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Ecology |
Plant Biology and, Ecology and Systematics, Cornell University, Ithaca, New York 14853-5908 USA; and Physical Ecology Laboratory, University of Northern British Columbia, Prince George, British Columbia, Canada V2N 4Z9
Received for publication August 7, 2001. Accepted for publication February 19, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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4 x 10–4 m/s; K 10–4 m2/s) compared to theoretical values that do not consider plant canopies. These findings support the concept that eelgrass canopies modify the fluid dynamics (i.e., reduced turbulent mixing) within their canopies. These results indicate that 1000–10 000 Z. marina pollen are required to pollinate a single flower. Similarly, it was estimated that under some conditions, the probability of particle impaction on eelgrass vegetation approaches certainty. These results provide insight into the evolution of filamentous pollen and submarine pollination, as well as dispersal and other mass transport phenomena within macrophyte canopies.
Key Words: canopy flow • dispersion • mixing • particle capture • particle transport • seagrass • submarine pollination • Zosteraceae • Zostera marina
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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) and, in some cases, the maintenance of local populations (Keddy and Patriquin, 1978
; see Philbrick and Les, 1996
). Like other flowering plants, the pollen of seagrasses must reach the stigmas of female flowers to pollinate (Faegri and van der Pijl, 1979
; Proctor, Yeo, and Lack, 1996
). However, unlike most flowering plants, the five families of seagrasses pollinate abiotically (i.e., without an animal vector) in the water (i.e., hydrophily), which is a condition that occurs among 2.7% of angiosperm families (Renner and Ricklefs, 1995
; Ackerman, 2000
). Not surprisingly, this functional group of 50 species, which occur within three clades (Les, Cleland, and Waycott, 1997
), have a number of characteristics that are associated and have evolved convergently with hydrophily (Pettit, 1984; McConchie and Knox, 1989
; Ackerman, 1995
). Significantly, seagrass pollen is filiform or functionally filiform in shape and is dispersed, transported, and captured underwater (Schwanitz, 1967
; den Hartog, 1970; Ackerman, 1995
). Since seagrass pollen is nonmotile, unlike the propagules of many algae (e.g., Reed, Amsler, and Ebeling, 1992
), seagrasses must rely on water currents for submarine pollination (Pettitt, 1984
; Ackerman, 2000
). The successful release, transport, and capture of pollen will, therefore, be influenced by the interactions between water currents and the seagrass canopy (Okubo, Ackerman, and Swaney, 2002
).
The study of pollination of seagrasses is limited in terms of taxa investigated and mechanisms examined (see review in Ackerman, 2000
). For example, the sole description of pollen capture at the floral scale exists for the north-temperate species Zostera marina L. (Potamogetonales; Zosteraceae; eelgrass) in which local flows around reproductive organs, principally the velocity gradient created by emergent flowers, causes filamentous pollen to rotate (Ackerman, 1997a
, 1997b
). This behavior increases the probability of pollination relative to "ancestral" spherical pollen (den Hartog, 1970), which would not cross through streamlines except due to diffusion (see Shimeta and Jumars, 1991
). Alternatively, the genetic implications of pollen movement at larger scales have been examined in a number of taxa across a wider geographic range (Alberte et al., 1994
; Ruckleshaus, 1995
; Waycott and Sampson, 1997
; Reusch, 2000
; Procaccini et al., 2001
), and there have also been some reports of pollen movement in the field (Verduin, Walker, and Kuo, 1996
; Smith, 2000
). Whereas these studies speak to the consequences of pollen movement in the field, the mechanics of pollen transport within the canopy flow environment remains largely unknown. The present study was, therefore, motivated by the need to examine the transport and capture of pollen within the canopy flow environment. The examination of these issues within an eelgrass canopy should provide valuable insight into submarine pollination as well as pelagic dispersals and recruitment that occur within macrophyte beds.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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5 m below mean low water within an eelgrass bed in Great Harbor, Woods Hole, Massachusetts, USA (Fig. 1; x-direction = nominally on/off shore flow; y-direction = nominally long shore flow; 10 m shoreward of the deep margin of the bed; Dennison and Alberte, 1982
, 1985
; Ackerman, 1986
). The sensor was mounted on a movable arm 40 cm from a vertical support and could be adjusted to a desired canopy height. Shielded cable from the sensor was carefully laid on the seafloor a distance of 8 m, where it was coiled several times and led to a boat on the surface.
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4 h before predicted low tide; tides are semidiurnal with a tidal range of <1 m; National Ocean Survey, 1985), beginning at a height z = 1.0 m above the seafloor (top of the canopy). At 6-min intervals, the deployment arm was carefully lowered 20 cm by a SCUBA diver. Care was taken to minimize the physical disturbances around the deployment area during recordings. Data were recorded on a two-channel portable DC chart recorder. A 250-s interval from the middle portion of each sampling period was digitized (1 datum/s) and used in the subsequent analysis.
A sample of ten 25 x 25 cm quadrats was used to estimate the canopy height (maximum leaf length per shoot) and shoot density near the meter. There was a mean (± SE) of 91 ± 7 shoots/m2 (N = 58) with a canopy height of 100 ± 3 cm. The mean height of reproductive shoots was similar (105 ± 6 cm; N = 14; Ackerman, 1986
). These observations were similar to previous surveys of this area (e.g., Dennison and Alberte, 1982
; Ackerman, 1985
).
Particle release and capture experiment
Because of the difficulty of acquiring and handling large quantities of Zostera marina pollen without distorting their shape or creating clumps, physical models were made from 1-kg test Stren fluorescent monofilament fishing line (E. I. Dupont de Nemours, Wilmington, Delaware, USA). The monofilament was cut into submillimeter pieces to serve as models of the hypothetical ancestral "spherical" pollen and 3–5 mm lengths to serve as models of Z. marina pollen. The 3–5 mm length was within the reported range for Z. marina and other seagrass pollen (see Ducker, Pettit, and Knox, 1978
; Pettitt, 1984
; Ackerman, 1997b
), but the diameter (1.524 x 10–4 m) and density (
1.17 x 103 kg/m3) were greater than Z. marina pollen (7.5 x 10–6 m diameter, Schwanitz, 1967
; 1.07 x 103 kg/m3, Ackerman, 1997b
). In addition, the aspect ratio of the spherical models was greater than one (the maximum length/diameter 
6.66); however the consistency of models, in terms of and diameter, provided an opportunity to evaluate the effect of filiform shape on particle dispersal and capture. The settling or terminal velocity (ws) of models was examined in a cylindrical vessel (1000 mL; 8 cm diameter) to address the differences between models and pollen. The duration of fall over an 8 cm distance was measured near the top, middle, and bottom of the vessel for each particle examined. Given the differences in presented above, it was expected that the ws of models would be greater than that of pollen, thus limiting the horizontal dispersal of models (see below). Consequently, Stren particles can only be considered as first-order approximations to pollen (see below), but they do provide an indication of the pattern of particle dispersal in the eelgrass canopy.
Three experiments were conducted over two field seasons: the first on 10 August 1986 during ebb tide (4 h before predicted low tide), the second on 31 July 1987 during slack water (30 min before predicted low tide), and the third on 2 August 1987 during ebb tide (4 h before predicted low tide; National Ocean Survey, 1985, 1986). In each experiment, approximately equal numbers of different colored filamentous models (3–5 mm) were released at the top (z = 100 cm) and middle (z = 60 cm) of the canopy to determine the effect of position of release on particle transport and capture (Fig. 2). The effect of model shape was also examined in the two 1987 experiments where approximately one-third the number of spherical particles were released at both heights in the canopy. Particles were released over a period of 10–15 s, by a diver, from modified 60-cm3 syringes fixed at the top and middle of the canopy, 1.5 m upstream from the center of a capture apparatus (Fig. 2).
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; Fitt and McCartney, 1986
). The poles, which were separated by 0.25 m, were arranged in five rows staggered 0.5 m apart to limit the overlap among rows and to minimize the fluid dynamic disruption of the eelgrass canopy (Fig. 2). The mean spacing among Z. marina reproductive shoots in this region of the canopy was 60 cm and the mean nearest neighbors were 17 cm apart (Ackerman, 1985
). Care was taken to disentangle plants from the poles and connecting cord to maintain the geometry of the bed. The capture apparatus was deployed in the same region of the bed where the velocity was measured and arranged with their coated surfaces perpendicular to the prevailing currents and the shoreline (Figs. 1 and 2). The prevailing currents above the canopy were approximately parallel to shore as indicated by the transport of material in the water. In the first experiment, two microscope slides (2.54 x 7.62 cm) were fixed at heights of 0, 20, 40, 60, 80, 100, and 120 cm on each pole. In the last two experiments, two plexiglass plates (5 x 120 x 0.5 cm) were hinged together and fixed to each pole. The slides and plates provided a definable surface on which to capture models and thus track the movement of particles within the canopy, but they were not flexible or similar in shape to Z. marina floral organs. At the end of an experiment (15 min in duration), the coated surfaces were either bagged (slides) or hinged closed (plates) for transport to the laboratory, where particles were identified and counted under UV illumination.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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) was estimated to range between 10 and 15 cm/s in a west and southwest direction (i.e., nominal long shore direction). The general features of the flow above the canopy did not change over the duration of the recordings (45 min). During this time, the canopy was more or less erect, although some monami ("flagging" or "waving" of the leaves) was also observed. The direction of flow (bearing from north) varied between 273° ± 4° at the top of the canopy and 211° ± 5° at a height of 5 cm (N = 250; 
t = 1 s; Fig. 3A). The direction varied little between z = 20 and 60 cm. In general, the longshore velocity (i.e., v) was greater in magnitude than the on/off shore velocity (i.e., u) (Fig. 3B). Four features were observed in the canopy flow: (1) a reduction in velocity at the top of the canopy; (2) a fairly uniform velocity in the middle of the canopy between z = 40 and 80 cm; (3) a slight increase in velocity at z = 20 cm; and (4) a decrease in velocity towards the seafloor (Fig. 3B). The relative magnitude of the variations in velocity in these four regions is presented as the turbulence intensity TI (TI = 100URMS/
, where URMS is the root mean square of the difference between the instantaneous speed and mean speed, ), which appears to complement the pattern in canopy velocity (Fig. 3C).
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5% at any location in the canopy (Fig. 4A). Stronger signals were seen on the 1987 dates, where between 20% and 30% of the particles were captured within the first set of collectors 1.5 m from the position of release. The decline with distance was very rapid on 31 July 1987 and reached 2% within the first 2 m downstream (Fig. 4B), whereas the captures on 2 August 1987 did not reach that level (i.e., 2%) until 3 m downstream of the release (Fig. 4C). The results from the different positions of release within the canopy were somewhat similar in pattern to what has been described from all particles, although some subtle differences were also evident. For example, on 10 August 1986 the decline with distance was smoother for mid-releases (Fig. 4D and G), and the captures from top releases were of lower magnitude that those from the mid-canopy releases (Fig. 4E vs. H and 4F vs. I). The pattern of particle captures declined with height in the canopy, although there was a bulge or plume-like feature evident between 80 cm and 100 cm, especially on 10 August 1986 and 2 August 1987 (Fig. 4A and C). This bulge-like pattern was evident several meters downstream from the position of release on 31 July 1987 (Fig. 4B). Importantly, it should be noted that particles were captured at 120 cm, which was 20 cm above the position of release and the top of the canopy. This represented a 20–60 cm upward movement for particles released at the top (z = 100 cm) and mid-canopy releases (z = 60 cm), respectively.
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5% on 31 July 1987 and to a lessor extent on 2 August 1987, where the decline extended 0.5 m downstream compared to filamentous particles (Fig. 5E–F). The bulge-like pattern of capture near the top of the canopy was evident for filaments on 10 August 1986 and 2 August 1987 but not on 31 July 1987 (Fig. 5A–C). However, this pattern was evident for spheres on 31 July 1987 at mid-canopy heights and on 2 August 1987 near the top of the canopy (Fig. 5E–F). Significantly, the settling velocity (ws) for filamentous particles in the laboratory was approximately twice that of spherical particles (e.g., 4.7 ± 0.1 mm/s [mean ± SE, N = 10] and 2.5 ± 0.1 mm/s, respectively).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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who showed that the velocity profile of the canopy matched the vegetative profile of the canopy (also see Van Keulen and Borowitzka, 2000; Okubo, Ackerman, and Swaney, 2002
). For example, the relative increase in velocity that was observed just below the maximum leaf area in the canopy (e.g., 30 cm height; Dennison, 1979
) may have some biological consequences, as the first flowering branches (rhipidia) of reproductive shoots were also found above this region (Ackerman, 1986
). Such canopy flow effects have been presented in velocity profiles of seagrasses (Fonseca and Kenworthy, 1987
; Gambi, Nowell, and Jumars, 1990
; Van Keulen and Borowitzka, 2000
; Okubo, Ackerman, and Swaney, 2002
) and terrestrial plants (e.g., Raupach, Antonia, and Rajagopalan, 1991
; Finnigan, 2000
). The vegetative profile of the canopy also affects the turbulence in the canopy by the cascade-like splitting of large canopy-scale eddies as they enter the canopy, by generating smaller-scale eddies through the turbulent shear caused by shoots and leaves, and through the hydroelastic-waving motion of the plants (Raupach, 1989
; Raupach, Antonia, and Rajagopalan, 1991
; Nepf, 1999
; Finnigan, 2000
). Spectral analysis of the velocity fluctuations in the eelgrass canopy revealed that the splitting of large eddies was less important than the monami (aquatic plant wave; Ackerman and Okubo, 1993
; cf. Honami or cereal wind wave; Inoue, 1955
) or flagging motions that were only observed during periods of moderate to high unidirectional flow (Ackerman, 1986
). The spatial and temporal resolution of the current data set and the lack of simultaneous observations at different canopy heights do not allow for further considerations of the canopy flow interaction. Fortunately, this is an area of active research (Nepf, 1999
; see reviews in Hurd, 2000
; Okubo, Ackerman, and Swaney, 2002
).
Particle dispersion and capture
It is apparent that the 31 July 1987 results differ from the other two experiments in a number of important ways, which influence the interpretation of these data. Firstly, the absolute capture rate of 0.295% was lower than the results obtained on 2 August 1987 even though more particles were released on 31 July 1987 (Table 1). Secondly, there were relatively more spherical particles captured than filamentous ones (P < 0.001; Table 1), and both the capture of filamentous and spherical particles declined rapidly with distance from release (Figs. 4 and 5). Thirdly, the bulge-like pattern near the top of the canopy was not seen for filamentous particles but was observed for spherical particles lower in the canopy (Fig. 5). It seems likely that many of these differences from the other experiment results were due to the different fluid dynamic conditions in the canopy, specifically the slack water conditions 30 min before the predicted low tide. This should be contrasted with more dynamic flow conditions during ebb tide (4 h before the predicted low tide) when the velocity profile and the 10 August 1986 and 2 August 1987 results were obtained. The lower flow rates during slack water probably led to the lower capture rates and different patterns of capture observed on 31 July 1986 (Table 1). In this case, the majority of filamentous particles had settled out of the water column prior to reaching the capture network, which is likely given a horizontal dispersal of 213 cm predicted for a filamentous particles (ws = 4.7 mm/s) released at the top of the canopy at a relatively slow ambient velocity of 1 cm/s. The capture of the remaining particles declined rapidly in the canopy as seen in Figs. 4 and 5. This explains why relatively more filamentous particles were captured closer to the position of release (Fig. 4). Moreover, this also explains why the rate of capture for filamentous particles, normalized for collector area, was lower for the 31 July 1987 experiment compared to the 10 August 1986 and 2 August 1987 experiments (Table 1).
Alternatively, the 2 August 1987 results can be considered to be representative of particle transport and capture during periods of moderate flow within the Z. marina canopy. Fortunately, the 10 August 1986 results, though relatively limited in the number of observations, do show similar patterns in terms of capture rates and distributions. The 2 August 1987 results indicate that significantly more particles were captured from those released at the top of the canopy (P < 0.001) and significantly more filamentous particles were captured than spherical ones (P < 0.001; Table 1). The former result seems likely given that the particles released at the top of the canopy would remain in the water column longer than those released closer to the seafloor (i.e., at mid-canopy). Ecologically, however, the spatial allocation and distribution of pollen grains within anthers in seagrass canopies remain unknown. The greater capture rate of filamentous particles may be related to the motion of these particles in the canopy flow (see below). It is apparent, therefore, that ambient flow conditions, specifically tidal conditions, have important effects on the transport and capture of particles in the seagrass canopy.
Horizontal and vertical mixing in the canopy
These results provide an opportunity to estimate the vertical and horizontal dispersion of particles within the Z. marina canopy and to verify independently the mixing rates estimated from fluid dynamic observations (Ackerman and Okubo, 1993
). A three-dimensional Gaussian model of diffusion in a patch (i.e., an instantaneous point release) is given by
). Under well-mixed conditions, the variation in the lateral direction can be neglected and Eq. 1 reduces to 
2 = 2Kt (e.g., Okubo, 1971
, 1980
; Okubo, Ackerman, and Swaney, 2002
). K was determined from the product of –1/4t (t = 900 s) times the least squares slope of the log-transformed capture data with respect to height plotted against the square of canopy height (regression statistics provided in Table 2).
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4 x 10–4 m2/s; Ackerman and Okubo, 1993
). The values reported here for vertical eddy diffusivity are less that the horizontal eddy viscosities (1 x 10–4–10–3 m2/s; Worcester, 1995
) estimated in a shallow eelgrass bed under slow flow conditions (<5 cm/s), which is consistent with expectations. Moreover, these values were also consistent with measurements from the deep ocean (e.g., 5 x 10–4 m2/s; Okubo, 1971
) and from crop plants (examples of other flows in vegetation), where K ranged from 1 x 10–4 to 1 x 10–1 m2/s, depending on canopy height and species (see review in Grace, 1977
). Interestingly, the lowest values for K in both aquatic and terrestrial canopies were more than an order of magnitude greater than the molecular viscosity for the media.
The dispersion in the horizontal plane (i.e., with distance) suggested a log-linear pattern of diffusion, which has been modeled using the Joseph-Sendner model given by
2 = 6P2t2 (Joseph and Sendner, 1962
; Okubo, 1971
, 1980
). In this case, P was determined from the product of –1/t times the least squares slope of the log-transformed capture data with respect to distance plotted against distance (regression statistics provided in Table 2). The estimated diffusion velocity (P) for spherical particles was greater than the P estimated for filamentous particles (8 ± 5 x 10–4 m/s vs. 3.6 ± 0.4 x 10–4 m/s, respectively; Table 2). In addition, the diffusion velocity of filamentous and spherical particles released at the top of the canopy was higher than those released at mid-canopy. Considering only the filamentous particles, this indicates that the horizontal mixing is on the order of 0.04 cm/s, which is well below the P of 0.5–1.5 cm/s measured in the open ocean and closer to the values for deep ocean layer (Okubo, 1980
). These low values are reasonable given the small t (900 s) compared to the oceanic studies, in which t is measured in days, although they are less easily interpreted relative to the more traditional Gaussian models. It should be noted that these calculations are among the first quantitative estimates of vertical and horizontal mixing of particles in a macrophyte canopy.
Implications for submarine pollination and other dispersals
The issue of similitude between experimental particles and pollen or other propagules is an important one that addresses the realism of the physical model. This can be understood through an evaluation of the dynamics of the system given in part by the Reynolds number (Re = Ul/
, where l is the length, U is the velocity, and is the kinematic viscosity). In this case, U is the settling velocity (ws), which can be calculated for particles and pollen using a shape-corrected Stokes' equation given by
p and f are the density of the particle and the fluid, respectively, µ is the dynamic viscosity, and 
is the coefficient of form resistance depending on particle orientation (cf. Vogel 1981
). The ws estimated from Eq. 5 were consistent with those measured experimentally for filamentous and spherical particles. Specifically, ws for filamentous particles ranged between 3.5 and 5.5 mm/s (based on = 0.48 and 0.304, respectively for a circular cylinder with aspect ratio of 50 : 1) vs. 4.7 ± 0.1 mm/s measured empirically, whereas ws for spherical particles ranged between 1.7 and 2.2 mm/s (based on = 0.993 and 0.772, respectively for a prolate spheroid with aspect ration 4 : 1) vs. 2.5 ± 0.1 mm/s measured empirically ( values from Table 13.1 in Vogel, 1981
). The ws of eelgrass pollen was estimated between 2.7 and 4.3 µm/s, given d = 7.5 µm (Schwanitz, 1967
) and p = 1.07 x 103 kg/m3 (Ackerman, 1997b
). The Re calculated for particles and pollen from Eq. 5 were at or just below unity (Re = 1.2 for spherical particles; 1.4 for filamentous particles; 1 x 10–2 for pollen), indicating that the conditions were relatively viscous, although well beyond the scale where Brownian motion is relevant (Russel, 1981
). This would imply that the motion of particles and pollen would likely be similar within the canopy flow (Vogel, 1981
; Niklas, 1992
). Notwithstanding these dynamic similarities, filamentous particles do settle faster than pollen and were captured on flat-adhesive surfaces rather than on eelgrass flowers and leaves. For these reasons, the experiments should be considered first-order approximations to pollination as they provide a means of tracking the dispersion and capture of pollen and other particles through the eelgrass canopy.
Based on the particle capture data in Fig. 4, pollen and other dispersing bodies (diaspores, larvae, etc.) released at or near the top of the canopy would be entrained into faster moving fluid above the canopy and thus travel farther than those released within the canopy. Once in the canopy, where both vertical and horizontal mixing are small, the transport of pollen and other bodies would be relatively limited. A measure of this transport, the "dispersal distance" was estimated from the integration of the distance a particle is advected horizontally at a given height interval given the velocity profile in Fig. 3. For example, eelgrass pollen with ws 3 µm/s would disperse 1.23 km under the canopy flow observed in this study (Fig. 3). Dispersal distance, or the time to settle to the bottom, is inversely proportional to ws. Given this prediction and the higher K and P for particles released at the top of the canopy, it would be reasonable to investigate whether seagrasses with tall canopies allocate more pollen to anthers near the top of their canopies relative to anthers lower within their canopies.
Submarine pollination (i.e., particle capture rates) was assessed through an analysis of the capture data within the region where flowers were normally found in the canopy (30 z 100 cm; Ackerman, 1986
). On 2 August 1987, 336 particles were captured in this region from the 25 704 particles released at the top of the canopy (Fig. 4), which, when normalized by length of the capture apparatus (i.e., 2 x 1 m; Fig. 2), provides a capture rate of 6.54 x 10–3 captures per meter of canopy (note that a 1 m width is considered). This rate was observed on collectors with an area of 7.70 x 103 cm2 per meter of canopy (i.e., 1.54 x 104 cm2 in 2 m). On average, there were 5 reproductive shoots/m2 (Ackerman, 1985
), 5 rhipidia per reproductive shoot (Ackerman, 1986
), 0.1 flowering inflorescences per rhipidium (Ackerman, personal observation), 8.5 carpellate flowers per inflorescence, or 21.25 flowers per meter of canopy. Each flower has a collector area of 0.01 cm2 (Ackerman, 1986
) leading to a floral collector area of 0.2125 cm2 per meter of canopy (i.e., 21.25 flowers/m x 0.01 cm2/flower). One would predict an equivalent capture rate by flowers of 1.80 x 10–7 captures per meter of canopy (i.e., 6.54 x 10–3 captures/m x 0.2125 collector cm2/m ÷ 7.70 x 10–3 collector cm2/m). Given the dispersal distance for pollen with an estimated ws of 3 µm/s from 30 z 100 cm within the canopy flow of Fig. 3 and the floral capture rate determined above, pollination is estimated at a rate of 1.83 x 10–4 captures (i.e., 1.01 x 103 m x 1.80 x 10–7 captures/m). In other words, 5.5 x 103 pollen would have to be released to successfully pollinate one flower (or 6.2 x 104 pollen given the 31 July 1987 results). These estimates of pollen capture are consistent with the pollen to ovule ratio of Z. marina (10 000 : 1; Ackerman, 1993
), which are one to several orders of magnitude less than what is reported for wind-pollinated species (e.g., Faegri and Van der Pijl, 1979
; Niklas, 1992
). Moreover, these estimates provide a mechanism that supports the high outcrossing rates determined through genetic analysis of Z. marina and other seagrasses (Ruckleshaus, 1995
; Waycott and Sampson, 1997
; Reusch, 2000
).
A similar undertaking for the recruitment of diaspores, larvae, and other propagules involves their initial contact with eelgrass leaves and shoots (Eckman, 1987
; Borowitzka and Lethbridge, 1989
; Newell et al., 1991
). In this case, a capture rate of 3.1 x 10–2 captures per meter is expected within the canopy given a leaf area index of 2 m2/m (Dennison and Alberte, 1982
) and a capture rate of 5.9 x 10–3 captures per meter of canopy (see above and Table 1). A propagule such as a dispersing postlaval mussel, which are common in eelgrass beds, released at the top of the canopy with ws 1 mm/s (reported ws range between 0.3 and 3 mm/s; Lane, Beaumont, and Hunter, 1985
) would disperse 37 m and be captured at a rate of 0.57 captures (i.e., 2 propagules released for one successful capture). It should be noted that since Z. marina pollen does not adhere to its own vegetation, it could be re-entrained in the canopy flow. Alternatively, bivalve recruitment on blades can be considerable (>90 spat/cm of blade; Newell et al., 1991
). Under some conditions (e.g., low ws), the capture rate approaches 1, but impaction on Z. marina vegetation is only the first phase in the recruitment process of many organisms (e.g., Eckman, 1987
). These calculations indicate that macrophyte canopies have the potential to affect strongly the dispersal and recruitment of pelagic and benthic organisms.
These results are also germane to the issue of the evolution of filiform seagrass pollen from the "ancestral" spherical state, although the dimensions (i.e., radius) and mass are unknown (den Hartog, 1970). If pollen mass has been conserved evolutionarily, then the ancestral spherical pollen would have been 63 µm in diameter and would, therefore, settle at ten times the rate of filamentous pollen. Under moderate flow conditions, filamentous pollen would disperse farther than spherical pollen and may also be entrained into faster moving eddies because they are longer than the smallest eddies (1 mm; Mitchell, Okubo, and Furman, 1985
). Importantly, there are also differences in the capture mechanism between spherical and filamentous particles. The reductions in mixing and turbulence described above indicate that viscous forces (i.e., laminar flow) may be relatively more important on the scale of "collectors" (slides, plates, flowers, and inflorescences). In this case, filamentous models would rotate in and cross through the velocity gradient around collectors in a similar manner that has been found for Z. marina pollen in a flume (Ackerman, 1997a
, 1997b
). Spherical particles also rotate but do not cross through the velocity gradients and therefore need to be directly upstream from a collector to be captured. Thus, one could envision a selective advantage for filamentous pollen given the greater expectation for capture (i.e., submarine pollination) within the canopy and explain the differences in particle capture seen here.
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FOOTNOTES |
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2 Phone: 250-960-5839; FAX: 250-960-5539; ackerman{at}unbc.ca
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LITERATURE CITED |
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Ackerman J. D. 1986 Mechanistic implications for pollination in the marine angiosperm Zostera marina. Aquatic Botany 24: 343-353[CrossRef]
Ackerman J. D. 1993 Pollen germination and pollen tube growth in the marine angiosperm, Zostera marina L. Aquatic Botany 46: 189-202[CrossRef]
Ackerman J. D. 1995 Convergence of filiform pollen morphologies in seagrasses: functional mechanisms. Evolutionary Ecology 9: 139-153[CrossRef][ISI]
Ackerman J. D. 1997a Submarine pollination in the marine angiosperm, Zostera marina. I. The influence of floral morphology on fluid flow. American Journal of Botany 84: 1099-1109[Abstract]
Ackerman J. D. 1997b Submarine pollination in the marine angiosperm, Zostera marina. II. Pollen transport in flow fields and capture by stigmas. American Journal of Botany 84: 1110-1119[Abstract]
Ackerman J. D. 2000 Abiotic pollen and pollination: ecological, functional, and evolutionary perspectives. Plant Systematics and Evolution 222: 167-185[CrossRef][ISI]
Ackerman J. D. A. Okubo 1993 Reduced mixing in a marine macrophyte canopy. Functional Ecology 7: 305-309[CrossRef][ISI]
Alberte R. S. G. K. Suba G. Procaccini R. C. Zimmerman S. R Fain 1994 Assessment of genetic diversity of seagrass populations using DNA fingerprinting: implications for population stability and management. Proceedings of the National Academy of Sciences USA 91: 1049-1053
Borowitzka M. A. R. C. Lethbridge 1989 Seagrass epiphytes. In A. W. D. Larkum, A. J. McComb, and S. A. Shepherd [eds.], Biology of seagrasses, 458–499. Elsevier, Amsterdam, The Netherlands
den Hartog C. 1970 The seagrasses of the world. North-Holland Publishing, Amsterdam, The Netherlands
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