Turbulence and Coral Reefs

Abstract

The interaction of coral reefs, both chemically and physically, with the surrounding seawater is governed, at the smallest scales, by turbulence. Here, we review recent progress in understanding turbulence in the unique setting of coral reefs—how it influences flow and the exchange of mass and momentum both above and within the complex geometry of coral reef canopies. Flow above reefs diverges from canonical rough boundary layers due to their large and highly heterogeneous roughness and the influence of surface waves. Within coral canopies, turbulence is dominated by large coherent structures that transport momentum both into and away from the canopy, but it is also generated at smaller scales as flow is forced to move around branches or blades, creating wakes. Future work interpreting reef-related observations or numerical models should carefully consider the influence that spatial variation has on momentum and scalar flux. Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 3, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Aquatic Biogeochemical Eddy Covariance Fluxes in the Presence of Waves

“…Assuming a Prandtl number of 1, the eddy diffusivity can be calculated by: where is the momentum flux (e.g., Equation ) and is the shear calculated from a velocity profile of mean flow (Holtappels & Lorke, 2011). Using the CPSD accumulation for determining the momentum fluxes will yield direct measurements of , which is superior to the assumption of a logarithmic boundary layer current profile (Holtappels & Lorke, 2011), especially in high‐roughness shallow environments such as shallow‐water coral reefs and seagrass beds where flow profiles often do not follow a logarithmic relationship (Davis et al., 2020; Nepf, 2012). For example, the momentum fluxes calculated from traditional covariance analysis produced substantially larger values (Table 2, Figure 4) and accumulating the CPSD below the wave frequencies will enable a more accurate determination of for the gradient exchange technique while removing the need to assume a logarithmic boundary layer current profile.…”

“…However, these data were rejected by the criteria developed by Rosman and Gerbi (2017) that determined that during low flow and high wave orbital velocities the low‐frequency part of the cospectrum is impacted by wave advection of turbulence. It is important to note that shallow‐water, high‐roughness environments, such as shallow reefs, are a very challenging location to determine turbulent fluxes (Davis et al., 2020). However, the high magnitude of biogeochemical activity in shallow waters (i.e., large vertical gradients) makes biogeochemical flux measurements feasible during some hydrodynamic conditions, especially when compared to momentum fluxes.…”

“…Gradient exchange fluxes are determined by taking the integral of Equation across ∂z and using the relationship (where is the friction velocity and is Von Karman's constant) (McGillis et al., 2009). The resulting relationship [] makes use of determined from logarithmic fits to benthic current profiles, which is an assumption that is often problematic in high roughness shallow systems during low flow conditions in the presence of waves (Davis et al., 2020). The gradient exchange technique has been primarily applied in shallow coastal seagrass and reef ecosystems (McGillis et al., 2009; Takeshita et al., 2016; Turk et al., 2016) and could benefit from the new analysis framework applied here by the direct calculation of .…”

An Experimental Investigation of Turbulence Features Induced by Typical Artificial M-Shaped Unit Reefs

“…Previous studies on the turbulent characteristics of natural coral reefs confirm that turbulent mixing controls the vertical coupling between the implantation layer and the overlying water, and it determines the vertical transport of heat, food, pollutants, larvae, or nutrients from the seabed community [40,[43][44][45]. At the same time, an improved understanding of turbulent processes on coral reefs is crucial for the prediction of momentum, energy, and scalar transport, and from an understanding of these basic fluxes, it could be possible to learn more insights about the physics that shape reef ecosystem processes [46,47], sediment suspension and transport [48], and larval settlement on reefs [49].…”

“…Turbulence represents the most effective mixing factor that can be met in nature on small as well as large scales [39]. In coastal areas, turbulence structures affect the chemical and physical interaction between the reefs and the surrounding features [40], and they have a strong impact on the typology of different aquatic organisms that may have dissimilar preferences for either high or low turbulent flow conditions. Therefore, turbulence is considered to be an extremely important factor affecting the larval survival and recruitment of pelagic fish [41,42].…”