Interactions between Fragmented Seagrass Canopies and the Local Hydrodynamics

Abstract

The systematic creation of gaps within canopies results in fragmentation and the architecture of fragmented canopies differs substantially from non-fragmented canopies. Canopy fragmentation leads to spatial heterogeneity in hydrodynamics and therefore heterogeneity in the sheltering of canopy communities. Identifying the level of instability due to canopy fragmentation is important for canopies in coastal areas impacted by human activities and indeed, climate change. The gap orientation relative to the wave direction is expected to play an important role in determining wave attenuation and sheltering. Initially we investigated the effect of a single transversal gap within a canopy (i.e. a gap oriented perpendicular to the wave direction) on hydrodynamics, which was compared to fully vegetated canopies (i.e. no gaps) and also to bare sediment. The wave velocity increased with gap width for the two canopy densities studied (2.5% and 10% solid plant fraction) reaching wave velocities found over bare sediments. The turbulent kinetic energy (TKE) within the gap also increased, but was more attenuated by the adjacent vegetation than the wave velocity. As expected, denser canopies produced a greater attenuation of both the wave velocity and the turbulent kinetic energy within an adjacent gap, compared to sparse canopies. Using non-dimensional analysis and our experimental data, a parameterization for predicting TKE in a canopy gap was formulated, as a function of easily measured variables. Based on the experimental results, a fragmented canopy model was then developed to determine the overall mixing level in such canopies. The model revealed that canopies with large gaps present more mixing than canopies with small gaps despite having the same total gap area in the canopy. Furthermore, for the same total gap area, dense fragmented canopies provide more shelter than sparse fragmented canopies.

Impact of anthropogenically created canopy gaps on wave attenuation in a Posidonia oceanica seagrass meadow

“…less resilient) and allow greater penetration of waves and currents into the canopy, which will enhance sediment resuspension and favor exchange between the meadow and the gap. These conclusions support those of , while Hendriks et al (2008), El Allaoui et al (2015 and El Allaoui et al (2016) reported the buffering of sediment resuspension by P. oceanica meadows. El Allaoui et al (2016) found that canopies with large gaps had a higher mixing level than canopies with small gaps, despite both canopies having the same total gap area.…”

“…While light climate and temperature were not measured in this study, we recognize that the change in seagrass structure among campaigns could markedly influence the observed hydrodynamic conditions in the vicinity of the seagrass bed. El Allaoui et al (2016) highlighted the increase in the mixing level that occurred in a patchy canopy compared with a canopy without gaps. An increase in mixing can cause an increase in sediment resuspension, which can result in a reduction in water clarity, threatening seagrass growth (Lawson et al 2007, van der Heide et al 2007, Carr et al 2012.…”

“…Spatial fragmentation of meadows, whether natural or anthropogenic, dramatically increases the frequency of edges; however, the impact of fragmentation on hydrodynamic edge effects has been poorly documented. In a laboratory experiment, El Allaoui et al (2016) studied the effects of canopy patchiness on the mixing level in a canopy and quantified the extent to which small gaps, oriented perpendicular to the wave direction, were sheltered by the canopy, and also the extent to which gap width impacted the degree of sheltering.…”

Fragmented Canopies Control the Regimes of Gravity Current Development

“…Sites with high silt content usually have a low diversity of species, while sites with coarse sediment particles have a much richer diversity of species (Thrush et al, ). As El Allaoui et al () pointed out, the presence of edges in canopies has been found to modify the hydrodynamics of the system, presenting a behavior that is neither that of the fully vegetated or the nonimpeded cases, but rather falls between. Ricart et al () also found that edges within a canopy change the sedimentary pattern, and they point out the importance of hydrodynamics to explain sediment pattern distribution.…”

“…These gaps may have both natural and/or human origins. For example, storms directly or indirectly may erode mattes by tearing whole sections away or scouring out their sediments, while laying cables or pipes in sea canopies favors the formation of gaps and changes the hydrodynamics in the canopy (El Allaoui et al, ). The persistence of coastal wetlands depends on the sediment deposition that controls the vertical position of the salt marsh surfaces (Bell et al, ).…”