Freshwater Biology (1999) 42, 315±328
Velocity gradients and turbulence around macrophyte stands in streams KAJ SAND-JENSEN AND OLE PEDERSEN Freshwater Biological Laboratory, University of Copenhagen, Helsingùrsgade 51, 3400 Hillerùd, Denmark
SUMMARY 1. Submerged macrophytes strongly modify water flow in small lowland streams. The present study investigated turbulence and vertical velocity gradients using small hot-wire anemometers in the vicinity and within the canopies of four macrophyte species with the objective of evaluating: (a) how plant canopies influence velocity gradients and shear force on the surfaces of the plants and the stream bed; and (b) how the presence and morphology of plants influence the intensity of turbulence. 2. Water velocity was often relatively constant with water depth both outside and inside the plant canopies, but the velocity declined steeply immediately above the unvegetated stream bed. Steep vertical velocity profiles were also observed in the transition to the surface of the macrophyte canopy of three of the plant species forming a dense shielding structure of high biomass. Less steep vertical profiles were observed at the open canopy surface of the fourth plant species, growing from a basal meristem and having the biomass more homogeneously distributed with depth. The complex distribution of hydraulic roughness between the stream bed, the banks and the plants resulted in velocity profiles which often fitted better to a linear than to a logarithmic function of distance above the sediment and canopy surfaces. 3. Turbulence increased in proportion to the mean flow velocity, but the slope of the relationships differed in a predictable manner among positions outside and inside the canopies of the different species, suggesting that their morphology and movements influenced the intensity of turbulence. Turbulence was maintained in the attenuated flow inside the plant canopies, despite estimates of low Reynolds numbers, demonstrating that reliable evaluation of flow patterns requires direct measurements. The mean velocity inside plant canopies mostly exceeded 2 cm s±1 and turbulence intensity remained above 0.2 cm s±1, which should be sufficient to prevent carbon limitation of photosynthesis in CO2-rich streams, while plant growth may benefit from the reduced physical disturbance and the retention of nutrient-rich sediment particles. 4. Flow patterns were highly reproducible within canopies of the individual species despite differences in stand size and location among streams. We propose that individual plant stands are suitable functional units for analysing the influence of submerged macrophytes on flow patterns, retention of particles and biological communities in lowland streams. Keywords: flow velocity, macrophyte patches, stream macrophytes, turbulence
Introduction Correspondence: Kaj Sand-Jensen and Ole Pedersen, Freshwater Biological Laboratory, University of Copenhagen, Helsingùrsgade 51, 3400 Hillerùd, Denmark. E-mail:
[email protected] ã 1999 Blackwell Science Ltd.
Submerged stream macrophytes often form a patchy mosaic of dense monospecific stands which undergo temporal changes in size and location (Butcher, 1933;
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Gessner, 1955; Sand-Jensen & Madsen, 1992). The stands usually have well-defined margins, leading to strong gradients in water velocity, turbulence and sediment composition at the plant-water boundaries (Chambers, Prepas & Gibson, 1992; Sand-Jensen, 1998). Within the macrophyte stands, bushy shoots strongly reduce the flow and promote sedimentation of fine particles, while between the stands, high velocities and particle erosion prevail (Sand-Jensen & Mebus, 1996). This spatial variability of the flow should be less pronounced for plant species forming open stands with streamlined leaves which permit the water to pass through the vegetation rather than being deflected around it. Water velocity can change vertically with distance from the sediment and the plant canopies because of their resistance to the flow, as well as horizontally along the length of the plant stands as the dimensions of the canopy and the main direction of the flow change (Sand-Jensen & Mebus, 1996). Vertical velocity profiles are traditionally used to characterize flow patterns and to calculate discharge (Statzner, Gore & Resh, 1988; Vogel, 1994; Carling, 1996). In these profiles, steep velocity gradients are associated with strong shear, and intense turbulence derives from the interaction between shear and roughness at solid surfaces. In deep streams without macrophytes, the stream bed provides the main resistance to the flow and relatively uniform logarithmic velocity profiles develop vertically above the sediment (Carling, Orr & Glaister, 1994). The slope of the logarithmic velocity profile provides a measure of shear velocity which is used to calculate the shear force and the roughness Reynolds number at the stream bed (Nowell & Jumars, 1984; Gordon, McMahon & Finlayson, 1992). In small, shallow streams with macrophyte stands distributed as isolated patches, ideal flows are unlikely to be fully developed. The complex spatial distribution of roughness elements among plant stands, stream bed and banks, and the shifting acceleration and deceleration of the flow outside and inside plant canopies, may result in variable velocity profiles and distortion of logarithmic profiles (Carling, 1996). Water flow has both a spatial and a temporal dimension. The spatial variability of the flow can be described by comparison of the average velocity for different locations. The temporal variability at any point in the stream leads to rapid fluctuations of the
velocity with the passage of turbulent eddies. If frequent measurements (several times per second) are made at a point, the turbulence intensity can be expressed as the standard deviation (SD) of flow velocity at this particular point (Gordon et al., 1992). Because turbulence intensity increases with the mean Å or a characteristic flow regime flow velocity ( u) (Morisawa, 1985), turbulence intensity is also often expressed as a proportion of mean velocity in the main direction of flow (i.e. the coefficient of variation, Å providing a simple measure of the CV = SD/( u), relative intensity of turbulence. In reality, turbulent fluctuations occur in all directions, but a common assumption is that, `turbulence intensities are the same in both horizontal and vertical directions since they arise from the same sets of eddies' (Gordon et al., 1992). Measurements of turbulence are few in stream habitats and no data describe the flow patterns in environments with patchy vegetation. Such measurements are needed to evaluate both the intensity of physical disturbance, and the flux of solutes and particles to support the metabolism of plants, animals and micro-organisms. Measurements of vertical velocity profiles in canopies of seagrasses in unidirectional flow in laboratory flumes as well as in marine tidal waters have shown a steep reduction of velocity immediately above the canopy surface, and relatively constant velocities with depth inside the canopies (Gambi, Nowell & Jumars, 1990; Koch, 1993). Under such conditions, the intensity of turbulence is high at the canopy surface and is dampened within the canopies. In contrast, in wave-exposed environments, the flapping movement of the seagrass leaves results in less steep velocity profiles and the transmission of turbulence further into the canopy (Koch, 1993). Flow patterns in streams are strongly modified by the presence of submerged plants both on the large scale involving entire stream reaches (Dawson, 1978; Larsen, Frier & Vestergaard, 1990; Thyssen et al., 1990) and on the local scale involving individual plant stands (Sand-Jensen & Mebus, 1996). The flow patterns generated or modified by the presence of the plants have strong implications for their own metabolism, physical resistance and development, as well as the sedimentation and resuspension of sediment particles, and the growth and survival of micro-organisms and invertebrates on plant surfaces or on sediments shielded by the canopies (Gregg & ã 1999 Blackwell Science Ltd, Freshwater Biology, 42, 315±328
Velocity gradients around macrophyte strands Rose, 1985; Tokeshi, 1986; Sand-Jensen, 1997). Different architecture, stiffness and density of plant species and individual plant stands will generate different flow patterns with strong implications for plants, macroinvertebrates and microbial processes. These physical aspects have remained unknown to date because of the lack of appropriate methods for measuring velocity and turbulence within the plant canopies. In the present study, we determined the vertical gradients of mean velocity and turbulence intensity in positions located in the free water upstream and downstream of plant stands, and in positions inside the canopies using small robust hot-wire anemometers to ensure high spatial and temporal resolution. We included four common plant species with different morphology and architecture in the analysis and examined six stands of each species with different size in several Danish streams to evaluate the inter- and intra-specific differences. The present study examines the same four stream species and uses the same measurement technique as reported by Sand-Jensen & Mebus (1996), but the current independent study focuses on vertical flow profiles measured with high spatial resolution and includes determination of turbulence. Our first objective was to use the shape and steepness of the velocity gradients above the canopy surfaces, and above the sediment surface inside and outside the stands to evaluate how plant canopies of different size and morphology influence flow patterns and shear forces. Our second objective was to determine how the intensity of turbulence changes with mean velocity outside the plant stands, and whether particularly high turbulence intensity is associated with regions of steep velocity gradients above canopy and sediment surfaces, as previously reported for the upper part of the boundary layer above leaf surfaces in air (Grace & Wilson, 1976) and above sessile benthic organisms in water (Fig. 10.8 of Denny, 1988). Our third objective was to determine the intensity of turbulence inside the plant stands, and evaluate whether systematic differences exist between plant species of different architecture and stiffness. Plant species which perform undulating movements may generate relatively high turbulence within the canopy, even though the mean velocities are low (Ackerman & Okubo, 1993). Finally, we discuss the implications of different flow environments among ã 1999 Blackwell Science Ltd, Freshwater Biology, 42, 315±328
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species and locations within the canopies for the plants and their attached fauna and micro-organisms.
Materials and methods Streams and plant species
Flow patterns were studied inside and outside plant canopies in five small Danish streams during midsummer. The stream sections were unshaded, 3±7 m in width, 0.4±0.8 m in mean depth and each section was dominated by one particular species. The downstream slopes of the water surface were between 1 and 4 m km±1, and the mean water velocities along the sections were between 20 and 60 cm s±1. The sediments were mainly composed of fine to coarse sand (0.063±2.0 mm grain size), and the contributions of fine silt and clay particles or coarser gravel and stones were only between 1 and 29% of the total mineral dry weight (Sand-Jensen, 1998). Flow patterns were studied in stands of four common species of submerged stream macrophytes with different morphology and resistance to the flow (Fig. 1 of Sand-Jensen & Mebus, 1996). Batrachium peltatum (Schrank) Presl has long flexible stems with leaves subdivided into 30±40-mm-long capillary filaments oriented in the flow direction. The longest trailing stems can be observed to move back and forth in the strong vortices formed behind large stands in rapid flow, and free water is found between the overhanging canopy and the sediment in the downstream part of the stands. Callitriche cophocarpa Sendter has short flexible stems and 15±20-mm-long leaves which form a dense network of tissue with relatively high flow resistance. Elodea canadensis L. C. Rich. forms a canopy of more erect stems with short, densely packed stiff leaves (5±10 mm long). All three species have apical meristems and relatively high densities of plant tissue per unit volume within the plant canopy [maximum = 4.7±12.1 mg dry weight (DW) cm±3; Jùrgensen, 1990). The three species are canopy formers with higher biomass density at the surface than deeper into the canopy (Sand-Jensen & Mebus, 1996). Sparganium emersum Rehman has relatively open stands (maximum = 3.7 mg DW cm±3; Jùrgensen, 1990) of 0.4±1.2-m-long, strap-formed leaves, while the stems are buried in the sediments. Sparganium emersum has a basal meristem and it is a
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meadow former with lower biomass density at the surface than deeper into the canopy (Sand-Jensen & Mebus, 1996). Flow measurements
The twenty-four plant stands studied for the four species were located in the middle of the streams with more than 2 m distance to upstream or downstream neighbouring stands. The stands had an ellipsoid form, and their length was between 0.6 and 4.6 m with seventeen stands measuring between 0.9 and 2.0 m. The range in length (in parenthesis) of the four species was: B. peltatum (100±460 cm), C. cophocarpa (60±200 cm), E. canadensis (110±200 cm) and S. emersum (60±130 cm). The width of the stands was 0.4±0.6 times their length. The longest axis of the stands was always oriented parallel to the banks and the main direction of the flow. Vertical velocity profiles were measured at 1±4-cm depth intervals at six positions along the length of the stands. The six positions were located along the centre line of the elongated stands parallel to the stream banks. Position I was located » 1 m upstream from the front of the stand. Position II was located 2 cm inside the stands front. Position III was located at one-third and position IV at two-thirds of the distance along the length of the stand. Position V was located 2 cm inside the downstream margin of the stand. Position VI was located about 1 m downstream of the stand. Water velocity was measured at each particular point with cylindrical hot-wire anemometers, 3 mm in length and 1 mm in diameter (Dantech Measurement Technology, Copenhagen, steel-clad probe 55 A76), which were positioned with an accuracy of 0.5 cm from the water surface and an accuracy of about 0.2 cm between consecutive locations along the vertical profile. Details of the equipment and principles of registration are provided by Sand-Jensen & Mebus (1996). The cylindrical hot-wire probe had a constant over-temperature of about 3 °C relative to ambient temperature, which remained constant during measurements on the individual stand, but differed somewhat (14±18 °C) between measurements on different plant species and stands in different streams. The steel-clad probe was calibrated in unidirectional flow of low turbulence at 15 °C in the laboratory before and after use. Calibration of the probe was not subject to measurable drift during use
and differences in ambient temperature have no consequences for the measurements. The electronic noise of the measurements is small relative to natural fluctuations caused by turbulence in the stream. The main flow in the stream is in the downstream direction, but the cylindrical hot-wire probe responds to the flow in all three dimensions, and therefore, the intensity of turbulence determined represents the integrated scalar changes in velocity around the probe. Measurements of water velocity were recorded with a frequency of 20 Hz over a time period of 50 s at each location of the probes. The mean velocity and standard deviation were calculated based on 1000 measurements over 50 s. To help the interpretation of the flow pattern around the macrophyte stands, we used a hypodermic needle to inject a dye (Rhodamin B) into the flow upstream and along the sides of some of the stands, as well as in the interior of the canopy, and observed the dye movements.
Results Vertical velocity profiles
Vertical velocity profiles measured in the free water upstream and downstream of the plant canopies had a variable shape for the different plant species and streams included in the present study (Figs 1 & 2). Most profiles measured outside stands of B. peltatum, C. cophocarpa and E. canadensis had almost constant velocities with depth until the velocity declined steeply immediately above the sediment surface. Several of the vertical profiles outside stands of S. emersum had a clear separation of the flow between a relatively thick zone of low velocity above the stream bed and a zone of high velocity closer to the water surface. We tested how well the decline of velocity fitted to either the linear distance or the logarithm of distance immediately above the stream bed involving between three and five data points over 4±10 cm. For the logarithmic description, we used Eqn 71 of Carling et al. (1994). Both the linear and the logarithmic models produced a high correlation coefficient (> 0.9) in more than 85% of the forty-eight vertical profiles examined, but the linear model provided a better fit in a significantly greater number (thirty-five) of the cases (x2-test, P < 0.01). Consequently, although ã 1999 Blackwell Science Ltd, Freshwater Biology, 42, 315±328
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Å bold lines) in six positions along the Fig. 1 Vertical profiles of mean velocity (thin lines) and relative turbulence intensity (CV = SD/u; centre line of three macrophyte stands of Batrachium peltatum. Bars represent the standard deviation of velocity fluctuations about the mean. The vertical profiles were positioned (I) upstream of the stands, (II) 1±2 cm inside the upstream front of the stands, (III) onethird of stand length into the canopy, (IV) two-thirds of the stand length into the canopy, (V) 1±2 cm inside the downstream front of the stand and (VI) downstream of the stands. The surface of the sediment and the canopy are indicated by the arrows; (X) CV-values larger than 0.5. The length of the three stands was (stand 1) 100 cm, (stand 2) 110 cm and (stand 3) 250 cm.
it was possible in many cases to estimate the shear velocity and shear forces at the sediment surface from the slope of the velocity profile as a function of the logarithmic distance in the boundary layer above the stream bed, the velocity profiles could better be characterized by linearly increasing velocities within short distances above the sediment followed by a sharp transition to constant velocities at greater distance above the sediment. If more data points were included, then the logarithmic model provided a better fit than a simple linear model, but the correlation coefficient dropped markedly because velocity remained relatively constant, and in some cases, even declined with further distance away from the stream bed. However, we acknowledge that more data points, a smaller probe and a higher accuracy in the location of the probe relative to the sediment surface would have been preferable for the evaluation of fine-scale velocity profiles immediately above the stream bed. Stands of B. peltatum, C. cophocarpa and E. canadenã 1999 Blackwell Science Ltd, Freshwater Biology, 42, 315±328
sis had a strong resistance to the flow, resulting in low velocities inside the canopy and high velocities outside the canopy (Figs 1 & 2). The deflected flow was gradually accelerated above the canopy as the thickness of the free water between the canopy surface and the water surface gradually diminished along the length of the plant stands. The flow velocity declined steeply in the transition zone between the free water and the canopy, and subsequently, remained relatively constant down through the canopy until the drop immediately above the stream bed. The average flow velocity inside the plant canopy in positions III and IV was between 8.0 and 13.2% of the free stream velocity measured upstream of the stands of the three species. For simplicity, we have only shown the vertical velocity profiles for three stands of B. peltatum (Fig. 1) and one stand of each of the other three species (Fig. 2), but the flow patterns were highly reproducible and closely resembled each other for all six stands of each species despite the variation in length and location of the stands.
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Å bold lines) in six positions along the Fig. 2 Vertical profiles of mean velocity (thin lines) and relative turbulence intensity (CV = SD/u; centre line of macrophyte stands of Callitriche cophocarpa (120 cm long), Elodea canadensis (180 cm long) and Sparganium emersum (100 cm long). The bars represent the standard deviation of velocity fluctuations about the mean. Further information is given in the legend to Fig. 1.
As with the evaluation of vertical velocity profiles above the stream bed, we tested how well the decline of water velocity in the transition from the free water to immediately inside the plant canopy fitted a linear and a logarithmic model based on between three and six data points located over a 3±8 cm vertical distance. In the thirty-six vertical profiles measured in positions III and IV in the middle of the longitudinal axis along the length of the canopies of B. peltatum, C. cophocarpa and E. canadensis, the linear model had a better fit than the logarithmic model in twenty-eight of the cases, providing a significant difference between the two descriptions (x2-test, P < 0.01). However, the correlation coefficient of the two description was higher than 0.9 for all thirty-six profiles apart from one case for the linear model and eight cases for the logarithmic model. Stands of S. emersum had relatively high velocities inside the open canopy composed of long, streamlined leaves (Fig. 2). The velocities varied markedly within short distances, depending on whether the point of measurement was located in the shelter
behind the strap-formed leaves or exposed to the freestreaming water between the leaves. The average velocity in positions III and IV inside the canopy was 72% of the velocity upstream of the stands of S. emersum. Special flow conditions existed in position V in the downstream part of the stands of B. peltatum (Fig. 1). This plant species has a dense overhanging canopy located close to the water surface, deriving from long shoots rooted further upstream in the stands, while free water is found above the stream bed. A region of reduced flow is associated with the overhanging canopy, while faster flow is found in the free water both above and below the canopy. Also, C. cophocarpa has an overhanging canopy in position V downstream in the stands, although it is less prominent because of the shorter plant shoots. A thin zone of accelerated flow is observed close to the stream bed (third data point at 4 cm distance above the bed) in positions V and VI (Fig. 2). For all four species, we examined the linear and the logarithmic decline of velocity over the 8-cm distance ã 1999 Blackwell Science Ltd, Freshwater Biology, 42, 315±328
Velocity gradients around macrophyte strands above the sediment in positions III and IV inside the stands. Because both the plants and the stream bed contribute to flow resistance, the velocity profiles were often irregular and only nine out of the linear and ten out of the logarithmic profiles produced high correlation coefficients above 0.9 among the fortyeight profiles examined. There was no significant difference between the linear and logarithmic models in their ability to fit the data. Velocity gradients above sediment and canopy surfaces
We calculated the linear velocity gradient from 2 to 1 cm above the stream bed and over the 2±4-cm-long distance across the canopy surface. The mean and standard error of the velocity gradients above the stream bed and canopy surfaces are shown for the six stands of each species (Fig. 3). The velocity gradient above the stream bed was much higher outside the stands than among the rooted shoots in positions III and IV inside the stands of B. peltatum, C. cophocarpa and E. canadensis. For the two former species, somewhat steeper gradients were observed above the stream bed in position V within the stands lacking rooted shoots. The velocity gradient was systematically higher at the canopy surface than above the stream bed for the three species. The velocity gradient
Fig. 3 Mean vertical velocity gradients ( SE) at 2±1 cm above the stream bed (X) and across the canopy surface (W) in the six positions along the centre line of six stands of four species of stream macrophytes. The bar shows the extension of the stands. ã 1999 Blackwell Science Ltd, Freshwater Biology, 42, 315±328
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at the canopy surface remained relatively constant along the length of E. canadensis stands, while it increased along the length of C. cophocarpa stands and reached a maximum at position IV for B. peltatum stands. These changes in the velocity gradient along the length of the stands were mainly related to changes in the thickness of the free-streaming water above the canopy because compressed flow leads to greater flow acceleration in the free water and steeper gradients towards the canopy surface (Figs 1 & 2). Velocity gradients were small above the stream bed and at the open canopy surface of S. emersum stands. The velocity gradients at the stream bed and at the canopy surface did not change significantly along the length of S. emersum stands, whereas differences in velocity gradients between positions inside and outside the patches, and between canopy surfaces and sediment surfaces were significant for the other three species (Fig. 3). Turbulence patterns
The intensity of turbulence was significantly reduced within the shielding canopy of B. peltatum, C. cophocarpa and E. canadensis to mean levels between seven and eighteen-fold lower than those in the free water upstream of the stands (Table 1). The intensity of turbulence was most reduced from outside to inside the stands of E. canadensis, and the levels were also significantly lower than in the canopies of B. peltatum and C. cophocarpa (Table 1). The intensity of turbulence was the same outside as inside the open canopy of S. emersum and this species experienced the highest intensity of turbulence in the canopy among the four species studied (Table 1). The intensity of turbulence increased linearly with the mean velocity in the free water upstream of the stands for the different species (Fig. 4). This pattern means that relative intensities of turbulence in the free water, expressed as the standard deviation of velocity Å relative to the mean velocity (i.e. CV = SD/u), remained constant for the individual species irrespective of absolute velocity and water depth (Figs 1 & 2). Whereas the relative intensity of turbulence was almost the same in the free water upstream of the stands of C. cophocarpa (average = 0.105), E. canadensis (average = 0.124) and S. emersum (average = 0.100), it was significantly higher upstream of the stands of B. peltatum (average = 0.152; Table 2).
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Table 1 Turbulence intensity (cm s±1) measured in different locations inside and outside the canopy of four stream macrophyte species from shallow Danish streams. Turbulence intensity at any given location of the flow probe was calculated as the standard deviation of 1000 measurements of velocity fluctuations about the mean during 50 s. The measurement locations were: (1) upstream of the stands at more than 4 cm above the stream bed; (2) upstream of the stands at 1 and 2 cm distance above the stream bed; (3) the surface of the plant canopy at positions II and III along the stands; (4) inside the canopy in positions III and IV at more than 4 cm above the stream bed; and (5) inside the canopy in positions III and IV at 1 and 2 cm distance above the stream bed. The values are presented as the means SE for six stands of each species. The numbers of measurement points were » 100 for locations 1 and 4, twelve for locations 2 and 3, and twenty-four for location 5. Significant differences (P < 0.05) among locations for each species were determined by a two-way analysis of variance and are marked by different superscript letters. At locations 1 and 4, turbulence increased significantly from S. emersuma, C. cophocarpab and E. canadensisb to B. peltatumc, and at locations 2 and 3, turbulence increased significantly from S. emersuma, C. cophocarpaa,b and E. canadensisb,c to B. peltatumc Macrophyte species Location (1) (2) (3) (4) (5)
Upstream bulk Upstream bottom Stand surface Stand bulk Stand bottom
B. peltatum 7.46 1.82 6.50 0.72 0.42
c
0.18 0.53b 0.29c 0.08a 0.20a
C. cophocarpa 3.62 1.61 3.01 0.53 0.31
c
0.16 0.33b 0.15c 0.05a 0.11a
E. canadensis 3.94 0.85 4.69 0.22 0.18
c
0.16 0.36b 0.15c 0.05a 0.11a
S. emersum 2.43 0.57 2.70 1.90 0.68
0.14c 0.28a 0.25c 0.06b 0.17a
Fig. 4 Relationships between average velocity and velocity fluctuations about the mean ( SD) in positions upstream (W, bold lines) and inside (X, thin lines) the stands of four stream macrophyte species. The locations in the bulk water were located at more than 4 cm above the stream bed. The locations inside the plant canopy were located more than 4 cm below the canopy surface and more than 4 cm above the stream bed in the middle of the plant stands (vertical gradients III and IV, viz. Figs 1 & 2). Inserts are shown to clarify the relationships inside the plant canopy. ã 1999 Blackwell Science Ltd, Freshwater Biology, 42, 315±328
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Table 2 Relative turbulence intensity (CV) measured in different locations inside and outside the canopy of four stream macrophyte species. The CV at any location of the flow probe was calculated as the standard deviation (SD) of 1000 measurements of velocity Å Further information is given in the legend to Table 1 fluctuations during 50 s as a proportion of the mean (u, i.e. CV = SD/u). Macrophyte species Location
B. peltatum
(1) (2) (3) (4) (5)
0.152 0.151 0.323 0.094 0.078
Upstream bulk Upstream bottom Stand surface Stand bulk Stand bottom
0.090b 0.018b 0.014c 0.006a 0.014a
C. cophocarpa 0.105 0.210 0.544 0.216 0.304
Water flow remained turbulent inside the plant canopy of all four species, although the absolute intensity of turbulence was markedly reduced for B. peltatum, C. cophocarpa and E. canadensis along with the profound reduction of mean velocity (Fig. 4, Table 1). The relative intensity of turbulence inside the plant canopies averaged 0.113 for S. emersum, 0.049 for E. canadensis, 0.094 for B. peltatum and 0.216 for C. cophocarpa (Table 2). The relative intensity of turbulence was not significantly different inside compared to outside the canopy of E. canadensis and S. emersum, but the relative intensity of turbulence was significantly lower inside than outside the canopy of B. peltatum and significantly higher inside than outside the canopy of C. cophocarpa. The high relative intensity of turbulence was observed both at the surface and inside the canopy of C. cophocarpa, and this may be related to the continuous vibrating movements of its flexible shoots observed in swift flow at the canopy surface (Fig. 2, Table 2). Turbulence above sediment and canopy surfaces
Water velocity continued to fluctuate with time at the reduced mean flow velocity measured 1 and 2 cm above the stream bed. The intensity of turbulence was strongly reduced close to the stream bed, particularly inside the stands (Table 1), but the relative intensity of turbulence was usually higher than in the water well above the sediment (Table 2). The mean flow velocity was reduced at the canopy surface of B. peltatum, C. cophocarpa and E. canadensis, but the intensity of turbulence was high and not significantly different from that observed at higher mean velocity upstream of the stands (Table 1). Therefore, the relative intensity of turbulence was significantly higher in the steep velocity gradient and high hydraulic roughness ã 1999 Blackwell Science Ltd, Freshwater Biology, 42, 315±328
0.011a 0.044b,c 0.051d 0.019b 0.036c
E. canadensis 0.124 0.217 0.216 0.049 0.549
0.011a 0.169a,b 0.227a,b 0.081a 0.160b
S. emersum 0.100 0.185 0.103 0.113 0.200
0.009a 0.033b 0.034a 0.009a 0.024b
associated with the canopy surface of the three species (Figs 1 & 2, Table 2). In contrast, the open canopy surface of S. emersum did not generate significant differences in the flow compared to that outside of the plant stands (Tables 1 & 2).
Discussion Flow patterns in the open water and interactions with macrophyte stands
Water flow in streams is driven by the influence of gravity on the downstream slope of the water surface. The fact that the flow patterns were almost the same for the six individual stands of each macrophyte species, although the stands differed in size and were examined in two to three streams, confirms that flow patterns are highly reproducible around and within plant canopies in streams, and that the regulatory mechanisms are well defined. This assertion is further supported by the fact that velocity patterns also resembled each other among the three stream macrophyte species forming a relatively closed canopy to the downstream flow (Figs 1±3). Mean velocity in the free water in the middle of the shallow streams was relatively constant with depth until it dropped immediately above the stream bed. In some cases, there was a separation between a relatively wide region of low velocity above the stream bed and a region of high velocity closer to the water surface. The decline of velocity close to the stream bed could be described as a linear or a logarithmic function of the distance from the sediment surface, but the linear model in most cases provided a better description than the logarithmic model. The submerged macrophyte stands will represent a
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large-scale roughness, which often extends to the water surface. The influence of macrophytes on both the mean flow velocity along stream sections and the vertical velocity profile varies with the areal cover and architecture of the plant species as well as with the height of the plant stands relative to the water depth (Larsen et al., 1990; Sand-Jensen, 1997). Macrophyte cover can range from 0% to almost 100% during midsummer among different stream sections, and at high density, it forms a much higher hydraulic resistance to the downstream flow than the stream bed and the banks (e.g. Sand-Jensen et al., 1989b; Thyssen et al., 1990). The variable acceleration and deceleration of the flow associated with the canopy of different plant species and the variable vertical distribution of plant biomass will influence the shape of the velocity profiles (Dawson & Robinson, 1984). Therefore, in shallow lowland streams with abundant plant growth the existence of logarithmic velocity profiles should be carefully checked before shear velocity and shear force at the sediment surface are estimated from velocity profiles. The intensity of turbulence, expressed as the standard deviation of the velocity fluctuations about the mean, increased linearly with the mean velocity in measurements outside the plant stands (Fig. 4). The intensity of turbulence relative to the mean velocity was usually of the same magnitude (0.10±0.15) as that reported by Morisawa (1985). However, the intensity of turbulence both in absolute and relative terms was significantly higher in the measurements outside stands of B. peltatum than in the open water surrounding the other macrophyte species (Tables 1 & 2). These difference may arise if stream sections with B. peltatum have higher slopes and higher hydraulic roughness leading to a more turbulent flow for the same mean velocity. Stream sections with B. peltatum did not have higher slopes than sections with the other species. However, stream sections with B. peltatum may tend to generate greater turbulence than stream sections dominated by other plant species because stands of B. peltatum are large and develop a strong shielding structure against the main flow, leading to strong acceleration of the deflected flow above and along the sides of the stands, and to deceleration of the flow behind the stands. Moreover, the long, trailing canopy of B. peltatum moves strongly back and forth in the vortices formed behind the stands, and thus, may transmit the turbulent eddies to
positions further downstream. The stream sections dominated by B. peltatum did experience the greatest variability in the topography of the sediment surface and the strongest erosion downstream of the stands (Sand-Jensen, 1998). These differences in relative intensity of turbulence outside of the plant stands were reproducible for the different plant species despite the fact that these were studied in two to three different streams. Hence, the dominant plant species at a particular stream section may influence the turbulent structure as a result of their characteristic morphology, architecture and hydro-elastic movements. Shielding canopy surface
All three species, B. peltatum, C. cophocarpa and E. canadensis, formed a relatively sealed canopy structure over which the water flowed. Steep vertical velocity gradients developed just above the surface of the canopy (Fig. 3), resulting in strong attenuation of the velocity and the intensity of turbulence within the plant canopy (Fig. 4). Visualization of the flow by injection of a dye confirmed this pattern, showing that most of the flow enters the stands through the upstream margin, whereas the deflected flow above and along the sides of the stands hardly enters the canopy. In fact, the water tends to flow from the canopy into the accelerated flow around the margin of the stands rather than in the opposite direction. The flow visualization showed that the main flow direction is from upstream to downstream through the vegetation, and that the turbulent eddies at the surface of the canopy are mainly carried downstream rather than being transmitted and dissipated into the canopy. The strong shielding at the surface of the canopy develops in the unidirectional flow of the shallow streams because the shoots and leaves bend over and form a high density of tissue at the canopy surface. Batrachium peltatum, C. cophocarpa and E. canadensis usually have an increased density of plant tissue close to the shoot apex because of apical growth and ramification (Sand-Jensen & Mebus, 1996). The fact that the plant tissue is positively buoyant, bends over and is further compressed by the flow above the canopy will markedly increase the tissue density at the surface relative to positions deeper into the plant canopy. In stands composed of long shoots of ã 1999 Blackwell Science Ltd, Freshwater Biology, 42, 315±328
Velocity gradients around macrophyte strands Potamogeton pectinatus in a shallow stream, more than 90% of the plant biomass was concentrated in the upper 10 cm of the stand (Bijl, Sand-Jensen & Hjermind, 1989). Steep velocity gradients at the canopy surface and little interaction between the open water and the interior of the canopy are also observed for seagrasses in unidirectional flow in tidal environments where the leaves bend over for extended periods of time (Koch, 1993). The sealed canopy structure and the low velocities inside the canopy are maintained for canopies of seagrasses and the three stream macrophytes at gradually higher velocities upstream of the stands because the canopy is further compressed against the sediment surface while the flow is deflected above the canopy surface (Fonseca et al., 1982; Gambi et al., 1990). In contrast, velocity profiles were much less steep across the canopy surface of S. emersum, forming open stands to the flow between the long strap-shaped leaves (Fig. 3). In marine seagrass beds, less steep gradients at the canopy surface and stronger interaction between the open water and the interior of the canopy are also observed in wave-dominated environments where the leaves flap back and forth, and the canopy `opens' and `closes' at regular time intervals (Koch, 1993). Strong turbulence and shear are associated with the steep vertical gradients above the canopy surfaces of B. peltatum, C. cophocarpa and E. canadensis. The relative intensity of turbulence at the canopy surface of the two former species was significantly higher than in any other position (Table 2). As a consequence of the intense turbulence, the apical leaf rosettes and the individual leaves exposed at the surface of C. cophocarpa stands were observed to vibrate rapidly and extensively in the turbulent flow. The same phenomenon is observed for apical parts of short shoots of B. peltatum in shallow water (< 20 cm), whereas the long shoots of the species in the somewhat deeper water studied here bend over, and the individual leaves moves little, because the long capillary leaf filaments are aligned parallel to the downstream flow. Flow and turbulence within canopies
Velocity and turbulence are only slightly reduced within the canopies of S. emersum, whereas the reduction is profound among the other three stream ã 1999 Blackwell Science Ltd, Freshwater Biology, 42, 315±328
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macrophytes studied (Fig. 4). Nonetheless, most measurements (> 88%) of mean velocity inside the canopies of B. peltatum, C. cophocarpa and E. canadensis exceeded 2 cm s±1, and most measurements (> 85%) of turbulence were above 0.2 cm s±1 (Table 1). The relative intensity of turbulence is highly reproducible and significantly different within the canopies of the different species, being about threefold higher for C. cophocarpa than for E. canadensis (Fig. 4, Table 2). We suggest that this difference is a result of the flexible tissue and the relatively long leaves of C. cophocarpa compared with the compact shoots with short stiff leaves of E. canadensis resulting in differences in the hydro-elastic movements of the plants and in the transmission of turbulence from the surface to deeper into the canopy. Stiff mesh-like structures are used experimentally to rescale highly turbulent flow to less turbulent flow with small eddies (Nowell & Jumars, 1984). This process has been suggested to take place in relatively stiff branch-ing structures of marine coralline algae (Anderson & Charters, 1982) and seagrasses (Koch, 1996), and it may also account for the comparatively smaller turbulence within stands of E. canadensis. Reynolds numbers (Re = L u/n) of 200±400 are estimated inside the canopies of stream macrophytes, based on assumed typical distances (L) between the individual leaves and shoots of about 1 cm (Mebus, 1993), typical flow velocities (u) of 2±4 cm s±1 and the kinematic viscosity (n) at water temperatures of 20 °C (Vogel, 1994). The low Reynolds numbers indicate that laminar flow conditions may exist. However, the flow is certainly turbulent within the plant canopies, and the evaluation is obviously not applicable, either because the uncertainty of estimates of typical lengths (movements of leaves/stems) and critical levels of transition from turbulent to laminar (roughness of the plants) are not taken into consideration, or because the flexibility of the plant canopies makes these unsuited to this theoretical evaluation since flow rates and distances to the plant surface change with time (Koch, 1994). Turbulent boundary layers were also maintained for blades of giant kelps at free stream velocities of about 2 cm s±1 and Reynolds numbers between 100 and 10 000, emphasizing that turbulent flow may continue down to relatively low Reynolds numbers (Hurd et al., 1997). Care should be taken predicting the transition from turbulent to
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laminar flow around plant surfaces from estimates of Reynolds numbers. Ecological implications of the flow patterns
Flow and turbulence inside the plant canopies are important for both the residence time of water and the exchange rate of solutes between the leaf surfaces and the surrounding water. Considering that most stands were between 1 and 2 m long, and the longest stands in streams are usually less than 5 m, the average water residence time within the stands will be short (less than a few minutes), and no major depletion of inorganic nutrients and carbon or accumulation of oxygen can take place within the plant canopies relative to the surrounding water as a result of photosynthesis and plant uptake (SandJensen & Mebus, 1996). Turbulence can also ensure an efficient exchange of solutes between the plant surfaces and the surrounding water, and thereby, counteract the tendency of photosynthesis and growth becoming limited by the supply rate of solutes from the water. For a given light intensity and water chemistry, the intensity of photosynthesis of stream macrophytes is usually saturated in laboratory experiments at water velocities of about 0.1±1.0 cm s±1 (Westlake, 1967; Madsen & Sùndergaard, 1983). Since the water velocity usually exceeded 2 cm s±1 inside the canopies studied here, it is not likely that photosynthetic rates are constrained by the reduced flow rates in the canopies relative to the free water. Substantial flow and turbulence within the canopies of stream plants will also stimulate the metabolism of bacteria, microalgae and invertebrates in the epiphytic community on the plant surfaces. Microbial degradation of dissolved organic matter and nitrification under the consumption of oxygen are important processes which can take place at high rates in the epiphytic communities on stream macrophytes based on the supply of organic matter, ammonium and oxygen dissolved in the water (Jeppesen et al., 1987; Sand-Jensen, Borg & Jeppesen, 1989a). Passive filterfeeding invertebrates among simuliids and chironomids are abundant on the surfaces of stream macrophytes (Iversen et al., 1985), and they can probably be constrained by the reduced supply rate of particles at low velocity and turbulence inside dense plant canopies (e.g. Chance & Craig, 1986; Lacoursiere &
Craig, 1993). Simuliid larvae are often particularly abundant on leaves of S. emersum (Iversen et al., 1985), where feeding may benefit from the unimpeded velocities and turbulence transmitted through the open plant stands. The implications of differences between relatively `closed' or `open' canopies of plant species are difficult to evaluate for the plants themselves because of the complexity of advantages and disadvantages changing gradually with the velocity in the open stream. The relatively dense canopies will have strong shear forces concentrated at the canopy surface where most of community photosynthesis will take place because of high light intensity and high tissue density (Bijl et al., 1989). The intense turbulence may physically damage the apical tissue and enhance respiratory costs because of leaf flapping (Madsen, Enevoldsen & Jùrgensen, 1993), while tissue deeper into the canopy will be physically protected and nutrient-rich fine particles can accumulate on shielded sediment surfaces (Sand-Jensen & Madsen, 1992; Sand-Jensen, 1998). The pressure drag will be high on the plant stands because of the profound pressure drop from the upstream front to the downstream front. On the other hand, the `open' canopies, like those of S. emersum, will have a better penetration of light into the canopy (Jùrgensen, 1990), high near-bed velocities (Fig. 4), and no major accumulation of organic matter and fine mineral particles on the sediments (Sand-Jensen, 1998). Pressure drag will be lower on the open plant stands, and the combined pressure and shear drag will be more evenly distributed among the individual leaves and shoots. The velocity range and the concentrations of inorganic carbon will determine the balance between the metabolic advantages (i.e. thinner diffusive boundary layers) and the mechanical disadvantages of enhanced turbulence. To address these aspects, the separate plant stands formed by clonal growth from single colonizing shoots are relevant functional units in the physical interactions with the environment and in the dynamics of plant recruitment, growth and mortality (Andersen & Andersen, 1990). We demonstrate here that flow patterns are highly reproducible for the separate stands of different species of stream macrophytes of characteristic morphology. Studies showing the interactions between the form of individual stands, and the hydrology and channel form of entire stream reaches are in progress. ã 1999 Blackwell Science Ltd, Freshwater Biology, 42, 315±328
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