Ghisalberti
Marco
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Retention time and dispersion associated with submerged aquatic canopies
Editorial: Canopies in aquatic ecosystems: integrating form, function, and biophysical processes
Ghisalberti
Marco
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Article
The shear layer at the top of a submerged canopy generates coherent vortices that control exchange between the canopy and the overflowing water. Unlike free shear layers, the vortices in a canopy shear layer do not grow continuously downstream but reach and maintain a finite scale determined by a balance between shear production and canopy dissipation. This balance defines the length scale of vortex penetration into the canopy, δ e , and the region of rapid exchange between the canopy and overflow. Deeper within the canopy, transport is constrained by smaller turbulence scales. A two-box canopy model is proposed on the basis of the length scale δ e . Using diffusivity and exchange rates defined in previous studies, the model predicts the timescale required to flush the canopy through vertical exchange over a range of canopy density and height. The predicted canopy retention times, which range from minutes to an hour, are consistent with canopy retention inferred from tracer observations in the field and comparable to retention times for some hyporheic regions. The timescale for vertical exchange, along with the in-canopy velocity, determines the minimum canopy length for which vertical exchange dominates water renewal. Shorter canopies renew interior water through longitudinal advection. Finally, canopy water retention influences longitudinal dispersion through a transient storage process. When vertical exchange controls canopy retention, the transient storage dispersion increases with canopy height. When longitudinal advection controls water renewal, dispersion increases with canopy patch length.
Article
This Research Topic presents new research investigating the coupling between physical (fluid dynamics, mass transport, and light availability) and biological (nutrient cycling, particle transport, ecosystem structure, and biodiversity) processes in aquatic canopies. The starting point for this topic was the observation that our notion of “canopy” in the aquatic sciences, in contrast to that of our terrestrially-focused colleagues, remains underdeveloped. Forest canopy studies have been considered a new field of science (Nadkarni et al., 2011) and the concept of forest canopy research is clearly documented in the literature (Barker and Pinard, 2001; Nadkarni, 2001; Lowman, 2009); we have not found similar mentions of the canopy concept in aquatic studies. Over the past decade, however, there has been an increase in the number of studies on underwater canopies, as well as a shift toward more multidisciplinary studies that consider more than just the physical impacts of the canopy's presence (Ackerman, 2007; Nepf et al., 2007; O'Brien et al., 2014).