Local hydrodynamics at edges of marine canopies under oscillatory flows

The role epiphytes play in particle capture of seagrass canopies

Seagrass epiphytic communities act as ecological indicators of the quality status of vegetated coastal environments. This study aims to determine the effect leaf epiphytes has on the sediment capture and distribution from outside sources. Thirteen laboratory experiments were conducted under a wave frequency of 0.5 Hz. Three epiphyte models were attached to a Zostera marina canopy of 100 plants/m2 density. The sediment deposited to the seabed, captured by the epiphytic leaf surface, and remaining in suspension within the canopy were quantified. This study demonstrated that the amount of epiphytes impacts on the sediment stocks. Zostera marina canopies with high epiphytic areas and long effective leaf heights may increase the sediment captured on the epiphyte surfaces. Also, reducing suspended sediment and increasing the deposition to the seabed, therefore enhancing the clarity of the water column. For largest epiphytic areas, a 34.5% increase of captured sediment mass is observed. The sediment trapped on the leaves can be 10 times greater for canopies with the highest epiphytic areas than those without epiphytes. Therefore, both the effective leaf length and the level of epiphytic colonization are found to determine the seagrass canopy ability at distributing sediment.

Spatial sedimentation and plant captured sediment within seagrass patches

Habitat degradation in coastal ecosystems has resulted in the fragmentation of coastal aquatic vegetation and compromised their role in supplying essential ecological services such as trapping sediment or sequestering carbon. Fragmentation has changed seagrass architecture by decreasing the density of the canopy or engendering small patches of vegetated areas. This study aims to quantify the role different patch sizes of vegetation with different canopy densities have in the spatial distribution of sediment within a patch. To this aim, two canopy densities, four different patch lengths, and two wave frequencies were considered. The amounts of sediment deposited onto the bed, captured by plant leaves, remaining in suspension within the canopy, and remaining in suspension above the canopy were used to understand the impact hydrodynamics has on sediment distribution patterns within seagrass patches. In all the cases studied, patches reduced the suspended sediment concentrations, increased the capture of particles in the leaves, and increased the sedimentation rates to the bed. For the lowest wave frequency studied (0.5 Hz), the sediment deposited to the bottom was enhanced at canopy edges, resulting in spatial heterogeneous sedimentation patterns. Therefore, restoration and preservation of coastal aquatic vegetation landscapes can help face future climate change scenarios where an increase in sedimentation can help mitigate predicted sea level rise in coastal areas.

Short-term sedimentation dynamics in mesotidal marshes

One of the key questions about wetlands resilience to sea-level rise is whether sediment supply will be enough to keep them coping with growing inundation levels. To address this question, researchers have put a lot of effort into field data collection and ecogeomorphic modelling, in an attempt to identify the tipping points of marsh survival. This study uses fieldwork data to characterize the sediment fluxes between the tidal flats and salt marshes, in the Ria Formosa lagoon (Portugal). Sediment fluxes were measured from the tidal channel towards the mid-upper marsh, during neap and spring tide conditions. The flow magnitude was measured, and induced transport was determined based on shear velocities. Deposition rates, instantaneous suspended sediment and near-bed velocities were linked through theoretical formulas and used to characterize time-averaged conditions for sediment delivery and deposition to the site. The results showed that suspended sediment concentrations and sediment deposition varied across the transect with no specific relation to elevation. Maximum water depths were recorded in the vegetated tidal flat, and the maximum currents were flood dominated, in the order of 0.20 m/s, in the low marsh due to flow-plant interactions and an increase of turbulence. Deposition rates ranged between 20 to 45 g/m²/hr, after a complete tidal cycle, and were higher in the mid-upper marsh. Hydroperiod was not the main contributor to sediment deposition in the study area. Sediment transport was tidally driven, strongly two-dimension during the cycle, and highly influenced by the vegetation. Measurements of marsh sediment flux obtained in our work are diverse from the ones found in the literature and evidence the importance of considering spatio-temporal variability of vegetated platforms in assessing overall marsh bed level changes.

Stem stiffness functionality in a submerged canopy patch under oscillatory flow

Seagrass canopies are coastal ecosystems that are able to modify the abiotic environment through their architectural structure. They have different structural parameters, such as plant stem stiffness, patch length and canopy density, all of which determine their overall functionality in modifying the seafloor hydrodynamics within coastal areas. To determine the interaction between hydrodynamics and the canopy structure, a set of laboratory experiments were carried out with both rigid and flexible stems for different canopy densities, patch lengths and wave frequencies. In the upper part of the canopy, flexible plants move with the flow without generating drag or producing turbulent kinetic energy, while rigid plants generate drag and produce turbulent kinetic energy. In the inner canopy layer, both types of plants behave like rigid stems and produce turbulent kinetic energy. A non-dimensional model based on the turbulent kinetic energy, the wave velocity and the plant characteristics is presented to describe the behaviour of flexible and rigid plants under an oscillating flow. Flexible plants behave in a stiffer manner under high wave frequencies than under low wave frequencies, thus making their behaviour closer to that of rigid plant stems. This difference between both canopy structures can explain their distribution in the environment, with rigid canopies being more extended in more sheltered regions while flexible plants are characteristic of more exposed regions with high flow energy.

Functional dynamics of vegetated model patches: The minimum patch size effect for canopy restoration

For the past two centuries coastal zones have been suffering seagrass loss resulting in a network of vegetated patches which are barely interconnected and which may compromise the ecological services provided by the canopy. To optimize management efforts for successful restoration strategies, questions need to be addressed about what appropriate canopy architectural considerations are required under certain hydrodynamic conditions. In this study, a set of laboratory experiments were conducted in which hydrodynamic conditions, plant densities and vegetated patch lengths were varied to determine minimum patch lengths for successful management strategies. Based on the TKE production, this study finds two possible canopy behaviours of seagrasses under oscillating flows: one where plants do not interact with the flow and the other where they interact with waves and produce TKE. A threshold from the first to second behaviour occurs for [Formula: see text] = 2, where C_D is the drag of the vegetated patch, n is the number of stems per m², d is the stem diameter and ϕ is the solid plant fraction. Therefore, high canopy densities, large patches of vegetation or moderate wave velocities will produce plant-wave interaction, whereas low canopy densities, small vegetation patches or slow wave velocities will produce a behaviour akin to the non-vegetated cases.

The role of seagrass vegetation and local environmental conditions in shaping benthic bacterial and macroinvertebrate communities in a tropical coastal lagoon

We investigated the influence of seagrass canopies on the benthic biodiversity of bacteria and macroinvertebrates in a Red Sea tropical lagoon. Changes in abundance, number of taxa and assemblage structure were analyzed in response to seagrass densities (low, SLD; high, SHD; seagrasses with algae, SA), and compared with unvegetated sediments. Biological and environmental variables were examined in these four habitats (hereafter called treatments), both in the underlaying sediments and overlaying waters, at three randomly picked locations in March 2017. Differences between treatments were more apparent in the benthic habitat than in the overlaying waters. The presence of vegetation (more than its cover) and changes in sedimentary features (grain size and metals) at local scales influenced the observed biological patterns, particularly for macroinvertebrates. Of note, the highest percentage of exclusive macroinvertebrate taxa (18% of the gamma diversity) was observed in the SHD treatment peaking in the SA for bacteria. Benthic macroinvertebrates and bacteria shared a generally low number of taxa across treatments and locations; approximately, 25% of the gamma diversity was shared among all treatments and locations for macrofauna, dropping to 11% for bacteria. Given the low overlap in the species distribution across the lagoon, sustaining the connectivity among heterogeneous soft sediment habitats appears to be essential for maintaining regional biodiversity. This study addresses a current scientific gap related to the relative contributions of vegetated and unvegetated habitats to biodiversity in tropical regions.