In-situ measurements of turbulent flow over intertidal natural and degraded oyster reefs in an estuarine lagoon

Abstract

Oysters are ecosystem engineers that form reefs with rough surfaces. The complex surface of a reference-condition reef contains clusters of live oysters that protrude into and interact with flows. A degraded oyster reef contains fewer live oyster clusters and subsequently has different bed roughness. Such morphological differences may translate to varied hydrodynamic function, with influence to local and larger-scale flow fields and sediment transport, as well as larval recruitment and long-term reef sustainability. The objective of this study is to compare a degraded and a reference-condition intertidal oyster reef, and assess how differences in structure (reef morphology and roughness) influence near-bed hydrodynamic function within the roughness sublayer defined by oysters. Sediment entrainment and bed deformation were observed only on the reference oyster reef, despite that mean velocities were lower than those at the degraded reef. This was attributed to slightly finer bed material and higher turbulent kinetic energy, k, at the reference reef (k = 29.2 ± 4.5 cm²/s² with mean velocity 11.5 ± 0.6 cm/s at 1 cm above the bed) compared to the degraded reef (k = 22.6 ± 4.2 cm²/s² with mean velocity 16.1 ± 1.7 cm/s at 1 cm above the bed). At 1 cm above the bed, shear turbulence production at the degraded reef was significantly larger than turbulence dissipation rate, while at the reference reef this pattern was observed only for time periods with low turbulent kinetic energy. For measurements 5 cm above the bed at the reference reef, shear turbulence production and turbulence dissipation rates exhibited a relatively balanced pattern, with dissipation exceeding shear production for the time period with elevated turbulence. Four bed shear stress calculation methodologies converged over the degraded oyster reef but deviated considerably at the reference oyster reef. The drag coefficient associated with bed shear stress at the reference reef was almost two times greater than that at the degraded reef and almost an order of magnitude greater than those observed at sand and mud beds.

Introduction

Oysters are found in shallow coastal waterbodies and constitute an integral part of many sustainable estuarine ecosystems. Often referred to as ecosystem engineers, oysters form complex reefs of three-dimensional clusters that protrude into the flow, augmenting hydraulic roughness and elevating flow resistance (e.g., Coen et al., 2007). The reef topography and relatively large surface area of oyster clusters exert drag to prevailing channel flows (Wright et al., 1990; Whitman and Reidenbach, 2012; Styles, 2015), altering local flow fields. In shallow profiles, such flow field alterations may direct estuarine flows at larger scales, influencing nearby shorelines through wave or current attenuation and sediment stabilization (Manis et al., 2015; Walters et al., 2017; de Paiva et al., 2018; Wiberg et al., 2018). The heterogeneous reef surface additionally creates differential zones of varied hydraulic habitat (Whitman and Reidenbach, 2012), leading to increased biodiversity of associated invertebrates and fishes (Lenihan and Peterson, 1998; Barber et al., 2010), and transfer of nutrients from pelagic to benthic environments (Kellogg et al., 2013; Chambers et al., 2018) that can increase light penetration for submerged vegetation (Volety et al., 2014). Grabowski et al. (2012) estimate the ecosystem services of oyster reefs to be as much as $99,000 per hectare per year.

Despite the acknowledged beneficial services provided by oysters (Lenihan and Peterson, 1998), there has been significant decline in oyster populations due to excessive harvesting, diseases and coastal degradation (e.g., Kennedy et al., 1996; Grizzle et al., 2002). Beck et al. (2011) estimate that oyster reef ecosystems have decreased globally by 85%, while in some bays, such as the upper Chesapeake Bay (Wilberg et al., 2011), the total oyster loss reaches 99%. While oyster restoration programs have demonstrated success in increasing oyster numbers at site and local scales (Nestlerode et al., 2007; Garvis et al., 2015), the scale of reef restoration is small as compared to historic habitat losses (Bersoza Hernández et al., 2018). To promote both successful reef restoration and natural recovery of degraded reefs, a more complete understanding of flow-reef interactions and how they vary across reef conditions is necessary.

Structural differences between intact, reference-condition reefs and degraded reefs may translate to varied function with respect to flow interaction. We consider a degraded reef to contain a low density of live oysters as compared to a reference-condition reef. Such ecological differences are likely to exert strong control to reef roughness and the dominant hydrodynamic interactions, which may in turn affect the provisioning of desired ecosystem services or may influence likelihood of reef recovery. For instance, interaction with prevailing flows, as mediated by reef structure, will influence the potential for reefs to modify flow energy, sequester organic matter, and maintain stability of sediments. Oyster clusters that protrude into the flow transform mean kinetic energy of the flow to turbulence, similarly to flows through arrays of obstacles (Chang and Constantinescu, 2015; Kitsikoudis et al., 2016; Yagci et al., 2017), with such turbulence cues attracting swimming larvae (Hubbard and Reidenbach, 2015) that propel themselves to the bed to settle (Fuchs et al., 2013). The complex reference-condition reef topography provides sheltered areas with low shear stresses (Whitman and Reidenbach, 2012), offers protection from predators (Nestlerode et al., 2007), and inhibits oyster burial from excessive deposition (Wall et al., 2005), which support the metamorphosis of larvae to spat and subsequent growth. The limited number of live oysters and the lack of structural complexity on a degraded reef may hinder the provision of such suitable boundary conditions and lead to an unfavorable flow environment for future oyster recruitment and proliferation.

Restoration or oyster reef enhancement efforts will benefit from better understanding of how reef characteristics influence turbulent boundary layer flows. Despite the potential for reef structure and ecological condition to control hydrodynamic interactions, this influence has not been demonstrated empirically. While previous studies have analyzed the flow field above the oyster roughness sublayer (Whitman and Reidenbach, 2012; Reidenbach et al., 2013; Styles, 2015), there are no detailed field measurements within this roughness sublayer on intact natural reef and comparison of these with similar measurements on degraded oyster reef. Such measurements are necessary to properly analyze the flow field within the roughness sublayer formed by oysters, where oyster larvae attach and grow. The objective of this study is to compare a degraded and a reference-condition intertidal reef to assess how differences in structure (reef morphology and roughness) influence near-bed hydrodynamics.