Canopy-Mediated Hydrodynamics Contributes to Greater Allelic Richness in Seeds Produced Higher in Meadows of the Coastal Eelgrass Zostera marina

Abstract

Seagrass meadows, which mediate ocean acidity and turbidity, sequester carbon, and increase biodiversity by providing shelter for larvae and small fish, are among the fastest disappearing ecosystems worldwide. Seagrasses are ecosystem engineers, creating distinct regions of enhanced and diminished flow and turbulent mixing, dependent upon canopy physical parameters, such as canopy density and blade morphology, which in turn impact the transport of pollen, sediment, and nutrients. The health and resilience of seagrass meadows increase with intraspecies genetic diversity, which depends on successful sexual reproduction and the transport of pollen particles between reproductive shoots, which in turn depends on the hydrodynamic conditions created by the meadow. This paper explored the transport of pollen grains in seagrass meadows using a random walk model. The model was parameterized with profiles of mean velocity and eddy diffusivity derived as functions of shoot density, canopy height, canopy shear velocity, canopy drag coefficient, and blade width, and validated with experimental measurements of a tracer plume evolving in a submerged model canopy. Model results showed that release at the top of the canopy led to significantly greater dispersal than release within the canopy, which was consistent with observed patterns of genetic diversity in Zostera marina seeds collected from coastal Massachusetts meadows. Specifically, seeds produced from upper inflorescences had greater allelic richness than seeds from lower inflorescences on the same reproductive shoot, and were the product of a greater number of fathers, reflecting the greater in-canopy pollen movement farther from the bed. Pollen grains modeled with a realistic elongated shape experienced significantly higher rates of capture by the canopy relative to spherical grains of the same volume. The effect of submergence depth (the ratio of water depth to canopy height) on pollen dispersal depended on the nature of the surface boundary: when Follett et al. Hydrodynamics Influence Eelgrass Allelic Richness pollen reflected off the water surface, the mean travel distance before pollen capture decreased with decreasing submergence depth. In contrast, when pollen accumulated at the water surface, surface transport increased pollen dispersal distances, especially at low submergence depths.

Transient Solute Dispersion in Wetland Flows With Submerged Vegetation: An Analytical Study in Terms of Time‐Dependent Properties

“…The above parameters can be estimated by the experiment or empirical formulas (Follett et al, 2019;Ghisalberti & Nepf, 2002, 2004Murphy et al, 2007).…”

“…Longitudinal velocity and vertical turbulent diffusion coefficient in equation are determined based on the existing experimental studies (Ghisalberti & Nepf, , ; Follett et al, ; Nepf & Ghisalberti, ), which can significantly affect the dispersion process (de Barros et al, ). A mixing layer velocity model (Ghisalberti & Nepf, , ; Follette et al, ), which matches the experimental data by Ghisalberti and Nepf () and a spatially dependent vertical turbulent dispersivity (Follett et al, ; Liang & Wu, ) are adopted, as illustrated in Figure b.…”

“…The velocity profile is affected by vegetation density, plant morphology, current speed, relative submergence, and the flexibility of vegetation (Boothroyd et al, ; Caroppi et al, ; Ghisalberti & Nepf, , , ; Lacy & Wyllie‐Echeverria, ; Murphy et al, ). Due to different flow conditions and types of vegetation, various velocity models have been proposed, including the logarithmic velocity profile (Lacy & Wyllie‐Echeverria, ; Singh et al, ), two‐layer model (Battiato & Rubol, ; Poggi et al, ; Rubol et al, ; Shi et al, ), three‐layer model (Huai et al, ; Liang & Wu, ; Nepf & Ghisalberti, ; Shucksmith et al, , ), and shear layer (or mixing layer) model (Ghisalberti & Nepf, , ; Follett et al, ).…”

“…Therefore, the aim of this work is to tackle the transient dispersion in a submerged vegetated flow by an analytical approach for an instantaneous source release at the top of the canopy on the basis of the previous experimental study (Murphy et al, ). The mixing layer velocity profile and four‐layer vertical turbulent diffusion coefficient profile are adopted (Ghisalberti & Nepf, ; Follett et al, ; Liang & Wu, ). As a classic method originated in Taylor dispersion problems in pure flows, Chatwin's () long‐time asymptotic expansion as extensively extended ever since (Guo, Jiang, et al, ; Jiang & Chen, ) is devised in this work.…”

“…The ‐zone model provides more accurate results than Murphy et al ()'s two‐zone model. To more precisely characterize the dispersion in vegetated flows, based on the previous experimental studies (Ghisalberti & Nepf, , ; Nepf & Ghisalberti, ), a spatially dependent vertical turbulent diffusion coefficient profile (here termed “four‐layer model”) is adopted by Follett et al () and Liang and Wu (). Due to the analytical difficulty, numerical methods were conducted to deal with this complicated model.…”