Consequences of shoot density and stiffness for ecosystem engineering by benthic macrophytes in flow dominated areas: a hydrodynamic flume study

Abstract

Within intertidal areas of European Atlantic coasts the distribution of the small seagrass Zostera noltii and the halophyte Spartina anglica can partially overlap, despite numerous biomechanical, demographic and ecophysiological differences. Both species are known to be ecosystem engineers that modify their habitat by reducing hydrodynamic energy within their canopies. In this study we investigate the influence of biomechanical (i.e. shoot flexibility) and demographic (i.e. shoot density) characteristics of these intertidal plants on their interaction with unidirectional currents to (1) understand their differences in ecosystem engineering capacity and (2) identify which physical traits explains these differences. In a flume tank, hydrodynamic variables were measured within transplanted S. anglica and Z. noltii meadows, and their corresponding simplified mimics. The results revealed that stiff canopies had a larger potential capacity (relative to flexible ones) to trap sediment, as in these vegetations velocity reduction within the canopy combined with a sufficient volumetric flow rate to provide sediment for settling. Flexible canopies were most efficient at reducing erosion by reconfiguration of their leaves. Shoot density increased the magnitude of these effects when values were moderate. However, the capacity to increase sediment accretion disappeared when the maximum velocity attenuation was reached and the flow of water was relocated on top of the canopy. These habitat modifications may provide ecological benefits for saltmarsh and seagrass species. For saltmarsh plants, the rigid shoots allow lateral expansion of their populations via increased sedimentation. For seagrasses, the dense and flexible shoots typical of temperate intertidal populations provide efficient protection from erosive forces, while at the same time helping to avoid stresses, such as drag forces and high sedimentation rates.

Key biogeochemical factors affecting soil carbon storage in <i>Posidonia</i> meadows

“…The SAR in seagrass meadows is mainly controlled by the canopy structure, which affects the trapping and retention of sediment particles (Gacia and Duarte, 2001;Peralta et al, 2008;Hendriks et al, 2010), the hydrodynamic energy, the availability of fine-grained suspended particles in the water column and the production of biogenic carbonates within the meadow (De Falco et al, 2000Mazarrasa et al, 2015). High plant biomass and density is associated with greater retention of particles (in particular, fine-grained sediments), lower hydrodynamic energy and higher production of biogenic carbonates within the meadow (De Falco et al, 2000), ultimately enhancing soil accumulation.…”

“…To account for this variability in seagrass tissue δ 13 C, the δ 13 C signatures of seagrass detritus measured directly in the seagrass detritus present in each core were used in the corresponding mixing model. Concentration dependence was incorporated into the model because elemental concentrations were different between sources (Phillips and Koch, 2002). We did not consider any fractionation with aging (0 ± 0 ‰) in the model because previous studies suggest small diagenetic shifts for δ 13 C during decomposition (Zieman et al, 1984;Mateo et al, 2010).…”

“…The rates of soil accumulation, the sediment structure and the biochemical composition of the organic matter buried may strongly influence C org accumulation and preservation, and are highly variable among seagrass meadows (De Falco et al, 2000;Kennedy et al, 2010;Duarte et al, 2013). Soil accumulation may be a function of the seagrass canopy structure (De Falco et al, 2000;Gacia and Duarte, 2001;Peralta et al, 2008;Hendriks et al, 2010), the availability of suspended particles to settle outside of the water column and the production of biogenic carbonates within the meadow (De Falco et al, 2000;Mazarrasa et al, 2015). If the accumulated sediments are fine, then they are likely to enhance the preservation of C org since they tend to limit oxygen exchange and redox potentials, which reduce remineralization (e.g., Keil and Hedges, 1993).…”

Multiple Stable States and Catastrophic Shifts in Coastal Wetlands: Progress, Challenges, and Opportunities in Validating Theory Using Remote Sensing and Other Methods

“…Net sedimentation is also promoted by plant roots improving sediment cohesion [170][171][172][173][174]. Larger diameter and less flexible plant stems or clusters are associated with more scouring [145,175], but less flexible vegetation traps more total sediment if not submerged [176]. Within patches of pioneer plants (e.g., Spartina anglica tussocks), tidal currents are reduced and sediment accumulates, raising the plant in the tidal range and resulting in a positive feedback on plant growth.…”

Comparing biodiversity effects among ecosystem engineers of contrasting strength: macrofauna diversity in Zostera noltii and Spartina anglica vegetations

“…Such reduction of hydrodynamic energy may lead to an enhanced accretion of organic materials and thereby an enhanced food supply to benthic animals that live from deposited debris (e.g., surface deposit feeders, grazers). However, because of the lower density of the Spartina than Zostera vegetation, Spartina maintains a higher canopy flux under unidirectional flow (Peralta et al 2008). Such flux through the canopy is needed both to provide food supply to benthic animals that live inside vegetations and depend on water refreshment for their food supply (e.g., filter feeders; Brun et al, this issue) and to provide materials to maintain high rates of accretion within the vegetated areas (Peralta et al 2008).…”