RIVER RESEARCH AND APPLICATIONS
River Res. Applic. (2011) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/rra.1561
BENCHMARKING RIVER HABITAT IMPROVEMENT I. BOAVIDA,a* J. M. SANTOS,b R. CORTES,c A. PINHEIROa and M. T. FERREIRAb a
Instituto Superior Técnico, Universidade Técnica de Lisboa, Lisboa, Portugal Centro de Estudos Florestais, Universidade Técnica de Lisboa, Lisboa, Portugal c Centro de Investigação e de Tecnologias Agro-Ambientais e Biológicas, Universidade de Trás-os-Montes e Alto Douro, Vila Real, Portugal b
ABSTRACT River ecosystems have witnessed a long history of human pressure, particularly the disruption of freshwater fish populations. The awareness of this situation has led to many habitat improvement projects, with a variable degree of success. In natural situations, fish populations co-inhabit throughout the hydrological cycle with different degrees of adequacy, and the sequence of favourable and unfavourable conditions dictates abiotic constraints and biotic interactions that shape the final biological assemblages. We postulate that a part of unsuccessful restoration results is related to insufficient closeness to the natural habitat conditions of the river type that is to be restored, including the naturally adverse periods. We used the RIVER2D model to predict habitat availability as weighted usable area (WUA) at a degraded site that is to be restored, for two native Mediterranean species and their life stages—the Southwestern nase Iberochondrostoma almacai and the Arade chub Squalius aradensis. We then analysed the yearly evolution of the natural WUA at a nearby reference site. Overall, the reference site exhibited the longest periods during which the WUA was continuously lower than the chosen WUA thresholds for each of the four bioperiods. Considerable divergences from natural habitat availability values can be seen for the spawning, rearing and growth bioperiods. Restoration outcomes can result in appreciable deviations—favourable or unfavourable to fish populations—from the WUA occurring under natural conditions over the course of the year. Restoration should therefore take account of local hydraulic and habitat patterns that govern population dynamics and result in the final fish assemblage. Copyright # 2011 John Wiley & Sons, Ltd. key words: benchmark; reference conditions; restoration; enhancement; freshwater fish Received 15 December 2010; Revised 20 May 2011; Accepted 23 May 2011
INTRODUCTION River restoration is emerging as a priority for water authorities (Bernhardt and Palmer, 2007; Jansson et al., 2007; Nilsson et al., 2007) and is expected to be an even more important issue in the coming years, because of the implementation of the Water Framework Directive (EC, 2000). Nowadays, restoration actions are designed to enhance longitudinal and lateral connectivity and to improve habitat heterogeneity and are not only a matter of improving river water quality (Ormerod, 2004). Researchers are also more aware of the need to ensure good practices in river restoration, as evidenced by the increasing number of articles that have addressed river restoration ecology in the last two decades (Nilsson et al., 2007; Palmer et al., 2010). Nonetheless, there is still no agreement as to what constitutes a successful river restoration project (Palmer et al., 2005). It is often not clear which techniques will be most successful for each specific restoration site and its ecological conditions (Roni et al., 2002), and follow-up
*Correspondence to: I. Boavida, Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal. E-mail:
[email protected]
Copyright # 2011 John Wiley & Sons, Ltd.
monitoring is rarely used to assess and correct the restoration outcomes (Alexander and Allan, 2007; Brooks and Lake, 2007). Many habitat restoration projects still pursue the idea that simply increasing physical heterogeneity will lead to adequate improvement of the species habitat (Jungwirth et al., 1995; Kondolf and Micheli, 1995; Montgomery, 1997; Palmer et al., 1997; Kemp et al., 1999). Effective river restoration requires a detailed definition of goals to clarify the resources and actions that will be needed (Pedroli et al., 2002) and to specify those that are necessary in order to understand how each river ecosystem used to function before it was degraded (Petts, 1984; Dynesius and Nilsson, 1994; Bernhardt et al., 2005; Parasiewicz, 2007). As idealistic as this concept may sound, restoration outputs should reflect naturalness—a concept embedded in the Water Framework Directive. However, river restoration is frequently conducted without considering the reference biological assemblages and ecological conditions found at undisturbed sites (Rheinhardt et al., 1999; Stoddard et al., 2006; Nestler et al., 2010), which should be considered when determining the success of the restoration (Pedroli et al., 2002; Palmer et al., 2005; Nilsson et al., 2007). It is not easy to benchmark restoration for a spatially heterogeneous and complex ecosystem such as a river, characterized
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as it is by the interplay of hydrology, geology, morphological conditions, woody debris or plant stands and topographic factors, which together determine hydraulic patterns and habitat mosaics. In order to ensure successful restoration, native fish species that occur at the reference site and their life stages should therefore be restored in approximate proportion (Strange, 1999). Because they are powerful tools with which to determine the hydraulic patterns in rivers (Leclerc et al., 1995; Ghanem et al., 1996; Crowder and Diplas, 2000), stream habitat studies have been using 2D models to predict the wetted usable area that results from restoration measures (Vehanen et al., 2003; Lacey and Millar, 2004; Pasternack et al., 2004; Jalón and Gortázar, 2007). To improve this analysis, it is important to account for the temporal variability of habitat suitability (Milhous et al., 1990) and the periods during which fish habitat conditions are unfavourable (Capra et al., 1995; Sánchez Navarro et al., 2007; Parasiewicz, 2008). However, during the natural annual hydrological cycle, there are always periods characterized by habitat harshness and unfavourable conditions, especially in more extreme flow regimes such as those that occur in Mediterranean regions, which are characterized by summer droughts and flash floods during autumn and spring storms (Gasith and Resh, 1999). These hydrological events shape the ecological processes governing fish populations and should also be considered when restoring rivers. In the present study, we aimed to understand the importance of benchmarking reference conditions for restoration planning. We used 2D habitat simulations to predict habitat availability at a degraded site that was scheduled for structural restoration, for two native Mediterranean species and their life stages. We then used 2D habitat simulations to analyse the yearly evolution of the natural usable habitat
area at a nearby reference site. Simulation results were compared for reference, degraded and restored situations, considering the consequences for restoration success.
MATERIALS AND METHODS Study area The study was conducted on the Odelouca River, the largest tributary of the Arade Basin (987km2), Southwest Portugal (Figure 1). The climate is Mediterranean, with more than 80% of the rainfall occurring from October to March in the form of flashy floods, with a variable period without flow between July and September that results in a succession of pools in the riverbed. The mean monthly temperature ranges from 9.6 C (January) to 22.4 C (July). The upper and middle part of the river is a Natura 2000 site (EU, 2000), because of well-preserved Mediterranean woodlands with scarce human population and human activities. Along this section, the river presents well-developed riparian galleries dominated by Alnus glutinosa L., Salix salviifolia Brot. ssp. australis Franco and Fraxinus angustifolia Vahl. Land use along the lower downstream reaches is agricultural, with extensive citrus groves replacing Mediterranean scrubland and cork-oak woodland vegetation (Quercus suber L.). Here, the river has been subject to reprofiling and straightening, although riparian vegetation is fragmented. There is no significant chemical or nutritive pollution. Two endemic species—the Southwestern Iberian nase (Iberochondrostoma almacai Coelho, Mesquita and CollaresPereira) (hereafter nase) and the Arade chub (Squalius aradensis Coelho, Bogutskaya, Rodrigues and CollaresPereira) (hereafter chub)—are the dominant natives in the basin. Other species, such as the Iberian barbel Luciobarbus
Figure 1. Map of the study area, showing the location of the Odelouca River and the two study sites: (1) degraded site and (2) reference site Copyright # 2011 John Wiley & Sons, Ltd.
River Res. Applic. (2011) DOI: 10.1002/rra
IMPROVEMENT HABITAT BENCHMARKING
sclateri Gunther, the European eel Anguilla anguilla L. and the Iberian loach Cobitis paludica de Buen, occur sparsely and in small numbers. The habitat preferences of the two key species had previously been obtained from reference sites in the region (Santos and Ferreira, 2008). Two study sites (Table I) were selected on the lower part of the river. Both sites have a Strahler’s order number of 4, and their distance to the river source is 59 and 66km, respectively. Geomorphology between sites is similar: open valley where the riverbed is dominated by schistose rocks, blanketed with alluvial deposits. The upstream site presented a high degree of naturalness and was considered to reflect the reference conditions for the downstream site, degraded site (Hughes, 1994). Main geomorphologic units include glide on the river side arms for high flows and a clear pool–riffle–run sequence. The reference site was 74-m long with an average bankfull width of 33m, was well structured with continuous riparian galleries on both banks and had a high complexity of habitat cover features (such as overhanging vegetation, submerged blocks, woody debris, logs and tree roots), which represented between 20% and 40% of the sheltered areas where fish could rest and hide. The degraded site was 250-m long and 58-m wide. Pool is the dominant geomorphologic unit followed by riffle. It presented unstable linear banks with scarce vegetation, woody debris or sheltering areas and a low diverse substrate composition, mainly gravel embedded with silt. Data collection The riverbed topography was surveyed at the reference and degraded site using a combination of a Nikon DTM310 total station (Mohave Instrument Co., Signal Hill, CA, USA) and a Global Positioning System (GPS) (Ashtech, model Pro Mark2, CRS Survey Equipment & Supplies Ltd., Concord,
Table I. Physical characteristics of the reference and degraded sites (Q=2m3 s1) in the Odelouca River Study site Geographic coordinates Total length (m) Average width (m) River slope (mm1) Depth (m) Maximum depth (m) Velocity (ms1) Froude number Dominant substrate Bank vegetation
Reference
Degraded
W 06 07′4700 W 06 11′1300 00 N 39 58′58 N 39 55′4400 74.1 286.2 33.1 58.3 0.0018 0.0025 0.410.21 0.250.14 0.86 0.73 0.700.56 0.410.23 0.350.34 0.260.14 Gravel Gravel Continuous in both banks Absent
Mean values are given for depth, velocity and Froude number followed by standard deviation. Copyright # 2011 John Wiley & Sons, Ltd.
ON, Canada), by sampling 1824 and 4129 spots, respectively. Water velocity and depth were measured at a series of points along cross-sections at both study sites where significant alterations in depth, water velocity, substrate composition and slope were noted. Depths were measured with a ruler, and water velocities were measured with a flow probe (model FP101, Global Water Instrumentation, Inc., Gold River, CA, USA) positioned at points that were 60% of the local flow depth from the surface (Bovee and Milhous, 1978). These data were used to calibrate the model bed roughness and to establish the boundary conditions, specifically the water surface elevation at the downstream and upstream cross-sections. The instream cover was also measured and defined as follows: (i) any submerged structure (other than substrate) in which fish could be hidden from overhead view; (ii) undercut banks or overhanging vegetation 1+), respectively (Magalhães et al., 2002). Sampling took place at undisturbed or minimally disturbed sites of the Odelouca basin, so that habitat associations reflected the optimal species habitat rather than an externally imposed displacement towards sub-optimal situations (Gorman and Karr, 1978). Fish sampling was performed during the flowing season— i.e. late May to early June—when habitats are fully connected and fish are therefore not confined to imposed pool habitats. Further details about site locations, sampling procedures and microhabitat measurements are given in Santos and Ferreira (2008). Data analysis The natural flow regime was computed using the daily flow data from a nearby gauging station (Monte dos Pachecos). The station is situated only 5km upstream from the degraded site, and daily flow data, which were available for a period of 29years (1962–2000), were considered to be representative of the local conditions. The daily flow data for the degraded and reference site were determined by multiplying the daily flow data available for the gauging station by an adjusting factor corresponding to the quotient between the study reach catchment area and the gauging station catchment area. The natural flow regime was then calculated considering the mean daily flow. Intra-annual seasons (bioperiods) with specific biological functions were selected, considering the Parasiewicz (2007) approach. In River Res. Applic. (2011) DOI: 10.1002/rra
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order to identify these bioperiods, the natural flow regime was compared with biological life-history information obtained from the literature (Magalhães et al., 2002; Coelho et al., 2005; Santos and Ferreira, 2008). The beginning and end of each bioperiod were adjusted to coincide with changes in natural flow patterns. Overall, four bioperiods were selected for each species (Figure 2). The winter floods (15 Oct–31 Dec for nase; 15 Oct–28 Feb for chub) bioperiod represents the time where the fauna will try to survive to flash flood characteristics of Mediterranean-type streams (Gasith and Resh, 1999). The spawning (1 Jan–30 Apr for nase; 1 Mar–31 May for chub) comes immediately after the winter floods and represents the time during which nase and chub will try to spawn in the Odelouca River. In this bioperiod, the fauna rapidly increased in abundance. At the end of the rearing and growth (1 May–15 Jul for nase; 1 Jun–15 Jul for chub) bioperiod, the abundance of biota is expected to be at its highest (Gasith and Resh, 1999). Finally, the summer droughts (16 Jul–15 Oct for both species) are the survival bioperiod, in which the fauna experiences reduced flows. Fish become increasingly confined to reduced spaces (isolated pools), thus increasing the competition for limited resources such as deeper areas and leading to a higher vulnerability to predators (Godinho et al., 1997). In order to illustrate flow conditions corresponding to a lowflow situation and a relatively high discharge, the mean monthly flows of the six months with lower mean monthly discharges and the six months with higher mean monthly discharges were calculated. The restoration action designed at the degraded site (hereafter restoration scenario) included the introduction of three islands in the mid-section of the river and the placement of two lateral bays on opposite banks. This restoration scenario was chosen as it has previously been found to maximize habitat for both species lifehistory stages (Boavida et al., 2010) and it is expected to be implemented in the riverbed in the near future.
A two-dimensional approach was chosen for habitat simulation modelling, as these models have been shown to accurately represent complex mosaics of depth and velocity distributions (Ghanem et al., 1996; Crowder and Diplas, 2000). Specifically, the 2D hydrodynamic and fish habitat RIVER2D models (University of Alberta, Edmonton, AB, Canada) (Steffler, 2000) were used. The weighted usable area (WUA)—i.e. the area that can potentially be used by a given fish life-history stage, computed as the product of depth, velocity and cover suitability indexes—was used to evaluate the adequacy of the restoration measures relative to the reference site conditions. Simulations were carried out in RIVER2D for discharges of up to 12m3 s1 for the reference and degraded sites and for the restoration scenario modelled at the degraded site. Histograms of velocity, depth and Froude number were used to compare habitat features at the three sites. The different conditions (i.e. reference, degraded and restored scenario) were then assessed by WUA versus discharge curves, where WUA was expressed as a percentage of the corresponding total wet area, measured in plan X Y using ARCGIS (ESRI, Redlands, CA, USA). In order to analyse the habitat variations between the degraded site with and without the implementation of the restoration scenario and the reference site in each bioperiod, continuous under threshold (CUT) habitat duration curves (Capra et al., 1995) were developed for the bioperiods that explain population dynamics. The curves evaluate durations and frequency of continuous events with habitat lower than a specific threshold as a proportion of the entire study period (Capra et al., 1995). The WUA versus discharge curves are combined with the hydrologic time series for each site to obtain the habitat time series for each bioperiod. Then the CUT curves are created, considering the durations for which the WUA is lower than a given threshold for each bioperiod. The periods during which WUA1.6ms1 high flows) than at the degraded site (0–0.8ms1 low flows; 0–1.6ms1 high flows) and for the restoration scenario (0–0.8ms1 low flows; 0–>1.6ms1 high flows), particularly during low-flow periods. In addition, the restoration scenario produced highly homogeneous velocities, with almost 90% of the values below 0.2ms1 for low flows. The depth distribution was similar in the different scenarios. However, at higher flows, the
reference site displayed a wider range of depths, with more than 50% of the values distributed in five classes. The variability of the Froude number increased from low flow, where more than 90% of the values for the restoration scenario correspond to a Froude number of less than 0.2, to high-flow periods. Overall, in all scenarios, the variability of velocity, depth and Froude number increased with increasing flows. Habitat gains for YOY nase relative to the degraded scenario were found to occur for discharges above 1.3m3 s1 [Figure 5(a)]. Contrary to the other life stages, the reference site provided higher WUA for YOY nase across the whole range of discharges. The restoration scenario produced habitat improvements for juveniles and adults of both species, across the whole range of simulated flows [Figure 5 (b), (c), (e) and (f)], relative to the degraded scenario. In general, for the juvenile and adult nase, the reference site provided lower WUA compared to the restoration scenario, with this difference being higher for discharges between 5 and 7m3 s1. For higher flows, WUA converged to similar values between both conditions. The results for chub showed a clear difference among the different life stages. Indeed, when the restoration scenario was modelled, juveniles and adults were offered a habitat gain in relation to the existing degraded conditions, whereas for YOY, this only happened for lower discharges (1
