Experimental assessment and modeling evaluation of the effects of the seagrass Posidonia oceanica on flow and particle trapping

Abstract

Retention of particles in seagrass canopies is usually attributed to only the indirect, attenuating effects canopies have on flow, turbulence and wave action, promoting sedimentation and reducing resuspension within seagrass meadows. Yet recent evidence suggests that seagrasses are also able to affect particle flux directly through loss of momentum and increased path length derived from collisions with leaves and binding of particles. We evaluated the role of Posidonia oceanica on flow and associated particle trapping through flume experiments. Our results confirm the existence of 2 dynamically different environments, viz. (1) the below-canopy habitat, with low shear stress and reduced turbulence, and (2) the canopy-water interface region, characterized by high shear stress and turbulence intensity, where vertical transport of momentum is enhanced. At relatively low free stream velocities (i.e. 0.05 and 0.10 m s -1 ) sediment concentration decreased much faster when a Posidonia meadow was present in the flume, indicating major particle trapping in the seagrass canopy. Fluxes to the sediment (as shown by large negative peaks in Reynolds stress inside the Posidonia meadow) indicated 2 to 6 times more sediment transport to the bottom when a meadow is present. However, calculations based on the experimental results point to loss rates an order of magnitude larger in a Posidonia meadow. We hypothesize that direct effects of particle collisions with leaves are responsible for this discrepancy, and we explore possible interactions with a simple model. Using only collisions as a loss factor, the model predicts that the probability a particle is lost from the flow upon a collision is 2 to 3%. Previously observed leaf density and flow velocity effects on particle loss rates were explained by the model. Fitting the model to our experimentally obtained particle disappearance rates in vegetation indicated that around 27% of particle momentum is lost upon each collision with a leaf. We hypothesize that physical filtration by sediment collisions with plant structures plays a role in particle removal in aquatic systems. KEY WORDS: Particle trapping · Physical filtering · SeagrassResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 356: 163-173, 2008 Explaining the full mechanism by which seagrasses enhance particle retention is less clear. Usually, it is assumed that the role of seagrasses is solely indirect, driven by their effects on flow (Fonseca et al. 1983, Ackerman & Okubo 1993, Lopez & Garcia 1998, Granata et al. 2001). Yet recent evidence suggests that once particles are brought near the canopy (through the effects on flow listed above), seagrasses also affect particle flux directly, leading to potentially high particle removal rates (Agawin & Duarte 2002). Possible mechanisms for such direct effects on particle trapping include (1) the loss of momentum and increased path length derived from collisions of the particles with leaves within the dense seagrass canopie...

Blue carbon stocks in Baltic Sea eelgrass (<i>Zostera marina</i>) meadows

“…In addition, it is well known that seagrasses modify sediments by reducing water flow and consequently increasing particle trapping and sedimentation and reducing resuspension (Fonseca and Fisher, 1986;Fonseca and Cahalan, 1992;Gacia et al, 2002;Hendriks et al, 2008;Boström et al, 2010). In this study, the DistLm analysis showed that contribution of Z. marina to the sediment surface carbon pool was an important driver (> 10.9 %) of the variation in the sediment C org stock (Tables 3 and 4, Fig.…”

“…Con- sequently, recent global estimates imply that seagrass sediments store almost 25 200 to 84 000 t C km 2 (Fourqurean et al, 2012). More importantly, carbon in submerged sediments is stored for timescales of millennia, while terrestrial soils are usually less stable and only sequester carbon up to decades (Hendriks et al, 2008).…”

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

“…The 3-to 11-fold lower C org storage capacity of bare sediments compared to P. sinuosa meadows at comparable depths is mainly due to the absence of seagrass inputs. However, it may also result from the absence of a canopy that would otherwise enhance the trapping and retention of organic-rich, fine sediment particles (Hendriks et al, 2008), as reflected in the low content of fine-grained sediments. Since all continental margins store C org , there is a need to account for the net C org storage capacity due to the presence of seagrasses when evaluating their role as carbon sinks.…”

Effects of seagrasses and algae of the Caulerpa family on hydrodynamics and particle-trapping rates

“…The large carbon burial capacity documented for seagrass meadows is strengthened by the role of their canopies in enhancing sediment deposition (Gacia et al 2002). Seagrasses alter their surrounding physical habitat, since their canopies attenuate turbulence and reduce water flow (Koch et al 2006), promoting fluxes of particles to the bed (Hendriks et al 2008) enhancing sedimentation (Koch et al 2006;Bos et al 2007) and preventing resuspension (Gacia and Duarte 2001). These intrinsic properties of macrophyte canopies act to stabilize sediments, reduce erosion and turbidity of the overlying water column (Madsen et al 2001), strongly influencing near-shore sediment dynamics (Marba et al 2002;van der Heide et al 2007).…”