BULLETIN OF MARINE SCIENCE, 71(3): 1269–1277, 2002

ESTIMATES OF POLLEN DISPERSAL AND CAPTURE WITHIN ANTARCTICA (LABILL.) SONDER AND ASCHERS. EX ASCHERS. MEADOWS

Jennifer J. Verduin, Jan O. Backhaus and Diana I. Walker

ABSTRACT The hydrodynamic micro-climate created within meadows and their immedi- ate surroundings has implications for pollen transport and settling within seagrass mead- ows. Quantitative estimates of and meadow architecture, flowering and pollination biology of indicated that the position of male and female flowers on a shoot coincided with areas of high turbulence in the canopy. A model on pollen dispersal and capture in an Amphibolis meadow was derived from both plant structure (meadow architecture) and in situ three dimensional velocity measurements. On a purely hydrodynamic basis, high pollen capture is expected in a more energetic and turbulent environment. However, the model results showed that the combination of flow dynamics and plant structure, i.e., plant-flow interactions, are more favorable for pollen capture in an area of less favorable conditions, e.g., less energetic. This suggests that, as a response to their hydrodynamic environment, the same of seagrass may have differing meadow and plant structures, such as different shoot length and shoot density.

Seagrass species in shallow coastal environments may rely, to varying degrees, on vegetative and/or sexual reproduction for the continuation of existing meadows (Orth et al., 1994). Information on pollen dispersal, germination and seedling success would prove useful for an understanding of dispersal strategies within . However, mecha- nisms and patterns of propagule dispersal such as seeds, spores or pollen in marine (Santelices, 1990; Kendrick and Walker, 1991) and most seagrass species (Orth et al., 1994; Ruckelshaus, 1996) are not well known. Seed dispersal of Zostera marina was studied by Orth et al. (1994) in unvegetated areas, however, this study viewed dispersal of propagules in the free water column and not from the source of pollen/propagule re- lease, i.e., excluding the interactions of plants with the flow. Many plant-flow studies have been carried out in uni-directional flume generated flows (e.g., Fonseca et al., 1982, 1983; Fonseca and Fisher, 1986, Fonseca and Kenworthy, 1987; Gambi,1990) or tidal flows (Fonseca and Cahalan, 1992; Ackerman, 1983, 1986). The uni-directional character of the flows allows turbulent boundary layers to build up. However, in wave-dominated environments the flow is highly dynamic and oscillatory (Verduin and Backhaus, 2000) resulting in a different flow character (Koch and Gust 1999). In comparison to uni-directional or tidal flow the time scale of wave induced motions is much shorter, therefore turbulent boundary layers have less time to develop and are expected to be much thinner (Denny, 1988). This may be of importance in terms of interpreting turbulent boundary layer dynamics in the presence of plants and its impli- cations in highly dynamic coastal environments. This study describes pollen dispersal from their point of release within Amphibolis antarctica meadows. A model, which utilizes field measurements as input data, was de- veloped. This model was applied to submerged A. antarctica meadows at two sites of different hydrodynamic, and highly energetic wave regimes off the coast of Western Aus- tralia.

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MATERIALS AND METHODS

STUDY SITES.—Shoalwater Bay [SWB] (32°16'S; 115°41'E) and Warnbro Sound [WS] (32° 19'S; 115° 43'E), are part of the Shoalwater Islands Marine Park off the coast of Western Austra- lia. Meadows of Amphibolis and Posidonia species occur in these bays. The A. antarctica mead- ows in WS are more patchy than in SWB. The dominant period of swell dynamics in SWB and WS is ca 15 s. Tides within SWB and WS are mainly diurnal with small semi-diurnal compo- nents. Diurnal tides are relatively low in the Indian Ocean and the existence of an amphidromic point off Perth is the reason for very small semi-diurnal tides. Maximum tidal currents are very low, inducing flows of around 1 cm/s (Steedman and Craig, 1983). In situ velocity measurements were taken within, adjacent and above A. antarctica meadows in SWB and WS. Measurements were carried out at a depth of 2 m below low water at the Shoalwater Bay site and 5 m below low water at the Warnbro Sound site. HYDRODYNAMIC MEASUREMENTS.—In situ water flow measurements were made in March, April and November 1995, using a three dimensional Acoustic Doppler Velocimeter (ADV) (Sontek 1995). The ADV was deployed vertically, attached to a pole with a sliding holder keeping the instrument 40 cm from the pole to ensure a true vertical position and to minimize the influence of the pole on the flow. Within canopy profiles of 11 points were obtained by sliding the ADV up- wards in 5 cm increments starting 10 cm above the seabed to a total distance of 80 cm above the seabed. Little vertical variation was apparent over bare sand, thus a coarser sampling rate was adopted (15 cm increments). Horizontally spaced measurements were made at positions within the canopy, the canopy edge and bare sand ca 1.5 to 2 m apart. Five to ten minute records at a sampling rate of 25 Hz were obtained for each of the positions (Verduin and Backhaus, 2000). The velocity data were used to compute vertical profiles of mean amplitude of flow velocities. Turbulent diffusion coefficients were computed from profiles of mean flow amplitudes according to a diagnostic turbulence closure scheme (Kochergin,1987). For unstratified, i.e., neutral condi- tions, the scheme estimates vertical turbulent diffusion coefficients from vertical velocity shear by assuming a constant mixing length. A mixing length of 50 cm was prescribed in the model indicat- ing the approximate vertical scale of a meadow (from field observations, Verduin, 1996). Horizon- tal transport over a given time results in a Lagrangian excursion of a particle (Tennekes and Lumley, 1972). Average advective excursions during a wave period were estimated by multiplying mean amplitudes of flow velocities with the time of half a wave period (Verduin and Backhaus, 2000). BIOLOGICAL MEASUREMENTS.—Shoots of A. antarctica were collected haphazardly from both field sites in WS and SWB by SCUBA divers. Canopy architecture was determined from the total length of the stems, the number, length and position of secondary branches and leaf clusters. Shoot density and leaf area (cm2) were determined from collections of shoots within six 20 × 20 cm quadrats from each of the sites. Shoots were subdivided into 5 cm length intervals and for each of these intervals leaf area and leaf dry weight were measured (Li-Cor Portable Area Meter, LI 3000). Number and position of male and female flowers along a shoot were recorded. Dyed seawater (seawater with red food coloring, neutrally buoyant) was released in situ to ob- tain an estimate of flow dispersal within and above a canopy. Two syringes were attached to a, vertically deployed, 1 m2 marked frame at the release levels of 30 cm and 50 cm, i.e., within and above the canopy respectively. Dye dispersal was recorded using an underwater video camera and stopwatch. THE MODEL CONCEPT.—Dispersal in a highly turbulent swell and wind wave dominated regime is governed by turbulent diffusion and advection due to wave motion. Short scale velocity fluc- tuations induced by wind waves and by wave reflections, are superimposed on oscillatory swell induced flow (Tennekes and Lumley, 1972). These motions show a random character compared to swell induced dynamics. In situ observations and velocity measurements (Verduin and Backhaus, 2000) would suggest that dispersal and capture of pollen within a meadow may also depend on the interaction of plants with the flow as was found for turbulent dissipation (Backhaus and Verduin, in press). VERDUIN ET AL.: AMPHIBOLIS ANTARCTICA POLLEN DISPERSAL AND CAPTURE 1271

In general, an instantaneous release of pollen would, initially, form a small particle-cloud, spreading out by increasing its size and decreasing its concentration due to turbulent diffusion (Verduin, pers. observ., 1995). The cloud would always follow the flow (advection). Shoots also follow the flow of a wave motion (flapping) which implied a very small relative velocity with respect to the ambient water. Shoots were observed to bend over with fully developed flow of a swell wave passing over the meadow until the shoots reached their maximum bending angle (ca 140°) governed by their geometry, i.e., shoot density and plant morphology, and by the local flow speed (van Keulen, 1987). Whenever plants were at their excursion limit hereafter referred to as ‘trapping’, there was a high relative motion with respect to the ambient water (Verduin, pers. observ., 1995; 1996) accounting for differential turbulent dissipation (Backhaus and Verduin, in press). A particle cloud near trapped shoots cannot stop moving as plants do, but instead follows the flow by passing through the shoots that are trapped at the end of their bending arch (Fig. 1A) The pulsating character of high and small relative motion around plants in a wave dominated hydrodynamic regime would have far reaching consequences on pollen dispersal (Fig. 1B) and, in particular, on pollen capture. In stagnant flow, the only means for pollen transport is diffusion. Hence, chances for pollen to reach a female flower would solely depend on the diffusive spreading of a particle cloud. This situation applies for the flapping phase where there is a low relative mo- tion. The likelihood of pollen capture is expected to be higher for a high relative motion, i.e. when plants are trapped, because then the pollen cloud is drawn (or ‘combed’, Fig. 1A) through a dense mesh that consists of shoots, their leaves, flowers, stems, etc. This can be demonstrated by an estimate derived from in situ observations of velocities and dye release. Under turbulent condi- tions, and with stagnant flow, turbulent diffusion may account for a spreading of the pollen cloud over a distance of ~1 m within a time in the order of several seconds up to minutes. Rapid advection during plant trapping, which always occurs at fully developed flow, would cover the same distance within a few seconds or even fractions of a second. FAR FIELD DISPERSAL OF POLLEN.—Once pollen gets dispersed out of the canopy its further dis- persal would be governed by turbulent diffusion and advection. In this case, plants will no longer play an active role in dispersal. This comprises the far field dispersal of pollen (Fig. 1C) which may eventually carry pollen away from a meadow. However, since only local measurements were made within and above A. antarctica meadows, no conclusive estimates of far field dispersal can be made here. The advection and diffusion processes have led to the definition of a general concept for a three- dimensional dispersal of pollen and its success of being caught within a meadow. Dispersal and capture of pollen by plants in a fully submerged meadow of A. antarctica is governed by a sequence of diffusion and advection phases which coincide with events of low and high motion relative to plants, i.e., plant flapping and trapping. This sequence and the respective duration of flapping or trapping events and, hence the success of pollen capture, depend on the dynamics of the incident waves interacting with the meadow. It was assumed that capture of pollen most likely occurred during trapping events. The trapping of plants depends on both the vertical velocity profile within a meadow and the bending angle of plants. The bending angle of plants, in turn, depends on plant morphology, density and flow speed. The current profile in turn depends on the dissipative effect of plants and, hence on plant-flow interactions (Verduin and Backhaus, 2000). The ratio of capture within the meadow and turbulent dispersal out of the meadow will govern the net loss of pollen of the meadow. This loss may be source for far field dispersal from meadow to meadow. THE MODEL.—The above described near field dispersal and capture of pollen was translated into hybrid, i.e., a combined deterministic-statistical model, which considered both dispersal and cap- ture of pollen (Verduin, 1996a). The diffusive part of the dispersal process was simulated by a numerical scheme for pollen diffusion which covered the entire water column (Press et al., 1992). This part of the model is the deterministic component. Pollen capture due to near field advection within a meadow was simulated by a statistical approach which considers probabilities of trapping caused by interaction of hydrodynamics and leaf area of A. antarctica. The efficiency of pollen capture was parameterized by a trapping frequency. This statistical component of the hybrid model 1272 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 3, 2002

Figure 1. 1a) Display of flapping and trapping within a geometry derived from in situ observations (excursion limits are shown by stippled lines). 1b) Trapping of the plants occurs when there is a high relative motion of the plants with respect to the flow. In this phase particles are ‘combed’ through the plant. It is at this stage that pollen will most likely be caught. Flapping, in contrast, occurs when there is low relative motion of the plants to the flow. 1c) depicts the situation once pollen get dispersed out of the canopy, further dispersal would be governed by turbulent diffusion and advection. In this case plants will no longer play an active role in dispersal. VERDUIN ET AL.: AMPHIBOLIS ANTARCTICA POLLEN DISPERSAL AND CAPTURE 1273

Figure 2. Plant flapping and trapping events in a plane. Traces of a seagrass shoot moving with the governing flow dynamics as attained from ADV measurements. Plant trapping occurs whenever traces reach the excursion limit derived from in situ observations. describes pollen capture as a result of rapid advection during trapping of plants. The only free parameter in the model is the trapping frequency, which mimics the stickiness of pollen ( number of pollen caught over time).The coupling of both components was achieved by a sink term in the diffusion model. It simulates the extraction of pollen caught by plants from the pollen cloud pre- dicted by the deterministic diffusion model. The diffusion coefficients (Kochergin, 1987) are de- rived from the field data. The number of trapping and flapping events, i.e., advection, is derived from the ADV flow measurements. Different plant geometries (excursion limits) were derived from visual observations and estimates of bending angles in dependence of flow speed. Plant geometry (bending arc from field observations) should be seen as a three-dimensional funnel. Measured flow data were used for the calculations of trapping and flapping times and excursions. Model input data for plant excursions therefore, represent the field situation. The model assumes that the plant is in the middle of the prescribed geometry funnel at the beginning of simulation and follows the flow and interacts with the flow as simulation progresses (Fig. 2).

RESULTS

The complexity of plant morphology in A. antarctica caused a variable and turbulent flow in the canopy, horizontally and with depth. The magnitude of the flow within the canopy itself was reduced by the canopy in comparison to the ambient flow. At SWB the mean kinetic energy (quantified as squared velocity) of ambient flow was 210 (cm/s)2. At 45cm above the seabed kinetic energy of up to 280 (cm/s)2 was observed. The lowest kinetic energy (20 (cm/s)2) was apparent ~25 cm above the seabed. The in situ velocity measurements revealed a highly turbulent regime near the sea bed which was caused by 1274 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 3, 2002

the canopy itself (Verduin and Backhaus, 2000). An increase in velocity, hence kinetic energy (125 (cm/s)2), was observed 10 cm above the seabed within the leaf-less stem area of the canopy. The bottom 15 cm of A. antarctica shoots has no leaves, therefore, less friction is exerted there. At WS the mean kinetic energy of the ambient flow amounted up to 35 (cm/s)2. At 45 cm above seabed a maximum of 50 (cm/s)2 was recorded. A minimum of 10 (cm/s)2 was measured 25 cm above seabed. Ten centimeters above the seabed an increase in kinetic energy of up to 20 (cm/s)2 was observed (leaf-less area). Mean leaf areas of A. antarctica in SWB and WS are given in Table 1. The highest measured leaf area occurs between 45 and 50 cm along the total length of plants in Shoalwater Bay, which translates to ~30 to 35 cm in situ. Highest measured leaf area at Warnbro Sound occurs between ~25–30 cm in situ. The shoot density at SWB was higher by ~60 shoots m2; whereas the mean total dry weight of epiphytes of 387 (g−2) was almost three times higher than for WS (Table 1). Female shoots were significantly taller than male shoots at WS (P = 0.041) (Table 1). The shoot lengths may be expressed as a per- centage of water column occupied by Amphibolis, i.e., 25% in SWB and 10% in WS. The mean number of flowers along the shoots was higher in WS than in SWB. Most flowers occurred between 30−35 cm along a shoot in WS and between 30 and 45 cm in SWB (Verduin, 1996b). The release of dyed seawater within the canopy showed marginal seepage of dyed water form the canopy into the water column above. These in situ observations would suggest that despite a loss of an estimated 30% to the ambient waters the dye remained within the canopy (Verduin, 1996b). The observations were conducted on a small scale (within 1 m2 frames) and did, therefore, not allow for an estimate of meadow to meadow dispersal. The above described model was supplied with field data, i.e., velocity mea- surements for determination of flapping and trapping events and turbulent coefficients. Model time series of integrated concentrations of pollen, dispersed within the open water column (above meadow) and within the meadow show that ~40 to 50% of pollen was lost from the meadow to the water column. Temporal evolutions of the integral loss from a meadow, either because pollen has been caught or was diffusively dispersed out of the meadow, were obtained from simula- tion data by carrying out a respective integration of predicted concentrations from the sea surface to the top of the meadow and over the height of the canopy. The hybrid model has only a one space co-ordinate (i.e., the vertical, equivalent to a seagrass plant). The pre- dicted probability of pollen capture for WS was systematically higher compared to SWB. The highest probability for pollen capture within A. antarctica meadows was predicted at 30 cm above seabed for WS and 45 cm for SWB.

DISCUSSION

Model predictions suggest that A. antarctica demonstrates a successful combination of its plant structure (i.e., distribution of leaf material and flowers on a shoot) and flow dynamics to enhance pollen capture. Capture of large numbers of pollen grains produced does, however, not guarantee fertilization and germination. The morphological charac- teristics of A. antarctica pollen are such that it tenaciously sticks to what it may encounter (Ducker et al., 1978; Pettitt, 1984). Pollen grains, aided by their filiform shape and forked VERDUIN ET AL.: AMPHIBOLIS ANTARCTICA POLLEN DISPERSAL AND CAPTURE 1275 appendages (Ducker et al., 1978; McConchie and Knox, 1989) may attach to leaves, epiphytes or stems. Pollination is reported to be a wasteful process and in terrestrial, wind pollinated, plants a mere 2% of the total number of released pollen grains may result in successful pollination (Faegri and van der Pijl, 1979; Richards, 1986). The differences between model predictions for Warnbro Sound (WS) and Shoalwater Bay (SWB) can be understood by examining the input data for the hybrid model. The input data reflected observed hydrodynamic conditions, plant-flow interactions and plant morphology. Turbulent diffusion coefficients between SWB and WS differ by a factor of ca 2 with smaller values at site WS. The same holds for flow velocities with mean veloc- ity amplitudes at WS about 50% smaller than at SWB. Plant-flow interactions were re- flected by the differences obtained for the respective trapping and flapping times. Plants appear to be trapped for longer times (factor of 3.5) at WS and their flapping times are much shorter (>50%) than at SWB. As a result hydrodynamic trapping probabilities at WS are about 50% higher than at SWB. Differences in leaf area (target size for pollen capture) between both sites are much smaller than for the hydrodynamic parameters. This explains why the combined hydrodynamic-biological capture probability, which deter- mines success of pollen capture in the hybrid model, is primarily governed by plant flow interaction, namely by the hydrodynamic trapping probability (Fig. 2). If capture would exclusively depend on hydrodynamics, i.e., advection and diffusion, capture probabilities are expected to be higher in the more energetic regime of SWB (Verduin, 1996a). Here both advective excursions and diffusion coefficients are higher than at WS. Capture probability however is a function of plant-flow interactions (plant trapping). It is the higher capture probability obtained for WS that accounts for better capture results at this site. This finding highlighted the importance of plant-flow interac- tions which are considered in this model. It also raises the hypothetical question if it would be possible for plants, inhabiting a regime that is less favorable in terms of its hydrodynamics (i.e., WS) have adapted to their local environmental conditions in order to optimize capture success? In this regard it is interesting to note that the number of pollen grains produced by the A. antarctica population in WS, was greater than in SWB by ~10 %. Ackerman (1989) defined a theoretical ‘canopy boundary layer’ by dividing the log- layer into two regions, according to theoretical approaches in studies of hydrodynamics in terrestrial plants. His measured current profiles, however, exhibited little or no evi- dence of a logarithmic dependency with distance from the seabed. This suggests that the classical boundary layer theory, and in particular the log-layer, might not be applicable for boundary layers where plants are present. In the profiles of mean amplitudes of flow velocities derived from in situ velocity measurements at SWB and WS there is no evi- dence of a logarithmic profile (Verduin, 1996; Verduin and Backhaus, 2000). Within both A. antarctica meadows at SWB and WS turbulent diffusion coefficients and shear stresses, exhibit a non-linear vertical variation (Verduin, 1996 a). Leonard and Luther (1995) and Koch (1996) arrived at the same conclusion for marsh and seagrass canopies in tidal flows. Therefore, theoretical concepts based on the boundary layer theory appear to be unsuitable for an estimation of pollen dispersal within and above the observed plant mead- ows. In consideration of the hypothetical question raised earlier, predicted probabilities of capture may be compared with the observed mean number of female flowers along a shoot, i.e., the targets for successful pollination. This comparison reveals a similarity of 1276 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 3, 2002 patterns. In particular at WS sharp peaks of both predicted capture and observed number of flowers coincide at the same height above seabed. In summary, the model predicted highest probabilities of pollen capture for the site where hydrodynamic conditions ap- peared to be less favourable for dispersal. Because of the favorable plant-flow interac- tions at this site (WS), a higher success of pollen dispersal and capture was obtained. According to this result, plants, through their interaction with the flow, take an active role in pollen dispersal.

ACKNOWLEDGMENTS

This research was funded by the Department of Botany, University of Western , Perth and the Australian Department of Employment, Education and Training (DEET). We gratefully thank E. Koch and M. van Keulen, and an anonymous reviewer for their comments on the manuscript.

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ADDRESS: (J.J.V., J.O.B.) Institute of Hydrobiology and Fisheries Science, University of Hamburg, Olbersweg 24, D–22767 Hamburg, Germany. (D.I.W.) Department of Botany, School of Plant Biol- ogy, University of Western Australia, Perth, WA 6907, Australia. E-mail: (J.J.V.) .