Flow-induced reconfiguration of buoyant and flexible aquatic vegetation

Plant posture can play a key role in the health of aquatic vegetation, by setting drag, controlling light availability, and mediating the exchange of nutrients and oxygen. We study the flow-induced reconfiguration of buoyant, flexible aquatic vegetation through a combination of laboratory flume experiments and theoretical modeling. The laboratory experiments measure drag and posture for model blades that span the natural range for seagrass stiffness and buoyancy. The theoretical model calculates plant posture based on a force balance that includes posture-dependent drag and the restoring forces due to vegetation stiffness and buoyancy. When the hydrodynamic forcing is small compared to the restoring forces, the model blades remain upright and the quadratic law, Fx 3 U2, predicts the drag well (Fx is drag, U is velocity). When the hydrodynamic forcing exceeds the restoring forces, the blades are pushed over by the flow, and the quadratic drag law no longer applies. The model successfully predicts when this transition occurs. The model also predicts that when the dominant restoring mechanism is blade stiffness, reconfiguration leads to the scaling Fx 3 U4/3. When the dominant restoring mechanism is blade buoyancy, reconfiguration can lead to a sub-linear increase in drag with velocity, i.e., Fx 3 Ua with a , 1. Laboratory measurements confirm both these predictions. The model also predicts drag and posture successfully for natural systems ranging from seagrasses to marine macroalgae of more complex morphology. The most obvious hydrodynamic effect of aquatic vegetation is that it provides resistance to flow. In the past, this has led to aquatic vegetation being removed from river channels to increase conveyance capacity and reduce flooding (Kouwen and Unny 1973). It is now recognized that aquatic vegetation provides many important ecosystem services by resisting flow and altering local flow conditions (Carpenter and Lodge 1986; Bouma et al. 2005; Peralta et al. 2008). By reducing the near-bed flow, benthic vegetation promotes the sedimentation of suspended material and inhibits sediment resuspension, thereby limiting erosion (Fonseca and Fisher 1986; Barko and James 1998). A reduction in suspended material leads to greater light penetration and enhanced productivity (Madsen et al. 2001; de Boer 2007). The ensuing low flow environment within vegetation beds serves as shelter for fish and aquatic invertebrates. However, these ecosystem services come at a cost—the vegetation must withstand the equal and opposite drag force exerted by the water, which can damage or dislodge the vegetation (Denny et al. 1998; Bouma et al. 2005). Many aquatic macrophytes are flexible. They are pushed over into more streamlined postures with increasing velocity. Relative to rigid, upright vegetation, this reconfiguration leads to significantly reduced drag for flexible vegetation (Koehl 1984; Vogel 1994). In addition to setting drag, posture influences other processes important to the health of aquatic vegetation. For example, vegetation posture controls light availability. An upright posture exposes the vegetation to higher light intensities, whereas a streamlined posture increases the projected leaf area absorbing the incoming light but makes self-shading among neighboring macrophytes more likely (Zimmerman 2003). Posture can also control nutrient and oxygen exchange between the vegetation and the surrounding water. Faster flows perpendicular to the vegetation lead to thinner diffusive boundary layers around the vegetation, which can enhance the rate of nutrient (Hurd 2000) and oxygen (Mass et al. 2010) transfer. In addition to regulating the health of the vegetation, nutrient uptake and oxygen production provide an important ecosystem service: aquatic vegetation prevents dangerous eutrophication and anoxia (Costanza et al. 1997). Previous studies show that the morphology of aquatic vegetation can change in response to the local hydrodynamic environment (Puijalon et al. 2005; Peralta et al. 2006; Stewart 2006), reflecting the feedbacks between flow, plant posture, and the biological processes described above. Because of its importance to flood and ecosystem management, the physical interaction between water flow and aquatic vegetation has received significant attention (Nikora 2010). There have been numerous attempts to characterize the drag generated by flexible vegetation in unidirectional currents starting with Kouwen and Unny (1973). However, a universal description of reconfiguration and drag for flexible aquatic vegetation remains elusive (see discussion of Sand-Jensen 2003 by Green 2005; Sukhodolov 2005; Statzner et al. 2006). Reconfiguration can also be important for terrestrial vegetation in wind-exposed environments (Harder et al. 2004). In a recent review concerning the effect of wind on plants, de Langre (2008) proposed a simple reconfiguration model balancing the opposing moments due to aerodynamic drag and plant stiffness that qualitatively reproduced the trends observed in experimental drag data. There is, however, an important distinction between terrestrial and aquatic vegetation— aquatic vegetation can be positively buoyant. Seagrass blades have gas-filled lacunae (Penhale and Wetzel 1983), and kelps and other macroalgae have gas-filled floats called pneumatocysts (Denny et al. 1997; Stewart 2006). As a *Corresponding author: [email protected] Limnol. Oceanogr., 56(6), 2011, 2003–2017 E 2011, by the Association for the Sciences of Limnology and Oceanography, Inc. doi:10.4319/lo.2011.56.6.2003