Hydrobiologia (2013) 712:129–143 DOI 10.1007/s10750-012-1226-6
FORM AND FUNCTION
Review Paper
Effects of hydromorphological impacts on river ecosystem functioning: a review and suggestions for assessing ecological impacts Arturo Elosegi • Sergi Sabater
Received: 7 November 2011 / Accepted: 9 June 2012 / Published online: 6 July 2012 Springer Science+Business Media B.V. 2012
Abstract Because of the serious effects of pollution on water supply much closer attention has been paid to water quality than to other aspects of river integrity. However, channel form and water flow are relevant components of river health, and recent evidences show that their impairment threatens the services derived from them. In this article, we review the literature on the effects of common hydromorphological impacts (channel modification and flow modification) on the functioning of river ecosystems. There are evidences that
even light hydromorphological impacts can have deep effects on ecosystem functioning, and that different functional variables differ in their responses. Three criteria (relevance, scale and sensitivity) in the selection of functional variables are suggested as a guide for the river scientists and managers to assess the ecological impacts of hydromorphological modifications. Keywords Hydromorphology River Impact Ecosystem functioning Biodiversity Management
Introduction Electronic supplementary material The online version of this article (doi:10.1007/s10750-012-1226-6) contains supplementary material, which is available to authorized users. Guest editors: A. Elosegi, M. Mutz & H. Pie´gay / Form and function: channel form, hydraulic integrity, and river ecosystem functioning A. Elosegi (&) Faculty of Science and Technology, The University of the Basque Country, PO Box 644, 48080 Bilbao, Spain e-mail:
[email protected] S. Sabater Catalan Institute for Water Research (ICRA), Edifici H2O, Emili Grahit, 101, 17003 Girona, Spain e-mail:
[email protected] S. Sabater Institute of Aquatic Ecology, Faculty of Sciences, University of Girona, Campus Montilivi, 17071 Girona, Spain
The rapid increase in human activities threatens the sustainability of services provided by ecosystems (Millennium Ecosystem Assessment, 2005), and some of the planetary boundaries for sustainable use have already been exceeded (Rockstro¨m et al., 2009). Rivers are a paradigmatic example of this situation: they provide key services to society, harbour a large part of the world biodiversity, but are highly threatened (Vo¨ro¨smarty et al., 2003; UNEP, 2007). Because of the serious effects of pollution on water supply and human health, legislators and water agencies have historically paid much closer attention to water quality than to other aspects of river condition (Moss, 1998). However, channel form and water flow are key components of river condition, and physical river impairment threatens the benefits these ecosystems provide to the society (Elosegi et al., 2010).
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Hydromorphology (or hydrogeomorphology), i.e. the complex interaction between the flow of water and channel form, is key to river condition (Poole, 2010), and tightly linked to water quality, biodiversity and river ecosystem functioning (Boix et al., 2010; Ricart et al., 2010). Biodiversity and functioning of river ecosystems depend on the conservation of aquatic habitats and on the preservation of natural flow regimes (Poff et al., 1997). The relationships between these components are often complex (Elosegi et al., 2010), and their interplay is essential to achieve a sustainable management of river ecosystems. Understanding the main principles underlying the relation between channel form, water flow and river ecosystems (including their biodiversity and functioning) is thus essential to design strategies to recover truly functioning rivers in the current, highly modified landscapes. A large number of studies on river ecosystem processes such as nutrient retention, organic matter breakdown, or river metabolism address the effects of hydromorphology. However, most of them focus on natural changes, such as comparing areas under different geological settings, or the effect of natural floods and droughts (e.g. Uehlinger & Naegeli, 1998; Acun˜a et al., 2004; von Schiller et al., 2011). Even less is known on the effects produced by human alterations of hydromorphology, such as channelization or water abstraction, on river ecosystem processes. In this article, we review the literature that could shed light on the links between hydromorphological impacts and river ecosystem functioning. Based on these evidences, we also suggest some criteria to select relevant functional variables to assess the ecological impacts of hydromorphological modifications.
River health, river integrity and ecosystem functioning Scientists and managers currently use contrasting conservation concepts that ultimately reflect two differing philosophies: compositionalism and functionalism (Callicot et al., 1999). Compositionalists, with a strong background on evolutionary ecology, tend to focus on concepts like biological diversity, ecosystem integrity or ecological restoration, and to pay special attention to the structure or patterns in the ecosystem. For instance, biological integrity is defined
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as native populations existing in their historic variety and numbers naturally interacting in naturally structured biotic communities (Angermeier & Karr, 1994). Functionalists, on the other hand, pay more attention to ecosystem ecology, and thus, tend to focus on concepts like ecosystem health, ecosystem services or ecological rehabilitation. Both schools of thought are in fact two ends of a continuum, and most river scientists move along this continuum depending on the circumstances, putting more or less emphasis on integrity or on functioning (Callicot et al., 1999). Ultimately, both approaches are complementary, and depending on the management objectives, integrity can be the main goal for some rivers, whereas ecosystem functioning can be especially important for others (Scrimgeour & Wicklum, 1996). Ecosystem health is defined as the occurrence of normal ecosystem processes and functions (Costanza et al., 1992). This term has been successfully used because it is readily interpreted by the general public and evokes societal concern about human impacts in rivers (Boulton, 1999). Karr (1999) stated that integrity applies to pristine rivers not affected by human activity, whereas health would also include rivers that are not pristine, but are still in good condition. Several authors insist that river health should include human values, uses and amenities derived from the system (e.g. Meyer, 1997; Boulton, 1999; Vugteveen et al., 2006). Such inclusiveness does not mean, for instance, that more productive rivers are necessarily healthier. In any case, many goals relating to river management and protection refer to ecosystem-level processes (Bunn et al., 1999), and thus ecosystem functioning should be included amongst the criteria to assess river ecological status and river health. Ecosystem function or ecosystem functioning is directly linked to ecosystem processes which basically involve the transfer of energy and materials from the combined activity of living organisms, such as primary production, decomposition of organic matter, or nutrient retention (Lyons et al., 2005; Wallace, 2007). Ecosystem functions are at the basis of ecosystem services, defined as the benefits that people obtain from ecosystems (Millennium Ecosystem Assessment, 2005), although there is an ongoing debate over the classification of these services (Wallace, 2007; Fu et al., 2011). Compositionalists and functionalists agree that biodiversity is important for river ecosystem
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functioning (Loreau et al., 2001; Hooper et al., 2005). Although there is some degree of redundancy in natural communities, biodiversity is nonetheless essential to maintain ecosystem functioning (Peter et al., 2011), and the number of species required to sustain ecosystem functioning increases with the number of processes considered (Ghilarov, 2000; Hector & Bagchi, 2007). Global biodiversity is decreasing as a consequence of environmental changes (Butchart et al., 2010), and freshwater ecosystems are amongst the systems suffering the fastest decrease (Dudgeon, 2010). The specific responses of biodiversity and ecosystem functioning are at the base of any theoretical consideration on the impact of hydromorphological modifications.
Effects of common hydromorphological impacts on ecosystem functioning: a review Hydromorphological impacts are diverse and can affect stream channels, riparian areas and floodplains either directly (e.g. extraction of gravel, protecting the banks with riprap and building levees), or indirectly (e.g. impervious surfaces in the drainage basin increase flashiness). Their effects on channel form and biological communities have been extensively reviewed (e.g. Brandt, 2000; Kingsford, 2000; Corenblit et al., 2007), but information on ecosystem functioning is scanty by comparison, and most of what is known on their effects on ecosystem functioning is often based on proxies. For instance, chlorophyll concentration is sometimes used to infer primary production, and the fate of aquatic communities is used to derive ecosystem condition (Boix et al., 2010). However, these proxies do not always reflect accurately ecosystem functioning (Fig. 1, data from Izagirre et al., 2008), and, therefore, should be used with caution. In this article, we show the results of a systematic search in the ISI Web of Knowledge, as well as on the main journals devoted to stream ecology and more generalist journals. The focus was on papers directly analysing the effects of channel or flow modification on river ecosystem processes such as retention of nutrients, retention, storage or breakdown of organic matter, or metabolism. We also considered some papers not dealing directly with these processes but showing large effects of hydromorphological impacts
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Fig. 1 Relationship between periphytic chlorophyll a and gross primary production calculated in winter and summer from diel changes in oxygen concentration following the single station, open-channel method in 21 Basque streams (data from Izagirre et al., 2008). Note that the relationship is weak and therefore makes untenable that one variable can be inferred from the other
on other variables, when it was certain that these would in turn affect ecosystem functioning (e.g. the effects of macrophyte abundance). Channel modification The direct modification of river channels, or channelization sensu lato is one of the most prevalent hydromorphological impacts on rivers. Rivers become channelized for a variety of reasons and in a variety of manners, from simple removal of boulders and logs to entire reconfiguration of the river channel or even construction of new artificial channels (Brookes, 1985; Liria Montan˜e´s, 2005). Nowadays, many river restoration projects involve actions to recover the physical habitat in the channel and riparian zone (Bernhardt et al., 2005), and some of the best evidence currently available of the impacts of river channelization derives from these projects. It must be noted, however, that the effects of restoration must not automatically be inverse to those of hydromorphological impacts, as there can be time lags and hysteresis effects in the recovery process. The response of ecosystem functioning to in-stream modifications of channel form (like removal or restoration of boulders and logs) is very clear for purely physical variables, but more complex for variables that are more influenced by biological activities (Table 1). For instance, wood jams slow down the water flow (Roberts et al., 2007; Elosegi et al., 2010), and their removal results in increased export of sediments (Beschta, 1979; Dı´ez et al., 2000).
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Table 1 Effects of instream hydromorphological impacts on river ecosystem functioning as deduced from experimental removal or addition of logs, boulders or gravel Ecosystem function
Response to impact
References
Sediment retention
Decrease
Beschta (1979), Bilby (1981), Dı´ez et al. (2000)
Nutrient retention
Decrease
Organic matter retention/storage
Decrease
Bilby (1981), Wallace et al. (1995), Webster et al. (2000), Ensign & Doyle (2005), Roberts et al. (2007) Bilby et al. (1998), Dı´ez et al. (2000), Muotka & Laasonen (2002), Negishi & Richardson (2003), Haapala et al. (2003), Lepori et al. (2005), Flores et al. (2011)
Litter breakdown
Variable
Entrekin et al. (2008), Flores et al. (2011)
Ecosystem respiration
Decrease
Mori et al. (2011)
Secondary production
Decrease
Entrekin et al. (2009)
However, the effects on the storage and use of organic matter are not so straightforward. Manipulating instream wood affects retention (Muotka & Laasonen, 2002; Negishi & Richardson, 2003), storage (Bilby et al., 1998; Dı´ez et al., 2000) and use of organic matter (Lepori et al., 2005; Flores et al., 2011), although some restoration experiments failed to detect significant differences (Entrekin et al., 2008). Wood removal reduced the retention of fish carcasses (Bilby et al., 1998) and plant propagules (Engstro¨m et al., 2009), which can have important effects on the dynamics of nutrients and riparian vegetation, respectively. Wood removal affects nutrient retention (Webster et al., 2000; Roberts et al., 2007), probably as an effect of decreased transient storage (Ensign & Doyle, 2005). In spite of these described patterns, the response of nutrient retention is less predictable than that of purely physical variables. Even less predictable are the potential effects of instream hydromorphological impacts on river ecosystem metabolism. Mori et al. (2011) reported gravel extraction to decrease hyporheic respiration and Entrekin et al. (2009) showed that restoring instream wood enhanced secondary production of benthic invertebrates. A number of studies addressed the effects of hydromorphological impacts occurring in the banks or in the riparian corridor. Most of them deal with channelization, building levees or reconfiguration of river channels (Table 2). In its most extreme form, changes in channel planform (such as converting braided reaches in single channels) represent shifts in the type of hydrogeomorphic patch (functional process zones sensu Thorp et al., 2006), which would in
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principle profoundly affect ecosystem functioning (Thorp et al., 2010), although empirical evidences are rare (Doyle & Stanley, 2006). Rivers subject to incision as a result of changes in riparian vegetation, and also channelized rivers, have been shown to retain less sediments (Bravard et al., 1997; Hupp et al., 2009), nutrients (Sabater et al., 2000; Sweeney et al., 2004), and organic matter (Petersen & Petersen, 1991; Quinn et al., 2007; Hupp et al., 2009). In the most extreme cases, the river is forced to run through a smooth canal lined by concrete, designed to be non retentive, and thus, lacking suitable habitats for most species, as well as affecting the development of hyporheos, an active component of river ecosystems (Valett et al., 1996; Boulton et al., 1998). One should therefore expect decreased efficiency for nutrient retention (Pinay et al., 2002), as well as changes in ecosystem metabolism. Nevertheless, the biological activity of the biofilm can sometimes override the effects of hydromorphological changes on nutrient retention, as can be seen when comparing artificial canals with natural channels (Fig. 2). Such situations are common in streams draining urban areas. Impacts in banks and riparian areas often have strong effects on floodplains, although results are less prone to generalization than those on river channels (Table 2). Common effects of these impacts are the changes in hydraulic connectivity between channel and floodplain, which can, in turn, trigger important changes in the retention and dynamics of nutrients (Verhoeven et al., 2006; Francis et al., 2009; Franklin et al., 2009; Racchetti et al., 2011) and organic matter (James & Henderson, 2005; Millington & Sear, 2007;
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Table 2 Effects of hydromorphological impacts in the banks or in the riparian corridor (channelization, levee building…) on river ecosystem functioning Ecosystem function
Instream
Floodplain
Response to impact
References
Sediment retention
Decrease
Bravard et al. (1997), Hupp et al. (2009)
Nutrient retention
Decrease
Sabater et al. (2000), Sweeney et al. (2004)
Organic matter retention/storage
Decrease
Petersen & Petersen (1991), Quinn et al. (2007), Hupp et al. (2009) Sweeney et al. (2004)
Litter breakdown
Decrease
Ecosystem respiration
Decrease
Sweeney et al., 2004
Nutrient retention
Variable
Verhoeven et al. (2006), Francis et al. (2009), Franklin et al. (2009), Lair et al. (2009), Racchetti et al. (2011)
Organic matter retention/storage
Decrease
James & Henderson (2005), Millington & Sear (2007), Samaritani et al. (2011)
Plant growth/recruitment
Variable
Kozlowsky (2002), Scott et al. (2004), Gonzalez et al. (2010)
Nitrification/denitrification
Variable
Lefebvre et al. (2004), Sheibley et al. (2006), Racchetti et al. (2011)
effects of channelization, in many instances, the dry soils retain more nutrients and pollutants than flooded soils, but when floodplain soils are used for agriculture, as is often the case, the reverse may occur (Lair et al., 2009). Changes in flooding patterns strongly affect also the recruitment and growth of riparian plants (Scott et al., 2004; Gonzalez et al., 2010), which in turn would affect organic matter and nutrient dynamics. Kozlowsky (2002) reported that lack of flooding reduces riparian forest productivity. Flow modification Fig. 2 Effects of discharge on uptake length of phosphate, measured on six occasions during spring in three reaches by means of slug nutrient additions. Simple Stream refers to a stream reach where wood has been historically removed. Complex Stream is a nearby reach where large wood loading has been restored. Canal is an artificial canal, made of concrete, to divert water to a nearby hydropower station. Note not only that the simple stream is less retentive for phosphate than the complex one, but also that no differences occur between the complex stream and the extremely simple concrete canal. See Electronic Supplementary Material for details on methods
Samaritani et al., 2011), as well as on processes like nitrification and denitrification (Lefebvre et al., 2004; Sheibley et al., 2006). Nevertheless, channelization seems to affect the spatial and temporal heterogeneity of carbon pools and fluxes more than their overall value (Samaritani et al., 2011). Regarding the overall
Flow regimes in most world rivers are currently altered by impacts such as dams (Dynesius & Nilsson, 1994; Nilsson et al., 2005), changes in soil imperviousness and groundwater abstraction, amongst others (Sabater, 2008). Changes in frequency, magnitude and duration of floods affect river geomorphology (Knighton, 1998) and the biota (Poff et al., 1997; Miller et al., 2007; Naiman et al., 2008; Brooks et al., 2011), often in complex ways. Although soil erosion has increased the sediment delivery to rivers, dams produce a 20% of decrease in the amount of sediments reaching the oceans worldwide (Syvitski et al., 2005). Sediment starvation below dams affects channel form and dynamics (Shields et al., 2000; Graf, 2006; Dade et al., 2011), and has important consequences for ecosystem functioning. For instance, reservoirs in the
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Ebro River (N Spain) retain 100% of bedload and 99% of suspended load (Tena et al., 2011), and the historically turbid lower reaches are currently dominated by submerged macrophytes (Sabater et al., 2008a), which can have important effects on retention of sediments and dissolved phosphorus (Julian et al., 2011). Other impacts of dams with potentially important effects on ecosystem functioning are the changes in water temperature, oxygen concentration, nutrients and suspended organic matter, barriers to the migration of organisms (Ward & Stanford, 1983; Pringle, 1997; Kunz et al., 2011), or reduction of the native riparian cover (Catford et al., 2011). Much of the available information on the effects of flow modification on river ecosystem functioning concerns processes occurring in the reservoirs (e.g. Alexander et al., 2002; Stanley & Doyle, 2002; Doyle et al., 2005; Bosch, 2008). Less is known on the impacts of reservoirs on downstream processes. Furthermore, different experimental approaches as well as different degrees of flow modification often result in studies yielding contrasting conclusions. While Orr et al. (2006) reported that experimental dam demolition had little or no effect in downstream nutrient retention, Doyle et al. (2003) reported a significant increase in retention (Table 3). Dewson et al. (2007) in a manipulative experiment in New Zealand streams showed that water
abstraction increased the retention, but not the breakdown of organic matter. Studying the effect of small dams on litter breakdown has also provided contrasting results. Casas et al. (2000) detected no effect, Muehlbauer et al. (2009), Mendoza-Lera et al. (2012) and Gonza´lez et al. (2012) reported a decrease in breakdown rate, whereas Short & Ward (1980) reported an increase. The differences are probably related to the specific patterns of dam water release (Gonza´lez et al., 2012). Regarding river metabolism, some studies suggest that it can be affected by flow regulation (e.g. Reid et al., 2006), but the experimental evidence often yields contrasting results. Munn & Brusven (2004) showed regulated reaches to have much higher primary production and slightly higher respiration than non-regulated ones, because of the growth of aquatic mosses in regulated reaches. On the contrary, Lucadamo et al. (2012) found primary production to be lower immediately below dams, and to increase downstream as the dam effect decreased. Experimental floods have been used to illustrate the potential effects of water regulation on river metabolism. Uehlinger et al. (2003) reported that primary production was more reduced than respiration after floods, but Chester & Norris (2006) reported increased production after the flood, whereas they found no change in respiration. These differences account for
Table 3 Effects of water abstraction or flow stabilization on river ecosystem functioning Ecosystem function Instream
Floodplain
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Response to impact
References
Nutrient retention
Variable
Doyle et al. (2003), Orr et al. (2006)
Organic matter retention
Increased
Dewson et al. (2007)
Litter breakdown
Variable
Short & Ward (1980), Casas et al. (2000), Dewson et al. (2007), Muehlbauer et al. (2009), Mendoza-Lera et al. (2012), Gonza´lez et al. (2012)
Primary production
Variable
Uehlinger et al. (2003), Munn & Brusven (2004), Ryder (2004), Chester & Norris (2006), Sabater (2008), Watts et al. (2010), Lucadamo et al. (2012)
Ecosystem respiration
Variable
Uehlinger et al. (2003), Munn & Brusven (2004), Chester & Norris (2006), Watts et al. (2010) Hupp et al. (2009)
Sediment retention
Variable
Nutrient retention
Variable
Hupp et al. (2009)
Plant growth/recruitment
Variable
Mahoney & Rood (1998), Johnson (2000), Kozlowsky (2002), Magdaleno & Ferna´ndez (2011)
Litter breakdown Denitrification
Decreased Increased
Ellis et al. (1999) Curie et al. (2009)
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other factors (e.g. adequate substrata) that also limit the growth of producers and consumers. Flow modifications, especially the frequency and duration of floods, can have paramount importance on ecosystem processes in the riparian areas or in the floodplains (Junk et al., 1989; Gregory et al., 1991; Kozlowsky, 2002). However, the trend is apparently complex. Hupp et al. (2009), when analysing the effects of large dams, channelization, and levee construction on USA Coastal Plain rivers, reported significant changes in the retention of sediments and nutrients, although the effects were site-specific depending on factors like river gradient and sediment size. Sediments size distribution is typically patchy in floodplains, and this may have important consequences for the type and productivity of forests (Kozlowsky, 2002). Therefore, changes in the inundation regime of floodplains have large consequences for growth of riparian forests (Thomas, 1996; Mahoney & Rood, 1998; Pollock et al., 1998; Johnson, 2000; Magdaleno & Ferna´ndez, 2011). In addition to changes in riparian vegetation, flooding of riparian areas increases groundwater recharge (Chen & Chen, 2003; Negrel et al., 2003), which in turn affects the fate and transformation of pollutants in the aquifer (Barbieri et al., 2011). Flooding also exerts a strong influence on ecosystem processes like nitrification and denitrification (Pinay et al., 2007; Curie et al., 2009; Fromin et al., 2010) or litter breakdown (Ellis et al., 1999), although the spatial variability characteristic of floodplains makes generalizations difficult.
Identification of research needs The present review shows that, despite increasing amounts of information on the effects of hydromorphological impacts on river ecosystem functioning, large uncertainties still remain. The response is rather straightforward for a few types of impact (e.g. channelization decreases instream retention), but for many others the response can be complex. In some cases, it might as well occur that the response of river ecosystems to specific hydromorphological impacts depends on river type, but the literature suggests that variability in response is large even within a given river type. This has been shown for metabolism (Tank et al., 2010), a metric that might be sensitive to humaninduced disturbances, but whose drivers and responses
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probably are stream specific. Here, we suggest some areas for future research that can be aimed at fulfilling some obvious knowledge gaps. A priority of research should be oriented toward detecting regularities beyond single case studies. Experimental manipulation is a powerful tool to gain insight on the variables that control rivers ecosystem form and functioning (Downes et al., 2002). Alternatively, multiple-site studies can often detect elusive patterns given the inherent variability of ecosystem functioning, especially if they are carried out in contrasting environmental settings (e.g. Quinn et al., 2007; Bernot et al., 2010; Sabater et al., 2008b). Metaanalyses in which a number of different experiments is jointly analysed in search for patterns are also interesting, but often hard to perform (Hladyz et al., 2011). Modelling can also provide powerful tools to gain knowledge on the mechanisms behind ecosystem functioning (e.g. Doyle & Stanley, 2006). Whatever be the approach, patterns and regularities will only emerge by a cumulative effort; more research is needed to define the role of physical complexity on river ecosystem functioning, and of the importance of processes generating physical complexity, like the maturing and fall of riparian trees (Palik et al., 1998). A special attention to the factors governing focal species sensu Dale & Beyeler (2001; e.g. keystone species, ecosystem engineers…) needs to be paid, since they are particularly important for ecosystem functioning. Although there is a general consensus on the factors governing some processes (e.g. primary production in freshwaters; Bernot et al., 2010), there exists still very little empirical evidence on the effects of hydromorphological impacts on them. As an example, the main groups of primary producers in freshwaters (phytoplankton, phytobenthos, and submerged and emerged macrophytes) differ in their preferences with respect to light (Horne & Goldman, 1994), and light regimes can be affected by hydromorphological factors like turbidity or channel width. It is therefore obvious that hydromorphological impacts can trigger shifts in the relative abundance of these groups, and thus, on the ecosystem functioning, even if the total GPP remains the same. Therefore, it would be interesting to determine those thresholds identified with respect to the respective dominance of each group, or their respective equilibrium states depending on factors such as turbidity or stability of bed sediments.
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Ecosystem respiration is another function response of which to hydromorphological impacts is little known. Respiration mainly depends on temperature and availability of labile organic matter, but factors like hydrology and nutrients can also be important (Bernot et al., 2010). Feedbacks exist between riparian cover and river temperatures (e.g. Johnson, 2004), and thus, cover can indirectly affect respiration. Similarly, hydraulics affect hyporheic flow and water temperature (Acun˜a & Tockner, 2009), as well as sediment clogging (Brunke & Gonser, 1997), which could have important effects on ecosystem respiration. Nutrient retention is another ecosystem function in which many questions remain to be answered. Although removal of dead wood and boulders are shown to decrease instream retention, much less is known on the response of floodplain retention to channelization or to different forms of regulation. Since nutrient retention depends not only on the contact between water and substratum (Mulholland & Webster, 2010) but also on the biological activity of substrata (Doyle et al., 2003), large shifts in the dominant community (e.g. changes from biofilm- to macrophyte-dominated rivers) will probably have important effects on nutrient dynamics. Finally, the scale or scales at which impacts can affect processes should be carefully considered. Many hydromorphological impacts, from channelization to straightening or water abstraction, directly affect the size (length and width) of the ecosystem, either the entire river channel, the wet channel, or the surface of associated elements like floodable forests or oxbow lakes. This calls for measuring ecosystem functioning and the associated services both at different scales, per m2 of the river bed, per linear km of the river channel, and per km2 of the landscape (e.g. Sweeney et al., 2004).
So, which functional variable should I use to assess the effects of hydromorphological impacts on ecosystem functioning? We have provided evidences showing that hydromorphological impacts strongly affect river ecosystem functioning. It is therefore fair to assume that it should be extensively used in ecosystem management (Rapport et al., 1998; Boulton, 1999; Xu et al., 1999), but this is rarely the case (Bunn & Davies, 2000). The use of functional variables for river management has been
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proposed for litter breakdown (Gessner & Chauvet, 2002), metabolism (Izagirre et al., 2008; Young et al., 2008), or nutrient uptake dynamics (Udy et al., 2006; von Schiller et al., 2008). However, there are only a few examples on the use of this sort of variables to assess the status of river ecosystems (Fellows et al., 2006; Feio et al., 2010). We hereafter discuss three criteria based on Cairns et al. (1993) to select functional variables potentially affected by hydromorphological impacts. Even though many papers have already discussed criteria to select monitoring variables (e.g. Fairweather, 1999; Dale & Beyeler, 2001; Jungwirth et al., 2002; Niemi & McDonald, 2004), this exercise has not yet been performed with variables addressed to river ecosystem functioning. The first selection criterion is relevance. Rivers across the world differ greatly in environmental and biological characteristics, and so does the relevance of different functional variables. Retention of nutrients and organic matter is considered to be especially relevant in small streams, although methods have been developed to measure the former also in medium-sized rivers (Tank et al., 2008). Similarly, litter breakdown is especially significant on streams (Tank et al., 2010). On the contrary, primary production usually is of small relevance in small, forested streams, except during short periods (Acun˜a et al., 2004), and thus, especially when analysed by the open-channel methods, it is especially suited for larger streams and for rivers. Sometimes, it can be interesting to analyse separately the metabolism of different biological elements. For instance, because macrophytes and floating macroalgae are very abundant in Pampean streams (Acun˜a et al., 2011) but not in Alpine streams (Uehlinger et al., 2003), measuring their production can be relevant in the former (and not so much in the latter) to assess their response to particular stressors. Given their nonlinear responses in relation to environmental variability, these variables should be measured through long periods of time (Sabater, 2008), The second criterion to select functional variables is the scale of response. For instance, metabolism, when measured with chambers, typically reflects what is going on at a spatial scale of cm2 (Acun˜a et al., 2011), while secondary production changes at a scale of metres, between riffles and pools (Entrekin et al., 2009), nutrient retention involves a scale of tens to hundreds of metres (von Schiller et al., 2008), and finally whole river metabolism can integrate several
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km of river (Reichert et al., 2009). On the temporal scale, whole ecosystem metabolism can inform almost on real time on the effects of disturbance (Izagirre et al., 2008), whereas litter decomposition can integrate the response of the ecosystem for weeks to years (Dı´ez et al., 2002). The scale of response of the indicator should be used in combination with the characteristic scale at which stressors change (Findlay & Zheng, 1997) to get the most reliable monitoring. The third criterion is sensitivity. As pointed by Dale & Beyeler (2001), while some indicators may respond to all dramatic changes in the system, the most useful indicator is the one that displays high sensitivity to a particular and subtle stress. As we have shown, organic matter retention is highly sensitive to changes in channel form (Flores et al., 2011), while nutrient retention or metabolism can also change with channel form, but are more likely to respond to eutrophication, to climate-related changes (temperature and water level), or to clearing of riparian forests, alteration of the size of the hyporheic zone and of dynamics of groundwater recharge (Boulton et al., 1998; Mulholland & Webster, 2010; Tank et al., 2010). These three criteria, as a whole or separately, can be combined to select variables with the highest likelihood of detecting impairment, depending on the size of the river and the type of impact (Table 4). Additional criteria for variable selection can include the possibility for frequent monitoring, the possibility
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of taking automatic measurements, or the cost of the analysis in relation to its diagnostic value.
Final remarks Hydromorphological impacts can have important effects on key functions in river ecosystems. Ecosystem functioning is key for river health and remains at the basis of important ecosystem services, thus showing that more emphasis should be placed on the measurement and assessment of selected descriptors. We discuss the suitability of different functional variables according to three criteria, namely, relevance, scale and sensitivity. There are still many uncertainties regarding the functioning of river ecosystems, but we detail some prospects for future research that could guide the implementation of ecosystem functioning measurements on future river management. Given the current trend of increased interest in ecosystem services, it is clear that ecosystem functioning should be directly measured in rivers instead of deduced from proxies, and a clear framework is necessary to identify the functions most likely to be affected by human actions, and the most appropriate variables to detect these impacts. Defining the best variables should be preceded by the establishment of the relevant research questions, to avoid unnecessary effort and costs.
Table 4 Variables linked to ecosystem functioning in river channels and their potential utility in assessing river health under different situations
Black cells mean a strong effect, white cells a weak effect, question marks insufficient information
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Coupling clarity of questions with selection of the best variables could define the best strategy to approach river functioning into everyday river management. Acknowledgments This paper has benefited from the support of the Spanish Ministry of Research and Innovation through projects METATOOL (BOS2003–04466), COMPLEXTREAM (CGL2007-65176/HID), and SCARCE (CONSOLIDERINGENIO CSD2009-00065).
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