Loreta Cornacchia
email: loreta.cornacchia@nioz.nl

Andrew Folkard
email: a.folkard@lancaster.ac.uk

Daphne Van Der Wal
email: daphne.van.der.wal@nioz.nl

Johan Van De Koppel
email: johan.van.de.koppel@nioz.nl

Sara Puijalon
email: sara.puijalon@univ-lyon1.fr

Tjeerd J Bouma
email: tjeerd.bouma@nioz.nl

Turbulence-mediated facilitation of resource uptake in patchy stream macrophytes

Keywords: submerged macrophytes, nutrient uptake, mass transfer 40

Many landscapes are characterized by a patchy, rather than homogeneous, distribution of vegetation.

Often this patchiness is composed of single-species patches with contrasting traits, interacting with each other. To date, it is unknown whether patches of different species affect each other's uptake of resources by altering hydrodynamic conditions, and how this depends on their spatial patch configuration. Patches of two contrasting aquatic macrophyte species (i.e., dense canopy-forming Callitriche and sparse canopy-forming Groenlandia) were grown together in a racetrack flume and placed in different patch configurations. We measured 15 NH4 + uptake rates and hydrodynamic properties along the centerline and the lateral edge of both patches. When the species with a taller, denser canopy (Callitriche) was located upstream of the shorter, sparser species (Groenlandia), it generated turbulence in its wake that enhanced nutrient uptake for the sparser Groenlandia. At the same time, Callitriche benefited from being located at a leading edge where it was exposed to higher mean velocity, as its canopy was too dense for turbulence to penetrate from upstream. Consistent with this, we found that ammonium uptake rates depended on turbulence level for the sparse Groenlandia and on mean flow velocity for the dense Callitriche, but Total Kinetic Energy was the best descriptor of uptake rates for both species. By influencing turbulence, macrophyte species interact with each other through facilitation of resource uptake. Hence, heterogeneity due to multi-specific spatial patchiness has crucial implications for both species interactions and aquatic ecosystem functions, such as nitrogen retention.

Introduction

In many ecosystems, vegetation shapes entire landscapes by interacting with physical processes [START_REF] Dietrich | The search for a topographic signature of life[END_REF][START_REF] Corenblit | Feedbacks between geomorphology and biota controlling Earth surface processes and landforms: a review of foundation concepts and current understandings[END_REF]. In coastal and fluvial aquatic ecosystems, vegetation modifies habitats through its effects on hydrodynamics and sedimentation [START_REF] Leonard | Flow hydrodynamics in tidal marsh canopies[END_REF][START_REF] Madsen | The interaction between water movement, sediment dynamics and submersed macrophytes[END_REF][START_REF] Schulz | The influence of macrophytes on sedimentation and nutrient retention in the lower River Spree (Germany)[END_REF][START_REF] Bouma | Spatial flow and sedimentation patterns within patches of epibenthic structures: Combining field, flume and modelling experiments[END_REF], hence acting as an ecosystem engineer [START_REF] Jones | Organisms as ecosystem engineers[END_REF]. Many studies first considered interactions between hydrodynamics and homogeneous vegetation [START_REF] Kouwen | Flexible roughness in open channels[END_REF][START_REF] Nepf | Drag, turbulence, and diffusion in flow through emergent vegetation[END_REF][START_REF] Nepf | Flow structure in depth-limited, vegetated flow[END_REF][START_REF] Järvelä | Effect of submerged flexible vegetation on flow structure and resistance[END_REF][START_REF] Chen | Flow adjustment at the leading edge of a submerged aquatic canopy[END_REF], and later focused on isolated or pairs of patches [START_REF] Sand-Jensen | Patch dynamics of the stream macrophyte, Callitriche cophocarpa[END_REF][START_REF] Folkard | Hydrodynamics of model Posidonia oceanica patches in shallow water[END_REF][START_REF] Bouma | Density-dependent linkage of scale-dependent feedbacks: A flume study on the intertidal macrophyte Spartina anglica[END_REF][START_REF] Vandenbruwaene | Flow interaction with dynamic vegetation patches: Implications for biogeomorphic evolution of a tidal landscape[END_REF][START_REF] Chen | The wake structure behind a porous obstruction and its implications for deposition near a finite patch of emergent vegetation[END_REF][START_REF] Zong | Vortex development behind a finite porous obstruction in a channel[END_REF]. Generally, vegetation patches locally reduce flow velocities, while increasing them in some adjacent areas [START_REF] Bouma | Spatial flow and sedimentation patterns within patches of epibenthic structures: Combining field, flume and modelling experiments[END_REF][START_REF] Chen | The wake structure behind a porous obstruction and its implications for deposition near a finite patch of emergent vegetation[END_REF][START_REF] Schoelynck | Selforganised patchiness and scale-dependent bio-geomorphic feedbacks in aquatic river vegetation[END_REF][START_REF] Meire | Interaction between neighboring vegetation patches: Impact on flow and deposition[END_REF].

In aquatic ecosystems, the interaction between vegetation and hydrodynamics regulates important ecological processes such as nutrient delivery and uptake by plants, as nutrients can be taken up from the water column through plant shoots [START_REF] Madsen | Sources of nutrients to rooted submerged macrophytes growing in a nutrient-rich stream[END_REF][START_REF] Bal | Influence of hydraulics on the uptake of ammonium by two freshwater plants[END_REF]). These processes are crucial for community primary productivity and ecosystem function [START_REF] Thomas | Effects of water velocity and canopy morphology on ammonium uptake by seagrass communities[END_REF][START_REF] Morris | Interaction between hydrodynamics and seagrass canopy structure: Spatially explicit effects on ammonium uptake rates[END_REF][START_REF] Levi | Macrophyte complexity controls nutrient uptake in lowland streams[END_REF]. Previous studies on uptake rates in relation to hydrodynamic conditions mainly focused on seagrasses, using flume experiments with dissolved 15 N-labeled ammonium or nitrate (often the main inorganic nitrogen sources in natural conditions; [START_REF] Haynes | Ammonium and nitrate nutrition of plants[END_REF]). These works identified the important effects of water velocity and flow alteration by seagrass canopies on resource uptake [START_REF] Thomas | Effects of water velocity and canopy morphology on ammonium uptake by seagrass communities[END_REF]Cornelisen and Thomas 2006), and the dependence of uptake rates on the rate of mass transfer to the leaf surface under unidirectional flow [START_REF] Cornelisen | Ammonium and nitrate uptake by leaves of the seagrass Thalassia testudinum: impact of hydrodynamic regime and epiphyte cover on uptake rates[END_REF]). Further, [START_REF] Morris | Interaction between hydrodynamics and seagrass canopy structure: Spatially explicit effects on ammonium uptake rates[END_REF] identified spatial patterns in ammonium uptake within seagrass patches, with higher uptake observed at the leading edge of the patch where the turbulent kinetic energy and velocity within the patch were highest. In a study of nutrient uptake by river macrophytes, [START_REF] Bal | Influence of hydraulics on the uptake of ammonium by two freshwater plants[END_REF] found that ammonium uptake increased with flow velocity. Because the diffusive boundary layer decreases with increasing velocity, the uptake rate also increases with increasing velocity [START_REF] Thomas | Effects of water velocity and canopy morphology on ammonium uptake by seagrass communities[END_REF][START_REF] Cornelisen | Ammonium and nitrate uptake by leaves of the seagrass Thalassia testudinum: impact of hydrodynamic regime and epiphyte cover on uptake rates[END_REF][START_REF] Morris | Interaction between hydrodynamics and seagrass canopy structure: Spatially explicit effects on ammonium uptake rates[END_REF][START_REF] Bal | Influence of hydraulics on the uptake of ammonium by two freshwater plants[END_REF].

However, most of the previous studies dealt with monospecific canopies or focused on a single species at a time, creating a monospecific community, while in reality natural landscapes are a diverse community made up of multiple species. Different patches of single species are heterogeneously distributed, and this patchiness is a common characteristic of aquatic habitats (Sand-Jensen and Vindboek [START_REF] Sand-Jensen | Patch dynamics of the stream macrophyte, Callitriche cophocarpa[END_REF]. A few examples are patchy seagrass meadows [START_REF] Fonseca | The role of current velocity in structuring eelgrass (Zostera marina L.) meadows[END_REF], and streams characterized by a 'pseudo-braided' distribution of plant stands between areas of faster flow [START_REF] Dawson | Submerged macrophytes and the hydraulic roughness of a lowland chalkstream[END_REF][START_REF] Cotton | The effects of seasonal changes to in-stream vegetation cover on patterns of flow and accumulation of sediment[END_REF][START_REF] Wharton | Macrophytes and suspension-feeding invertebrates modify flows and fine sediments in the Frome and Piddle catchments, Dorset (UK)[END_REF]). This additional level of complexity has just started to be integrated in studies of hydrodynamic-vegetation interactions. For instance, [START_REF] Weitzman | The attenuation of current-and wave-driven flow within submerged multispecific vegetative canopies[END_REF] focused on hydrodynamic implications of multi-specific canopies, but considered canopy heterogeneity in the vertical dimension. [START_REF] Adhitya | Comparison of the influence of patch-scale and meadow-scale characteristics on flow within seagrass meadows: a flume study[END_REF] focused on hydrodynamics and spatial configurations of seagrass patches with different densities, but did not test the consequences for resource uptake. [START_REF] Bal | Influence of hydraulics on the uptake of ammonium by two freshwater plants[END_REF] focused on nutrient uptake rates within monospecific patches of two species next to each other, but they only tested a single spatial configuration and therefore did not investigate the effects of spatial patchiness. To date, it is still unknown how patches of different species interact with each other by altering hydrodynamics and uptake of resources, and how this depends on their landscape configuration.

Multispecies effects could be important for hydrodynamics and nutrient uptake because the density, flexibility and canopy structure of different species affect hydrodynamics differently [START_REF] Peralta | Consequences of shoot density and stiffness for ecosystem engineering by benthic macrophytes in flow dominated areas: a hydrodynamic flume study[END_REF][START_REF] Bouma | Organism traits determine the strength of scale-dependent bio-geomorphic feedbacks: A flume study on three intertidal plant species[END_REF]). As we cannot easily predict the flow alteration by heterogeneous species distributions, our understanding of the implications for species interactions and nutrient load reduction in aquatic ecosystems is limited. Generally, the hydrodynamic controls on uptake rate are expected to be dependent on the macro-scale rate of delivery (mean flow velocity; e.g. Cornelisen and Thomas (2006)), or on the micro-scale processes that determine the concentration gradient at the leaf boundary layer (turbulence; e.g. [START_REF] Morris | Interaction between hydrodynamics and seagrass canopy structure: Spatially explicit effects on ammonium uptake rates[END_REF]). However, in a diverse community there might be cases where a single hydrodynamic parameter is not sufficient to describe uptake rates for multiple species with different traits and effects on hydrodynamic conditions. For instance, turbulence can have a significantly smaller scale in very dense canopies, compared to sparser ones [START_REF] Nepf | Flow and transport in regions with aquatic vegetation[END_REF]. On the other hand, the mean flow speed can be relatively constant within sparse canopies, but turbulence might be locally variable. Therefore, understanding the interaction between multiple species in terms of nutrient uptake, mediated by their hydrodynamic effects, is essential to gain a more realistic understanding of species interactions and productivity in heterogeneous, multi-specific communities.

In this study, we use streams colonized by aquatic macrophytes as a model system. We investigate how patches of two different species with contrasting morphological traits interact with each other by influencing hydrodynamics, and thereby ammonium uptake. Moreover, we test how this depends on their spatial configuration (patchiness). Here, we define multi-specific patchiness as a community composed of patches of different species. Specifically, we study the interaction between two macrophyte species that co-occur under field conditions and have contrasting density and canopy structure. Callitriche platycarpa Kütz., 1842 forms very dense patches that exhibit increasing canopy height with increasing patch length ('dense' species). Groenlandia densa (L.) Fourr. has a more open canopy, and its canopy height is constant along the patch length ('sparse' species). In the field, the dense patches of Callitriche are distributed quite regularly at a distance of about 8 meters, and Groenlandia patches tend to aggregate around them [START_REF] Cornacchia | Landscapes of facilitation: how self-organized patchiness of aquatic macrophytes promotes diversity in streams[END_REF]. Given the differences in shoot density and canopy architecture between the two species (Table 1), we hypothesize that the effects of the dense Callitriche patches on hydrodynamics may facilitate the delivery and uptake of resources by the sparse Groenlandia patches. To test this hypothesis, patches of the two species were arranged in different configurations in a laboratory flume. To investigate the role of spatial configuration and reciprocal species effects on nutrient uptake, both the species upstream and the relative location of the species downstream were varied. We discuss the implications of multi-specific spatial patchiness on facilitation and aquatic ecosystem functions, such as nitrogen retention.

Materials and methods

Plant material

We tested the effect of macrophyte patch species and configuration on ammonium uptake rates using two submerged macrophytes species, Callitriche platycarpa and Groenlandia densa. Both species were collected in February 2015 from a wetland on the Ain River, France (5.2825 E, 45.9855 N). Plants were stored in plastic bags and transported to the laboratory in NIOZ Yerseke (The Netherlands) within 24 hours from collection. Until installation in the flume, the two macrophyte species were stored in a green house, in tanks filled with freshwater that was continuously aerated, and exposed to natural light.

The macrophytes were allowed to recover for two days in the green house before starting the experiments. In order to be used for the experimental setup, individual plants were transplanted in stainless steel trays (30 29.5 5 cm). The trays were filled with a bottom layer of river sand (4.5 cm) and a top layer (0.5 cm) of fine gravel (0.2 cm grain size). A false bottom in the flume allowed the trays to be inserted with the soil surface at the same level as the flume bed. Based on the naturally occurring densities of the two species in the field, we constructed patches of 97 ± 28 g DW m -2 (mean ± SD) for Groenlandia ('sparse' species) and 318 ± 67 g DW m -2 for Callitriche ('dense' species) (Figure 1; Table 1). The biomass of the constructed patches was estimated by collecting all plant material within randomly selected 0.1 0.1 m quadrants (n = 3). The mean patch biomass for Callitriche is close to the range of 208 -256 g DW m -2 reported in Sand-Jensen and Vindboek [START_REF] Sand-Jensen | Patch dynamics of the stream macrophyte, Callitriche cophocarpa[END_REF]. The mean patch biomass for Groenlandia corresponded to ca. 500 shoots m -2 , which represents a relatively low shoot density in natural plant beds [START_REF] Sheldon | Maximum depth inhabited by aquatic vascular plants[END_REF]. We used a different patch length for each species to resemble the typical lengths observed in the field, i.e. 2.7 m and 1.2 m on average for Groenlandia and Callitriche respectively. These values are representative of average patch sizes observed in field conditions (2.5 ± 1.7 m for Groenlandia, n = 20; 1.4 ± 0.8 m for Callitriche, n = 20; Supporting Information Fig. S1). We used a total of 9 trays for Groenlandia, for a total patch coverage of 2.7 0.3 m. For Callitriche, plants were rooted in two trays (0.6 0.3 0.05 m). When Callitriche was placed upstream, three trays (filled with the same soil as the plant trays) were placed between the two patches, to account for the presence of the typical overhanging canopy for this species. That is, when the flume was running, a total coverage of 1.20 0.3 m 2 was observed due to shoots bending; this region was considered as part of the Callitriche patch (see Figure 2). A distance of one tray (0.3 m) between the two patches was used for the configurations in which Groenlandia was in the upstream position. The flume section next to each patch was left open (without plants, but filled with the same soil substrate used in the plant trays) in all configurations. Thus, the patches occupied one half of the flume, rather than extending across the width of the flume. As patches in the field do not span the whole channel, this configuration is representative of the typical distribution of vegetation patches in streams, with an empty (unvegetated) zone next to the patch into which water flow is deflected and accelerated around the patch [START_REF] Fonseca | The role of current velocity in structuring eelgrass (Zostera marina L.) meadows[END_REF][START_REF] Gambi | Flume observations on flow dynamics in Zostera marina (eelgrass) beds[END_REF][START_REF] Bouma | Spatial flow and sedimentation patterns within patches of epibenthic structures: Combining field, flume and modelling experiments[END_REF][START_REF] Follett | Sediment patterns near a model patch of reedy emergent vegetation[END_REF]. The canopies of both species were fully submerged during the experiments. The relative depth of submergence (H/h, ratio of water depth to maximum canopy height; Nepf (2012)) was relatively constant along the Groenlandia patch due to its uniform height. Values of H/h ranged between 8.75 and 4.3, corresponding to shallow submerged (H/h < 5). Instead, canopy height along the Callitriche patch varied from H/h = 17.5 at the leading edge (i.e. deeply submerged, H/h > 10) to 3.0 in the middle of the patch (indicating shallow submergence (H/h < 5), and even 1.30 at the downstream end of the patch, which is very close to emergent conditions (H/h = 1).

Flume setup and experimental configurations

All experiments were performed within a unidirectional racetrack flume using a water depth of 0.35 m and with a cross-sectionally-averaged velocity of 0.24 ± 0.03 m s -1 . This is a moderate flow velocity, representative of the summer flow conditions in streams typically colonized by Callitriche and Groenlandia (0.21 ± 0.01 m s -1 , based on our field measurements in 2014 and 2015; [START_REF] Cornacchia | Landscapes of facilitation: how self-organized patchiness of aquatic macrophytes promotes diversity in streams[END_REF]). For a more detailed description of the flume, see [START_REF] Bouma | Trade-offs related to ecosystem engineering: A case study on stiffness of emerging macrophytes[END_REF]. To test for the effects of patch spatial configuration on ammonium uptake rates, the two patches were arranged one downstream of the other, either on the same side of the flume ('aligned' configurations), or on opposite sides ('staggered' configurations) (Figure 2). These different spatial configurations are both commonly observed in natural streams, where patches grow downstream of other patches, or in a staggered arrangement (L. Cornacchia, pers. obs.). Moreover, patches of different species in the field can be found co-occurring at very short distances from each other, at the scale of 0.5 m [START_REF] Cornacchia | Landscapes of facilitation: how self-organized patchiness of aquatic macrophytes promotes diversity in streams[END_REF]. To test for interactions between the two species, in terms of reciprocal effects on ammonium uptake rates, we also switched the species located upstream for each of these configurations ('Sparse-Dense' or 'Dense-Sparse' configurations).

Measuring spatial patterns in 15 N-NH 4 + uptake rates and canopy hydrodynamics

To determine spatial patterns of ammonium uptake rates by the macrophyte species, we measured uptake rates at selected locations within the patches (Figure 2). Nutrient uptake rates were determined inside the two patches at 10%, 50% and 90% of the patch length (0.27, 1.45 and 2.43 m from the leading edge in Groenlandia; 0.12, 0.6 and 1 m from the leading edge in Callitriche) and, for each location along the patch length, at 0.15 and 0.25 m of the patch width. For each incubation experiment, macrophyte individuals were randomly selected from the tanks where they were kept with freshwater and were transplanted into plastic pots (5 shoots per pot). Before transplantation in the flume, roots were removed from the plants at the selected test locations, to prevent ammonium uptake by that means from the labeled water that inevitably penetrated into the sand (following [START_REF] Bal | Influence of hydraulics on the uptake of ammonium by two freshwater plants[END_REF]). Since the sediment was not changed in between treatments (as that would have meant destroying and recreating the patches), this avoided an effect of treatment order on uptake rates. While some nutrients are obtained through roots in field conditions, the nutrient demands of many macrophyte species can be satisfied by shoot nutrient uptake alone [START_REF] Madsen | Sources of nutrients to rooted submerged macrophytes growing in a nutrient-rich stream[END_REF]. Although root removal may have potentially affected the plant response to the flow, it was unlikely to affect their ability to resist the flow during a short-term experiment (6 hours). Each plastic pot was then placed in one of the patch locations described above, and inserted in the trays so that their upper part was in line with the sediment level to avoid scouring effects. The pots were replaced after each incubation experiment and new plants were transplanted. The renewal of plants between each run allowed us to avoid an effect of treatment order on uptake rates, due to plants being exposed to the labeled water for a longer time.

Moreover, it provided natural variability in plant structure, while maintaining a constant patch structure by keeping the trays forming the plant patches in the same positions between runs. The treatments were run in the order shown in Figure 1.

In the incubation experiments, 15 N-NH4 + was added to the water creating a 20 to 30 mol-NH4 + L -1 solution, with 30% of the N as 15 N abundance, following [START_REF] Bal | Influence of hydraulics on the uptake of ammonium by two freshwater plants[END_REF]. This range of values is representative of nutrient concentrations found in natural ecosystems: in summer 2015, average ammonium levels recorded in the studied region were 24.5 ± 28.54 mol-NH4 + L -1 , with a maximum value of 79.6 mol-NH4 + L -1 . Such a high enrichment (30%) was chosen to minimize dilution effects of the 15 N source pool over the course of the experiment. At the start and end of the experiment, three replicate water samples were taken to measure NH4 + concentration in the water. The same labeled water was used to perform four experiments, before replacing it with freshwater and a new label for the next runs (based on [START_REF] Bal | Influence of hydraulics on the uptake of ammonium by two freshwater plants[END_REF]). Incubations were performed under artificial light conditions. Lamps were mounted above the flume tank throughout the test section to provide 14 h d -1 of light (photosynthetic photon flux density of 550 mol m -2 s -1 ; measured 35 cm above the sediment surface). The stable isotope was added near the paddles that drive the flow in the flume to ensure mixing. Each incubation experiment lasted for 6 hours, and two replicate runs were performed for each configuration. At the end of the 6 hours, macrophytes were collected from the test positions, rinsed with tap water to remove excess isotope from the plant surface, and folded into aluminum foil.

In addition to the samples collected (n = 30 for each species), five specimens per species were randomly selected during the experiments from our species stock, to determine the background 15 N signal. The plants were dried in the oven for 48 hours at 60° C, and individual biomass was weighed.

Dried macrophytes were ground to a fine powder using a ball mill (MM 2000, Retsch, Haan, Germany). A subsample of about 3 mg of powder per plant was sent to the laboratory for mass spectrometry analysis of the isotope ratio. The samples were analysed for total N content and 15 Natomic percentage (as ( 15 N/total N) × 100) with an Elemental Analyser (Thermo Electron FlashEA 1112) and subsequent isotope ratio mass spectrometry (Thermo Delta V -IRMS). For recent guidelines on stable isotope notations, see [START_REF] Coplen | Guidelines and recommended terms for expression of stable-isotope-ratio and gasratio measurement results[END_REF].

The 15 N atomic fraction of the dissolved N source pool in the water column (x( 15 N)aq.N) was calculated according to [START_REF] Morris | The role of hydrodynamics in structuring in situ ammonium uptake within a submerged macrophyte community[END_REF]:

𝑥( 𝑁) . = ([𝑁𝐻 ] 𝑥( 𝑁) ) + ([𝑁𝐻 ] 𝑥( 𝑁) ) [𝑁𝐻 ] + [𝑁𝐻 ] (1)
where x( 15 N)tracer is the atomic fraction of the added tracer (0.98), [NH4 + ]water is 20.7 ± 3.1 μmol-NH4 + L -1 (mean ± SE), and x( 15 N)initial is the atomic fraction of the initial water column (assumed to reflect 15 N of atmospheric N, 3.7 × 10 -3 ). To provide an estimate of the change in 15 NH4 + ( mol-NH4 + L -1 ) concentrations between runs (i.e., [NH4 + ]tracer), mean 15 NH4 + uptake rates (µmol g -1 (DM) h -1 )

for each species were multiplied by their total estimated biomass (g DM m -2 ) and by the incubation time (6 hours). Given the large volume of the flume water, the estimated 15 NH4 + tracer concentrations remained high, between 8.0 and 8.8 mol-NH4 + L -1 , corresponding to an increase in total water column [NH4 + ] of between 27.5% and 29.7%.

To calculate the NH4 + uptake rate (V in µmol g -1 (DM) h -1 ) of each sample, we followed the equation in [START_REF] Morris | The role of hydrodynamics in structuring in situ ammonium uptake within a submerged macrophyte community[END_REF]:

𝑉 = [𝑁] _ • 𝑥 ( 𝑁) _ 𝑥( 𝑁) . • ∆𝑡 (2)
where x E ( 15 N)DM_sample is the 15 N excess atom fraction, calculated as the difference between the atomic fraction measured in the biomass of the sample after incubation (x( 15 N)DM_sample) and the background 15 N abundance x( 15 N)DM_nat.ab measured on five background specimens for each species ((3.70 ± 0.008) 10 -3 for Callitriche, (3.69 ± 0.018) 10 -3 for Groenlandia); ∆𝑡 (h) is the incubation time (6 h) and

[N]DM_sample is the N content of the dry biomass (µmol g -1 (dry mass (DM)) of each sample.

Hydrodynamic measurements

To test the relationship between hydrodynamic parameters and nutrient uptake, vertical profiles of velocity were measured with a 3D acoustic Doppler velocimeter (ADV, Nortek) over 30 s at 10 Hz.

Within each profile, velocity was measured at seven vertical locations at 2, 5, 10, 12, 15, 17 and 27 cm above the channel bed. The profiles were measured in the same streamwise and lateral locations as the plant samples collected for nutrient uptake estimation, i.e. at 10%, 50% and 90% of the length of each patch in the streamwise (x) direction, and at 0.15 and 0.25 m of the patch width in the spanwise (y) direction (Figure 2). To minimize interference by vegetation structures within the sampling volume of the ADV probe, the probe started at the lowest measuring point for each vertical profile within the vegetation. This prevented the canopy from being compressed as the probe moved towards the bed. To ensure that measurements were based on reliable data points, spikes and low-quality data points (i.e., correlation below the standard quality threshold of 70%) were removed during post-processing. The height of the vegetation canopy in each location was measured with a ruler in cm. The canopy height (h) was 0.17 ± 0.08 m for Callitriche, and 0.07 ± 0.01 m for Groenlandia (Table 1). Freshwater macrophytes are flexible, mesh-like structures (Sand-Jensen 2005), often with highly branched stems that get entangled in each other. Their flexibility and complex morphology make it very challenging to measure individual plant parameters like shoot height, frontal area, shoot density or average distance between shoots. Thus, the frontal area per water volume (a, m -1 ) and frontal area per bed area (ah, dimensionless) for the two species were calculated from water depth (H) and canopy length (l), width (y) and height (h) measurements (Table 1) through the following equations:

𝑎 = ℎ 𝑦 𝐻 𝑦 𝑙 = ℎ 𝐻 𝑙 (3) 𝑎ℎ = ℎ 𝑦 𝑦 𝑙 = ℎ 𝑙 (4)
As the frontal area of flexible submerged macrophytes is highly variable (i.e. their shape and canopy height change with flow velocity; Sand-Jensen ( 2005)), the values are indicative for the incoming flow velocity of 0.24 m s -1 used in the experiments.

The instantaneous velocity (𝑢(𝑡), 𝑣(𝑡), 𝑤(𝑡)) measured in the streamwise, lateral, and vertical directions, respectively, were separated into time-averages (𝑈 , 𝑉 , 𝑊 ), and instantaneous turbulent fluctuations (𝑢′(𝑡), 𝑣′(𝑡), 𝑤′(𝑡)), e.g. as 𝑢 (𝑡) = 𝑢(𝑡) -𝑈, and similarly for v and w. The vertical average of the time-averaged velocity in the streamwise direction was used to calculate the depth-averaged velocity (<Ū>, m s -1 ) at each profile position. The Total Kinetic Energy (Total KE) per unit mass is defined from the instantaneous velocities (𝑢, 𝑣, 𝑤), defined as:

𝑇𝑜𝑡𝑎𝑙 𝐾𝐸 = 𝑢 + 𝑣 + 𝑤 = 𝑇𝑢𝑟𝑏𝑢𝑙𝑒𝑛𝑡 𝐾𝐸 + 𝑀𝑒𝑎𝑛 𝐾𝐸 (5)
which can be partitioned into turbulent kinetic energy (Turbulent KE) and mean kinetic energy (Mean KE), defined as (e.g. [START_REF] Kundu | Fluid mechanics[END_REF]):

𝑇𝑢𝑟𝑏𝑢𝑙𝑒𝑛𝑡 𝐾𝐸 = 1 2 𝑢 + 𝑣 + 𝑤 (6) 𝑀𝑒𝑎𝑛 𝐾𝐸 = 1 2 𝑈 + 𝑉 + 𝑊 (7)
The Total Kinetic Energy (m 2 s -2 ) provides a better metric for the instantaneous velocity, because it reflects both the time-mean and turbulent fluctuations, and as such it is more relevant to boundary layer dynamics, especially in cases with low time-mean velocity but high Turbulent KE.

Specifically, previous studies have suggested that strong instantaneous velocity and/or plant motion can periodically strip away the diffusive sub-layer, which, if frequent enough, will enhance flux to the plant surface [START_REF] Koch | Hydrodynamics, diffusion-boundary layers and photosynthesis of the seagrasses Thalassia testudinum and Cymodocea nodosa[END_REF][START_REF] Stevens | Boundary-layers around bladed aquatic macrophytes[END_REF][START_REF] Huang | The motion of kelp blades and the surface renewal model[END_REF].

Reynolds shear stress (τxz, Pa) at the top of the canopy at each location was calculated as:

= -𝑢 (𝑡)𝑤′(𝑡) (8)
in which =1000 kg m -3 is the density of the flume water.

Volumetric flow rate of water through the patches (Qc, m 3 s -1 ) was calculated as:

𝑄 = 𝑄 and 𝑄 = 𝑦(ℎ -ℎ )𝑢 (9)
in which ℎ is the canopy height, 𝑄 the volumetric flow rate of water through the layer (ℎ -ℎ ), y is the patch width (0.3 m) and 𝑢 the double-averaged u component (i.e., averaged in time and spatially averaged in the two lateral positions) of the velocity at depth ℎ .

Measuring channel-scale patterns of ammonium uptake

To investigate how the relationship between hydrodynamic parameters and ammonium uptake develops at the scale of a whole channel, we tested the correlation between the total in-patch NH4 + uptake rates and in-patch average hydrodynamic parameters (mean velocity, Turbulent KE and Total KE). This allowed us to test whether spatial patch configurations that generated higher mean flow velocity, Total KE or Turbulent KE levels within the canopies promoted higher uptake at the channel scale. The total in-patch NH4 + uptake rate for each configuration was calculated as the sum of the uptake rates estimated in all sampling points (n = 6 per species; Figure 2). This total uptake was used as an estimate of channel-scale uptake, but is not necessarily a measure of total ammonium uptake rates per biomass or aerial cover.

Statistical analyses

The flume incubation experiments yielded n = 30 samples per experimental run (i.e. from 3 x-positions 2 y-positions 5 shoots per position) for each species, and two replicate runs were performed for each configuration. As we were interested in differences in uptake rates among positions within patches, the average uptake rates of the 5 shoots per each position were used in subsequent analyses.

Friedman's rank-sum test was used to test for the presence of trends in NH4 + uptake rates from the upstream to downstream positions along the patches in all replicate runs. The tests were run separately for each species, both for the centerline and for the edge measurement points. To account for the fact that within-patch measurements were not independent from each other, statistical differences in NH4 + uptake rates for each species under four flume spatial configurations were tested using nested ANOVA (with replicate run as a nested factor within the configuration treatment). As Friedman's rank-sum tests showed no significant trend in ammonium uptake neither along the centerline nor along the edge positions within patches, measurement position was considered a random effect in the model. The data were log-transformed to meet the ANOVA test assumptions of normality and homogeneity of variance.

Pearson's correlation coefficient was used to test for significant correlation between NH4 + uptake rates (µmol g -1 (DM) h -1 ) and hydrodynamic parameters (depth-averaged velocity <Ū> (m s -1 ); Reynolds shear stress τxz (Pa); Turbulent KE (m 2 s -2 ); Total KE (m 2 s -2 ); and volumetric flow rate, Qc (m 3 s -1 ), and between channel total NH4 + uptake rates and average hydrodynamic parameters within both species patches (mean velocity, Turbulent KE and Total KE). P values of less than or equal to 0.05 were considered to be significant. All statistical analyses were performed in R 3.1.2 (R Core Team

2015)

.

Results

Relationship between canopy hydrodynamic parameters and nutrient uptake

We found that the two macrophyte species affected each other's ammonium uptake rates by altering mean flow velocity (<Ū>) and turbulence (Turbulent KE). Ammonium uptake rates depended on either mean flow velocity (Callitriche) or turbulence (Groenlandia), but Total Kinetic Energy (Total KE) was the single best descriptor of uptake rates for both species (Figure 3; Table 2). Specifically, NH4 + uptake rates for the sparse Groenlandia were significantly correlated with Turbulent KE (r = 0.68, p < 0.001),

but not with mean flow velocity (r = -0.20, p = 0.35) (Figure 3; Table 2). The opposite was true for the dense Callitriche: uptake rates were significantly correlated with mean flow velocity (r = 0.42, p = 0.04), but not with Turbulent KE (r = 0.34, p = 0.1) (Figure 3; Table 2). However, Total KE, which is more representative of the instantaneous velocity, described uptake for both species (r = 0.79, p < 0.001 for Groenlandia; r = 0.45, p = 0.03 for Callitriche; r = 0.54, p < 0.001 for both species together) (Figure 3A; Table 2). No significant relationship was found between ammonium uptake rates and either Reynolds shear stress or Qc (Table 2).

Effects of patch spatial configurations on nutrient uptake

When located upstream, the dense Callitriche patch increased turbulence and thereby enhanced the uptake of resources by the sparse Groenlandia patch located downstream. The ammonium uptake rates were influenced by both macrophyte species and spatial patch configuration (order and alignment).

Importantly, the Dense-Sparse (D-S) configurations led to higher uptake rates for both species. The NH4 + uptake rates for the sparse Groenlandia were 2.63 ± 1.33 µmol g -1 (DM) h -1 , almost double than for the dense Callitriche (1.44 ± 0.78 µmol g -1 (DM) h -1 ). Testing for the presence of patterns in NH4 + uptake rates from upstream to downstream within the patches showed no significant trend in uptake rates along the patch centerline (Friedman = 3, p = 0.22 for Callitriche; Friedman = 1, p = 0.60 for Groenlandia), nor along the patch edge (Friedman = 0.25, p = 088 for Callitriche; Friedman = 3, p = 0.22 for Groenlandia). This indicates there was no significant pattern in stable isotope concentration from upstream to downstream within patches.

The upstream-downstream order and spatial patch alignment of the species significantly affected uptake rates for both the sparse Groenlandia (nested ANOVA, F3,4 = 6.87, p = 0.04) and the dense Callitriche (nested ANOVA, F3,4 = 12.57, p = 0.017; Figure 4). We generally found that when the denser species (Callitriche) was located upstream of the sparser one (Groenlandia), ammonium uptake rates for both species increased significantly, compared to patch configurations in the Sparse-Dense order (Figure 4). This significant increase in uptake rates was related to the hydrodynamic effects of different configurations, and particularly the traits of Callitriche (i.e. density and canopy height, which blocks a larger fraction of flow depth). When the dense patch of Callitriche was upstream, it generated higher Turbulent KE that influenced the downstream patch of Groenlandia (Figure 2), enhancing its uptake rates (Figure 3). Also, when the dense Callitriche was upstream, its leading edge was exposed to higher mean velocity compared to when it was trailing behind the sparse patch (Figure 2), thereby increasing its uptake rates (Figure 3). Specifically, for the dense Callitriche, uptake rates within the Sparse-Dense order were higher in the staggered than in the aligned configuration (Tukey's HSD, z = -2.66, p < 0.05). In the Dense-Sparse configurations, no significant difference in uptake rates was found between the staggered or aligned arrangement (Tukey's HSD, z = -0.50, p > 0.05). However, uptake rates were significantly higher in the Dense-Sparse staggered configuration than in both the S-D configurations (Tukey's HSD, z = -2.83, p < 0.05). For the sparse Groenlandia, uptake rates within the Sparse-Dense aligned and staggered configuration were not significantly different from each other (Tukey's HSD, z = -1.92, p < 0.05). In the Dense-Sparse configurations, no significant difference in uptake rates was found between the staggered or aligned arrangement (Tukey's HSD, z = 0.51, p > 0.05). Uptake rates in the D-S aligned configuration were significantly higher than in the S-D aligned configuration (Tukey's HSD, z = -2.69, p < 0.05), but were not significantly different from the S-D staggered case (Tukey's HSD, z = -0.77, p > 0.05).

We found that the vegetation distributions that generated higher Total Kinetic Energy levels within the patches promoted higher total uptake at the channel scale (Figure 5). Testing for the hydrodynamic parameter-uptake relationships at the channel scale revealed a significant positive relationship between the in-patch Total Kinetic Energy (average of both patches in each configuration) and the channel total ammonium uptake (r = 0.98, p = 0.01; Figure 5). Channel total ammonium uptake was also significantly related to in-patch Turbulent KE (r = 0.97, p = 0.03), but not to mean flow velocity (r = 0.93, p = 0.07).

Discussion

The interaction between vegetation and hydrodynamics regulates important ecological processes such as nutrient delivery and uptake by aquatic plants, which are crucial for community primary productivity [START_REF] Thomas | Effects of water velocity and canopy morphology on ammonium uptake by seagrass communities[END_REF]Cornelisen and Thomas 2006;[START_REF] Morris | Interaction between hydrodynamics and seagrass canopy structure: Spatially explicit effects on ammonium uptake rates[END_REF]. We found that, by generating turbulence, dense macrophyte patches facilitate resource uptake by neighboring sparse patches. Flume measurements showed that the dense Callitriche had a strong hydrodynamic effect, creating highturbulence regions in its wake that facilitated nutrient uptake by the sparse Groenlandia, which had a weaker hydrodynamic effect. While the sparse vegetation benefited from the high turbulence generated in the wake of a dense patch, the dense vegetation benefited from being located at a leading edge, where it was exposed to higher mean velocity, compared to when it was located downstream of another patch (Figure 2; Figure 6). We identified Total Kinetic Energy as the best descriptor of the nutrient removal capacity of streams, especially in heterogeneous multi-species communities. Overall, spatial configurations that lead to higher Total Kinetic Energy within the patches were the ones that led to higher total ammonium uptake. Hence, our results highlight the importance of turbulence as an agent of interaction between different species. Moreover, this study suggests that accounting for interactions between heterogeneous, multi-specific patchy vegetation is crucial to understand aquatic ecosystem functions such as nitrogen retention.

Implications of resource uptake in mono-and multi-species communities

Previous studies of macrophytes generally found that nutrient uptake rates increased with mean flow velocity (Cornelisen and Thomas 2006;[START_REF] Bal | Influence of hydraulics on the uptake of ammonium by two freshwater plants[END_REF]. [START_REF] Morris | Interaction between hydrodynamics and seagrass canopy structure: Spatially explicit effects on ammonium uptake rates[END_REF] found that Turbulent KE was associated with spatial variation in uptake, and volumetric flow rate explained differences in uptake between contrasting species. Yet, in our study, neither of these traditional hydrodynamic parameters could accurately describe uptake rates for both species. However, direct comparison with these previous studies is limited to some extent by differences in treatments. While previous experiments tested the effect of changing bulk flow velocity on uptake rates, we tested a single bulk flow velocity that represented the average flow conditions in the study sites where Callitriche and Groenlandia coexist. We thus focused on within-patch flow variability, and how the interaction between the two species was determined by their spatial arrangement and flow modification. Using a single bulk flow condition resulted in a relatively narrow range of velocities experienced by the sparse Groenlandia (0.13 -0.27 m s -1 ), whereas the flow alteration ability of the dense Callitriche led to a higher flow variability (0.002 -0.24 m s -1 ). This factor may partly explain the lack of significant correlation between flow velocity and uptake rates for Groenlandia. Further research is needed to test how changes in mean flow conditions affect the interaction between the two species.

We identified Total Kinetic Energy as the parameter that explained most of the variability in uptake rates for both species. To our knowledge, this parameter has not been related before to nutrient uptake rates by aquatic vegetation. Previous studies have suggested that Turbulent KE may influence nutrient uptake [START_REF] Anderson | A fluid dynamics study of seawater flow through Gelidium nudifrons[END_REF][START_REF] Koch | Hydrodynamics, diffusion-boundary layers and photosynthesis of the seagrasses Thalassia testudinum and Cymodocea nodosa[END_REF], and the total energy parameter captures this influence. Specifically, when Turbulent KE is weak, flux is controlled by the time-mean diffusive sublayer thickness, which is a function of the time-mean velocity (e.g. [START_REF] Hansen | Photosynthetic oxygen flux by Macrocystis pyrifera: a mass transfer model with experimental validation[END_REF]; [START_REF] Rominger | Effects of blade flexural rigidity on drag force and mass transfer rates in model blades[END_REF]; [START_REF] Lei | Impact of current speed on mass flux to a model flexible seagrass blade[END_REF]). However, when the Turbulent KE is high, periodic disturbances of the diffusive sub-layer by the turbulence can create instantaneously higher concentration gradients at the surface and, thus, higher flux (e.g. [START_REF] Stevens | Boundary-layers around bladed aquatic macrophytes[END_REF]; [START_REF] Huang | The motion of kelp blades and the surface renewal model[END_REF]; [START_REF] Rominger | Effects of blade flexural rigidity on drag force and mass transfer rates in model blades[END_REF]). By reflecting the magnitude of both the time-mean velocity and Turbulent KE, the total kinetic energy captures both regimes of flux. The Total Kinetic Energy is particularly suitable in heterogeneous systems where upstream Turbulent KE generation (e.g. by larger, denser patches) can influence flux downstream, i.e. the Turbulent KE is not locally generated and thus uncorrelated with the local time-mean velocity. In the dense Callitriche patches, the canopy is often too dense for turbulence to form within the patch or to penetrate from the free stream. Under these low Turbulent KE conditions, the flux is correlated by the local time-mean velocity, which sets the scale of the diffusive sub-layer. For the sparse Groenlandia, time-mean flow velocity is relatively constant in the canopy and we found no correlation between within-patch flow variations and uptake rates. However, the withinpatch Turbulent KE is elevated both by local stem generation and the penetration of turbulence generated upstream. Under these high Turbulent KE conditions, the uptake rates have a high correlation with the Turbulent KE intensity. In landscapes made up of patches of different species, regions with flux controlled by <Ū> and regions controlled by Turbulent KE are heterogeneously distributed, so that neither <Ū> nor Turbulent KE can capture the channel-scale nutrient uptake. Because it can describe both regions of low Turbulent KE (uptake controlled by mean velocity) and high Turbulent KE (uptake controlled by Turbulent KE intensity), we propose Total Kinetic Energy as a useful parameter to describe nutrient uptake capacity in heterogeneous landscapes, which could be used to estimate ecosystem services of nutrient retention by vegetation.

In contrast to the findings of [START_REF] Morris | Interaction between hydrodynamics and seagrass canopy structure: Spatially explicit effects on ammonium uptake rates[END_REF] and [START_REF] Bal | Influence of hydraulics on the uptake of ammonium by two freshwater plants[END_REF], we did not find a significant relationship between ammonium uptake rates and volumetric flow rate, likely because the two species have different flexibility and density traits that affect patch compression (Table 1). The canopies of the two species, and the relative importance of flow velocity and turbulence within them, are consistent with the sparse and dense canopy regimes described in [START_REF] Nepf | Flow and transport in regions with aquatic vegetation[END_REF]. As expected for a dense canopy condition (ah > 0.1), canopy-scale turbulence in Callitriche is generated at the top of the canopy and can be transported downstream, while stem-scale turbulence is much smaller. Instead, Groenlandia is representative of a sparse canopy condition (ah < 0.1), where stem-scale turbulence is generated within the canopy, but the velocity profile remains logarithmic.

Our results reveal the important role of patch spatial configuration and the resulting heterogeneity in influencing species interactions and the nutrient uptake capacity of the landscape. The generation of turbulence by a Callitriche patch led to a 59% nutrient uptake enhancement effect on Groenlandia, increasing it from an ammonium uptake rate of 2.03 ± 0.85 µmol g -1 (DM) h -1 in the Sparse-Dense configurations (where Groenlandia was not exposed to turbulence generation by Callitriche), to 3.23 ± 0.37 µmol g -1 (DM) h -1 in the Dense-Sparse configurations (where it was exposed to turbulence generation by Callitriche). These findings are in line with a field study showing that spatial heterogeneity, created by the interaction of canopy morphology, sediment topography and hydrodynamics, controlled nutrient transport and uptake rates in a patchy seagrass landscape [START_REF] Morris | The role of hydrodynamics in structuring in situ ammonium uptake within a submerged macrophyte community[END_REF]). While we found clear effects of the upstream-downstream patch arrangement on ammonium uptake rates, we only observed a significant effect of patch arrangement (staggered vs. aligned) in the Sparse-Dense configurations. Vegetation patches in the flume experiment did not span the whole width of the channel, leading to water flow deflection and acceleration around the lateral edges of patches.

This flow acceleration effect around the patches is in general agreement with experimental evidence in field and laboratory studies [START_REF] Vandenbruwaene | Flow interaction with dynamic vegetation patches: Implications for biogeomorphic evolution of a tidal landscape[END_REF][START_REF] Schoelynck | Selforganised patchiness and scale-dependent bio-geomorphic feedbacks in aquatic river vegetation[END_REF][START_REF] Bouma | Organism traits determine the strength of scale-dependent bio-geomorphic feedbacks: A flume study on three intertidal plant species[END_REF],

suggesting that the conclusions of this work could be generally applied to field conditions. In addition to the role of spatial configuration, it is likely that the distance between the patches governs the intensity of the interaction between them. The stronger interactions between patches likely occur when the distance between them is less than the wake length of the upstream patch [START_REF] Folkard | Hydrodynamics of model Posidonia oceanica patches in shallow water[END_REF]. It might be expected that the wake length is in turn related to patch density, because density determines at what distances the patch effects will dissipate [START_REF] Zong | Vortex development behind a finite porous obstruction in a channel[END_REF]. Further studies should be undertaken to investigate the detailed hydrodynamic consequences of different spatial patch configurations, testing for the effects of a wider range of distances and its interactive effect with patch density.

Turbulence-mediated species interactions: implications for species distributions and nutrient load reduction

The study of turbulence-mediated interactions between macrophyte species suggests a possible mechanism behind the co-occurrence of Groenlandia patches around Callitriche in the field. Recently, it has been shown that Groenlandia shoots grow better around Callitriche patches than on bare, unvegetated sediment [START_REF] Cornacchia | Landscapes of facilitation: how self-organized patchiness of aquatic macrophytes promotes diversity in streams[END_REF]. The wake of the Callitriche patches is both a highturbulence and low-velocity region (Sand-Jensen 1998). Thus, a combination of enhanced resource uptake by turbulence, and reduced biomass losses by flow velocity, might be the conditions behind the improved growth rates of Groenlandia plants around Callitriche patches. As the sparse Groenlandia tends to surround the dense Callitriche patches in regularly spaced aggregations every 8 m [START_REF] Cornacchia | Landscapes of facilitation: how self-organized patchiness of aquatic macrophytes promotes diversity in streams[END_REF], the interaction between the two species might enhance the overall nutrient removal capacity of the river. The facilitative effect of Callitriche on Groenlandia could switch to competition, as the high biomass Callitriche might have a competitive advantage by reducing resource availability for the sparser species (Groenlandia). On the other hand, competition could be lessened through root uptake from the nutrient pool in the sediment. The balance between facilitation and competition can be clarified by considering additional variables such as root uptake and nutrient availability in the water column and in sediment. Moreover, care must be taken when upscaling the relationship between hydrodynamics and resource uptake at the channel scale. In our incubations, we focused only on uptake rates of a single nutrient (ammonium), which is energetically less costly, but some species might invest in nitrate uptake. This is an interesting aspect that should be explored in future studies of channel-scale nitrogen uptake by vegetation. As a future perspective, we might be able to use the knowledge on these types of species interactions as tools to enhance restoration success of degraded (eutrophic) sites. Numbers indicate mean (± SE) water velocity Ū (m s -1 ) and Turbulent Kinetic Energy (Turbulent KE) (m 2 s -2 ) within each species patch.

Figure 3: Scatter plots of NH4 + uptake rates (μmol g -1 (DM) h -1 ) against Total Kinetic Energy (Total KE) (m 2 s -2 ), depth-averaged velocity <Ū> (m s -1 ) and Turbulent Kinetic Energy (Turbulent KE) (m 2 s - 2 ) for the sparse Groenlandia (black circles) and the dense Callitriche (white diamonds). Black lines are linear regression lines for the Groenlandia (solid line) and Callitriche (dotted line) data separately and represent significant relationships (p 0.05).

Figure 4: Boxplots of the distribution of NH4 + uptake rates (μmol g -1 (DM) h -1 ) within patches of the dense Callitriche (a) and the sparse Groenlandia (b) in each spatial configuration (S indicating sparse vegetation, D indicating dense vegetation, see Figure 2). Letters denote significant differences (Tukey's HSD, p < 0.05).

Figure 5: Scatter plots of channel total NH4 + uptake rates (μmol g -1 (DM) h -1 ) in each spatial configuration against Total Kinetic Energy (m 2 s -2 ) averaged within patches of Callitriche and Groenlandia in each spatial configuration (S indicating sparse vegetation, D indicating dense vegetation, see Figure 2). Error bars represent standard error of the mean.

Figure 6: Schematized drawing of the effects of multispecific spatial patchiness on hydrodynamics and nutrient uptake rates. In Sparse-Dense configurations (a), the sparse vegetation is exposed to high mean flow but low turbulence, and does not benefit from being located at the leading edge. Similarly, the dense vegetation is exposed to low mean flow speed due to sheltering by the patch upstream, and hence has lower uptake rates. Instead, in Dense-Sparse configurations (b), uptake rates of both species are higher: the dense vegetation benefits from being at the leading edge and exposed to high mean flow speed (which increases uptake rates); at the same time, the sparse vegetation benefits from the high turbulence created in the wake of the dense patch. nutrient uptake rates. In Sparse-Dense configurations (a), the sparse vegetation is exposed to high mean flow but low turbulence, and does not benefit from being located at the leading edge. Similarly, the dense vegetation is exposed to low mean flow speed due to sheltering by the patch upstream, and hence has lower uptake rates. Instead, in Dense-Sparse configurations (b), uptake rates of both species are higher: the dense vegetation benefits from being at the leading edge and exposed to high mean flow speed (which increases uptake rates); at the same time, the sparse vegetation benefits from the high turbulence created in the wake of the dense patch.

Figure 1 :

 1 Figure 1: (A) Natural patches of Callitriche and (B) Groenlandia in the field. (C, D) Lateral view of the

Figure 2 :

 2 Figure 2: Schematic diagram of the four spatial configurations of aquatic macrophytes in the test

FiguresFigure 1

 1 Figures

Figure 2

 2 Figure 2 Schematic diagram of the four spatial configurations of aquatic macrophytes in the test

Figure 3

 3 Figure3Scatter plots of NH4 + uptake rates (μmol g -1 (DM) h -1 ) against Total Kinetic Energy (Total

Figure 4

 4 Figure 4Boxplots of the distribution of NH4 + uptake rates (μmol g -1 (DM) h -1 ) within patches of the

Figure 5

 5 Figure5Scatter plots of channel total NH4 + uptake rates (μmol g -1 (DM) h -1 ) in each spatial

Figure 6

 6 Figure 6 Schematized drawing of the effects of multispecific spatial patchiness on hydrodynamics and

Table 2

 2 Pearson's correlation coefficients between NH4 + uptake rates (μmol g -1 (DM) h -1 ) and canopy height (cm) or hydrodynamic parameters (depth-averaged velocity <Ū> [m s -1 ]; Reynolds shear stress τxz [Pa]; Turbulent Kinetic Energy (Turbulent KE) [m 2 s -2 ]; Total Kinetic Energy (Total KE) [m 2 s -2 ] and canopy water flow Qc [m 3 s -1 ]) for Groenlandia, Callitriche and both species considered together.Correlation coefficients in bold are significant at p ≤ 0.05.

		Groenlandia (n = 24)	Callitriche (n = 24)	All (n = 48)
	Height	0.17	-0.18	-0.30
	<Ū>	-0.20	0.42	0.40
	τ xz	0.01	-0.18	0.03
	Turbulent KE	0.68	0.34	0.53
	Total KE	0.79	0.45	0.54
	Q c	-0.07	0.19	-0.09

Acknowledgments

The authors gratefully acknowledge Bert Sinke, Lennart van IJzerloo, Lowie Haazen and Jeroen van Dalen for their technical assistance in the flume. We thank Peter van Breugel and members of the analytical lab of NIOZ-Yerseke for the stable isotope analyses. We thank Siebren Wezenberg for helping during the experiments. This work was supported by the Research Executive Agency, through the 7th Framework Programme of the European Union, Support for Training and Career Development of Researchers (Marie Curie -FP7-PEOPLE-2012-ITN), which funded the Initial Training Network (ITN) HYTECH 'Hydrodynamic Transport in Ecologically Critical Heterogeneous Interfaces', N.316546. Data associated with this study is available from 4TU.Centre for Research Data at: 10.4121/uuid:e4678efa-426f-483a-b74a-c6ab4df53354

Tables

 Table 1Summary of patch characteristics [mean ± SD (n)] of the two species at the incoming flow velocity of 0.24 m s -1 used in the flume experiments: biomass (g DW m -2 ; measured), canopy height (h, m; measured), frontal area per water volume (a, m -1 ; calculated from Eq. 3), frontal area per bed area (ah, dimensionless; calculated from Eq. 4).