Diffusivity in a marine macrophyte canopy: implications for submarine pollination and dispersal

The dispersion and capture of differently shaped particles within a Zostera marina L. (eelgrass; Zosteraceae) bed were examined to understand submarine pollination and other dispersals. During periods of moderate flow in the canopy, the capture rate of 'spherical' (the shape of ancestral pollen) and 'filamentous' (the shape of eelgrass pollen) particles was greater for particles released at the top of the canopy (3.07 and 4.53% x [10.sup.-5] [cm.sup.-2] of collector; i.e., percentage of particles captured normalized to collector area) and greater for filamentous than for spherical particles (4.51% x [10.sup.-5] [cm.sup.-2] vs. 2.01% x [10.sup.-5] [cm.sup.-2]). Estimates of the horizontal P (Joseph-Sendner diffusion velocity) and the vertical diffusivity (Gaussian K) of filamentous particles were small (P [approximately equal to] 4 x [10.sup.-4] m/s; K [approximately equal to] [10.sup.-4] [m.sup.2]/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 1000-10000 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.