a n n e l t y p o l o g y a l o n g d r a i n a g e n e t w o r kR e g i o n a l b i o d i v e r s i t y S e e d l i n g e s t a b l i s h m e n t ;e x c h a n g e o f n u t r i e n t s a n d O ME x t e n s i v e l a n d u s e c h a n g e ;u r b a n i z a t i o nC o n n e c t i v i t y L o n g i t u d i n a lC o n t i n u i t y L a r g e -s c a l e m o b i l i t y ;m e t a p o p u l a t i o n d y n a m i c s ;r e s i l i e n c e ,c o m m u n i t y p e r s i s t e n c eB a r r i e r e f f e c t ;s e d i m e n t t r a n p o r t ;p o p u l a t i o n s t r u c t u r eD a m s ,e n g i n e e r e d r e a c h e s ;flo w r e g u l a t i o n ,flo o d c o n t r o l ,a n d d r a i n a g eL a t e r a lC h a n n e l p l a n f o r m ;h y d r a u l i c c o n n e c t i v i t y ;flo o d i n g R e f u g e (flo o d s );b r e e d i n g i n flo o d p l a i n s ;t e r r e s t r i a l a n d a q u a t i c o r g a n i s m i n t e r a c t i o n sS e d i m e n t a n d O M r e t e n t i o n ;e n h a n c e s v e r t i c a l c o n n e c t i v i t yC a n a l i z a t i o n ;d i k e s ;flo w r e g u l a t i o n ;v e g e t a t i o n r e m o v a lV e r t i c a lV a r i a b i l i t y o f c h a n n e l s l o p e a n d c r o s s -s e c t i o n ;h y p o r h e o s ;w o o d j a m s R e f u g e (flo o d s ,d r o u g h t s );l i f e c y c l e s ;b i o d i v e r s i t y h o t s p o t sD i v e r s i t y /f u n c t i o n l i n k s ;n u t r i e n t r e t e n t i o n ;r i v e r m e t a b o l i s m S i l t a t i o n ;c a n a l i z a t i o n ;d i k e sD y n a m i s m H o u r s /d a y sS t o r m r u n o f f ;flo o d s ;d r o u g h t s D i s t u r b a n c e ;o p p o r t u n i t y f o r c o l o n i s t s ;a v o i d a n c e o f c o m p e t i t i v e e x c l u s i o nR e s e t s s u c c e s s i o nR e g u l a t i o n ;d r e d g i n g ,b a n k fix a t i o n ;flo o d c o n t r o lM o n t h s /y e a r sH y d r o l o g i c r e g i m e ;s e d i m e n t b u d g e t ;c h a n n e l m i g r a t i o nD i s t u r b a n c e r e g i m e s h a p e s l i f e c y c l e s ;h a b i t a t ,r e f u g eC o n n e c t s r i v e r w i t h flo o d p l a i nF l o o d p l a i n f o r e s t r y ,u r b a n i z a t i o n ;r e g u l a t i o nC e n t u r i e sI s l a n d f o r m a t i o n ;p l a n f o r m a d j u s t m e n t b y b i o g e o m o r p h i c i n t e r a c t i o nK e y h a b i t a t s (o x b o w s …),L W i n p u t ;i m p o r t a n t f o r e v o l u t i o n ;h a b i t a t ,r e f u g eH y d r o g e o m o r p h i c p a t c h e s ;L W h o t s p o t f o r m e t a b o l i s m F l o o d p l a i n o c c u p a t i o n ;c h a n n e l a r t i fic i a l i z a t i o n 202Hydrobiologia (2010)657:199–215123channel complexity,since the variability of hydraulic pressure at the bed results from interaction of flow with bed forms or slope discontinuities at reach scale (Savant et al.,1987;Harvey &Bencala,1993;Elliott &Brooks,1997;Kasahara &Hill,2006)and the porosity is determined by sorting and packing of the sediments (Freeze &Cherry,1979).On all scales vertical exchange is highly variable in space and time.For instance,in a reach-scale flume study,wood input caused sudden sediment deposition and increase of the vertical water flux through the bed (Mutz et al.,2007).Vertical water flux increased again after bed forms extended due to elevated flow velocity (Fig.2).Hence,rivers exhibit shifting mosaics of patches of hyporheic exchange produced by the variable arrangement of morphological features such as wood,riffles,pools and bars.Vertical water flux through the bed sediments also causes physical retention of fine sediments and colloids,which can form clogging layers and reduce vertical connectivity (Brunke,1999).Sustainability of vertical connectivity and morphological dynamism are coupled because flush-ing floods and sediment redepostion are needed to periodically remove clogged layers and regeneratevertical connectivity (Scha ¨lchli,1992).DynamismRiver hydromorphology is far from static becauseriver channels are shaped by the transport of water and sediments that varies from periodic tohighlyFig.2Short-term alteration of bed forms and vertical connectivity by experimental wood addition to plane sand bed.A Wood distribution and resulting bed forms in experiment Wood-1with bed form generated by flow of 0.13ms -1and Wood-2with bed forms generated by 0.20ms -1;B Vertical water flux across the bed for the control without wood and the experiments with wood (Mutz et al.,2007)Hydrobiologia (2010)657:199–215203123episodic.Channels are constantly shifting and adjust-ing to changes in stream power,sediment yield and valley features,making rivers highly dynamic eco-systems.Dynamism is an intrinsic characteristic of rivers that shapes both channel form and connectiv-ity,processes often modified by human activities.Dynamism is linked to channel complexity and connectivity,as for instance,when meander migra-tion results on formation of oxbow lakes or when floods reconnect them to the main channel.Channel dynamics depend on many basin and reach charac-teristics,butflow is of paramount importance.Such dynamism is important for river ecology at a broad range of temporal scales,from days to centuries (Frisell et al.,1986).At a scale of hours to days,flood events resulting from storm runoff produce extensive movement and rearrangement of sediments and litter packs.They also cause substantial changes in lateral and vertical connectivity,driving transport of water and sediments in both dimensions.At a scale of months to years,channel form is shaped by seasonal and interannual variations in discharge.Bankfull discharge,which occurs on average once every 2years,is extremely important shaping channel dimensions.Additionally,landslides,severe storms and other events produce mass failures in the riparian forest,resulting in sporadic inputs of sediments and large wood,crucial to channel morphology(May& Gresswell,2004).At a scale of centuries,lateral migration of river channels leads to meander cutoff, formation of oxbow lakes,and terrestrialization of abandoned paleochannels,and there is a cycle of formation,growth and decay of islands in braided channels(Gurnell et al.,2001).Relevance for biodiversity and functioning Physical habitat shapes biological communities and ecosystem functioning.Smith&Powell(1971)depict its effect as a series of environmental‘screens’(from large-scale,biogeographic factors tofine physiolog-ical and biotic interactions)that eventually shapes the composition of local communities.Southwood(1977) stressed the idea that the environment forms the template to which organisms have adapted through natural selection,and therefore,affects life traits. Several authors(Tomanova&Usseglio-Polatera, 2007)showed that habitat characteristics at the mesoscale determine invertebrate biological traits, and this idea is in the core of the concept of functional habitats(Harper et al.,1992),that is still in use(Harvey&Clifford,2008).Nevertheless, attempts to identify one-to-one connections between surfaceflow types,units of channel morphology and functional habitats oversimplify a complex and dynamic hydraulic environment,and some authors (Harvey et al.,2008)proposed to use instead a nested hierarchy of reach-scale physical and ecological habitat structures,characterized by transferable assemblages of functional units.ComplexityThe relationship between physical complexity of ecosystems and diversity of biological communities leads to the concept of ecological niche,borrowed from thefield of architecture and reminiscent of the old saying that the more niches in a church,the more saints couldfit in.In a classical paper,Hutchinson (1959)already stressed the role of what he called‘the mosaic nature of the environment’in maintaining a large number of species,and thereafter,the role of architectural complexity on community diversity has become more and more evident(Oldeman,1983). Less evident but also important is the effect of physical complexity on ecosystem functioning.At the microhabitat scale,high diversity of sedi-ment size is clearly linked to biodiversity,because patches differing in grain size often constitute different functional habitats(sensu Harper et al., 1992).Many invertebrate taxa are tightly linked to grain size(Fig.3).For example,in Basque streams Capnioneura and Limnephilinae are found almost exclusively in organic matter and Rithrogena mainly on sand and stones.Many studies report dependencies of macroinvertebrate diversity,abundance,traits or productivity with substrate diversity or surface-perimeter ratio(Beisel et al.,1998,2000;Lancaster, 2000).Among the elements of the stream bed, organic material plays a key role as a substratum for ecosystem functioning(Aldridge et al.,2009). The physical complexity of dead wood affects abundance and diversity offish and macroinverte-brates(Crook&Robertson,1999;Scealy et al., 2007).Invertebrate taxa use wood directly as habitat and for food(Dudley&Anderson,1982;Hoffman&204Hydrobiologia(2010)657:199–215 123Hering,2000),while many fish seek refuge and cover.At the reach level,channel complexity is related with the diversity of riffle/pool sequences or with the abundance and diversity of gravel bars and features such as undercut banks.Again,this level of com-plexity is related to biodiversity and functioning,and loss of these features can result in local extinctions.As an example,many fish species require different habitats through their life cycle.Brown trout spawn in gravel-bed riffles or runs,young fry concentrate in shallow areas,and adults prefer deep pools and wood accumulations.Because predatory fish species have large effects on invertebrate food chains,the absence of key habitats can decrease fish populations,with effects cascading through the food webs (Katano et al.,2006).At the catchment scale,natural rivers usually show large differences in physical structure,species associ-ations and ecological functions both from headwaters to lower river,and among different tributaries of similar order.Dependence of biotic communities on local hydromorphological setting lead scientists to propose different schemes of zonation,based for instance on invertebrates (Ilies &Botosaneanu,1963),or fish (Huet,1962),or associated changes in ecosys-tem function (Vannote et al.,1980).Therefore,at the catchment scale,biodiversity is linked to diverse hydrogeomorphic patterns throughout the river network.ConnectivityConnectivity is important in all communities and ecosystems for many reasons,from maintaining gene pools in populations to recolonizing an area after a major disturbance.Since disturbance,mainly in form of floods or droughts,is so common,connectivity might be especially important in river ecosystems.All three dimensions of connectivity,longitudinal,lateral and vertical,are important for communities and ecosystem processes.Longitudinal connectivity with the entire drainage net is essential for migratory species that live in different reaches along their life cycles,and for organisms in general to recolonize a reach after disturbance.Colonists from adjacent reaches influ-ence the success or failure of river restoration projects (Kail &Hering,2009).The dendritic char-acteristics of river networks determine the location of refuges (Meyer et al.,2007),and thus,affect recol-onization trajectories.Furthermore,because river communities are always subject to a continuous supply of propagules from upstream reaches,changes in connectivity can affect local communities.Not all natural rivers have high longitudinal connectivity.Indeed,large waterfalls can block entire sections of the catchment and act as barriers to organism dispersal for millennia,resulting in different com-munities up and downstream.Longitudinal connec-tivity also can affect ecosystem functioning.ForFig.3Densities ofselected invertebrate taxa on microhabitats at theheadwater of the An˜arbe stream,Basque Country,Spain.Vertical bars are standard errorsHydrobiologia (2010)657:199–215205123instance,Taylor et al.(2006)showed that decreases in abundance of a detrital-feedingfish reduced downstream transport of organic carbon and increased primary production and respiration in a South American river.Upstream migration of salmon periodically carries energy and nutrients from the ocean into reaches where carcasses fertilize the stream and,mediated by predation and lateral trans-port by bears,provide N influx to riparian forests (Helfield&Naiman,2006;Quinn et al.,2009). Characteristics that make a reach a suitable corridor are highly species-dependent and not necessarily limited to channel variables.For instance,Roberts& Angermeier(2007)showed experimentally that ripar-ian cover significantly affects the movement offish along a reach.Lateral connectivity,or the ability for materials and organisms to cross the border between river channel and the riparian areas andfloodplains,is often linked to hydrological connectivity,and to the physical charac-teristics of banks and riparian areas.Steep,undercut banks form more difficult barriers than smooth ones, and the transition between aquatic and terrestrial habitats is probably easiest in complex channels with dead arms and fallen teral sloughs,alcoves, and side channels increase the interface between channel andfloodplain and act as preferential paths for organisms between these two habitats.Log jams and other large wood structures(including beaver dams)increase water stage and makeflooding more teral connectivity is important for in-stream biodiversity,at least in large rivers(Paillex et al.,2007),increasing the survival of species that spend a part of their life cycle on thefloodplain and another in the channel,like tropicalfish species reproducing onfloodplains(Welcomme,1985;Agost-inho et al.,2004).It is equally important for ecosystem function because it regulates the transport of nutrients and organic matter betweenfloodplain and channel. For instance,experimental cover of stream channel eliminated input of terrestrial insects into streams, changedfish and macroinvertebrate populations,and also affected terrestrial spider and bat populations because it decreased aquatic insect emergence,which is subsidy to terrestrial predators(Baxter et al.,2005). Additionally,lateral connectivity may increase the chances offinding refuge during disturbances,as when fish move into low velocity river margins and inver-tebrates crawl tofloodplain trees duringflood time.Through these complex interactions,lateral connec-tivity influences biodiversity at thefloodplain scale (Ward&Tockner,2001).Vertical connectivity across the bed and the banks occurs through the hyporheic zone,the dynamic ecotone between the surface stream and the shallow groundwater aquifer(Gibert et al.,1990)in which both waters mix(White,1993).Quantity and quality of water exchange control the magnitude of the hyporheic pore space and surfaces with the associated micro organisms and habitat of the hyporheos,which contribute significantly to stream biodiversity (Williams&Hynes,1974;Bretschko,1991;Boulton et al.,1998).It is also a temporary habitat for early instars,and pupae of many benthic invertebrates (Boulton,2000),as well as for the development of eggs and embryos of manyfish species(Vaux,1962; Malcolm et al.,2005).Further,the hyporheic zone can strongly influence metabolism at the reach scale (Grimm&Fisher,1984;Naegeli&Uehlinger,1997). Organic carbon,dissolved oxygen and other electron acceptors,enter the hyporheic zone through down-welling surface water or upwelling groundwater (Jones et al.,1995).Additionally,organic particles can be stored within the subsurface zone during sediment deposition(Metzler&Smock,1990;Fuss &Smock,1996).Because hyporheic metabolism may be limited by supply of organic carbon or exhaustion of terminal electron acceptors from the surface stream,vertical connectivity may limit stream metab-olism and regulate biogeochemical transformations. Vertical connectivity is spatially complex;flow direction,water residence time,and hyporheicflow path length vary with channel form and sediment structure.Abundant and shortflow paths may result in higher overall metabolic rates(Malard et al., 2002).On the other hand,longer hyporheicflow paths and small scale heterogeneity in vertical connectivity create contrasting conditions that permit the coexistence of oxidation and reduction processes within small spatial scales and maintain lower water temperatures.Bacterial production can be enhanced in downwelling zones whereas in upwelling water anaerobic conditions can promote ammonification, denitrification and sulphate reduction(Hendricks, 1993).Thus,upwelling water containing nutrients released by hyporheic metabolism can promote patches of high benthic primary production(Grimm &Fisher,1989).The proportion of total discharge in206Hydrobiologia(2010)657:199–215 123。