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.
Highlights
Vegetated systems cover less than 0.5% of the sea bed but account for up to 70% of the carbon storage in ocean sediments [1]
Our model predicts that when wave velocities approach those that plants can withstand, a fragmented canopy in deeper water will be more vulnerable than a fragmented canopy in shallow water [57]. This has implications for the management of human activities that might trigger the fragmentation of canopies. These results have quantified the complex interactions between canopy density, the proportion of canopy that is fragmented, gap width, and the sheltering experienced within the gap
The presence of a single transversal gap within a canopy increased both the wave velocity and the turbulent kinetic energy within the gap, compared to what is found within a continuous canopy
Summary
Vegetated systems cover less than 0.5% of the sea bed but account for up to 70% of the carbon storage in ocean sediments [1]. As ecosystem engineers, aquatic plants play a critical role in protecting coastal areas . Paul and Amos [14] showed that canopies need a minimum shoot density to initiate wave attenuation, and Paul et al [15] found that leaf length, canopy density and blade stiffness were crucial in determining the degree of wave attenuation. They found that the presence of a tidal current reduced considerably the effectiveness of wave attenuation by a canopy. Aquatic plants are recognized as both ecosystem engineers and water quality indicators, and their restoration and protection is a priority given that their regrowth is slow and variable [26,27]
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