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Oysters are ecosystem engineers that shape coastal aquatic environments through hydrodynamic influence, which is governed by the reef structure. Hydrodynamic studies have investigated effects of oyster reefs as whole systems, overlooking the spatial variability inherent in canopy structures. In this research, a field investigation was undertaken to characterize spatial variability of flow dynamics within a single eastern oyster (Crassostrea virginica) reef and query how the local canopy density influences the hydrodynamic environment. High-resolution flow measurements were taken within and above the oyster canopy over a range of flow conditions. Hydrodynamics were compared across reef locations as the oyster canopy density increased from sparse to very dense. Unique hydrodynamic behaviors were observed within and above all canopies. For example, flow attenuation was more complete within the canopy (70%-99% attenuation of incident flows) as compared to above (48%-65%). Canopies consisting of moderately dense to dense clusters of oysters exhibited similar hydrodynamic behaviors, characterized by high levels of flow attenuation (64%-97%) and turbulent mixing (mean turbulence intensity up to 30 times the shear velocity). Locations with sparse canopy density and the greatest density, where oysters were packed homogeneously rather than clustered, also greatly attenuated flows (70%-99% attenuation of incident flows) but were characterized by lower turbulent mixing (mean turbulence intensity 1-5 times the shear velocity). Flow augmentation (60% increase in flow speed relative to incident flow) was observed only above the homogeneously dense canopy, indicating development of a shear layer above the canopy. Quadrant analysis revealed that turbulent patterns observed in the vicinity of the homogeneous dense canopy were relatively orderly as compared to the sparse, moderately dense, and dense clustered canopies. Study findings quantify hydrodynamic variability found within natural intertidal oyster reefs, with applications to the design of reef-based natural infrastructure and prediction of how reefs may affect flow and sediment transport.
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Benthic Flow and Mixing in a Shallow Shoal Grass (Halodule wrightii) Fringe
Mean flow and turbulence measurements collected in a shallow Halodule wrightii shoal grass fringe highlighted significant heterogeneity in hydrodynamic effects over relatively small spatial scales. Experiments were conducted within the vegetation canopy (~4 cm above bottom) for relatively sparse (40% cover) and dense (70% cover) vegetation, with reference measurements collected near the bed above bare sediment. Significant benthic velocity shear was observed at all sample locations, with canopy shear layers that penetrated nearly to the bed at both vegetated sites. Turbulent shear production (P) was balanced by turbulent kinetic energy dissipation (ϵ) at all sample locations (P/ϵ≈1), suggesting that stem-generated turbulence played a minor role in the overall turbulence budget. While the more sparsely vegetated sample site was associated with enhanced channel-to-shore velocity attenuation (71.4 ± 1.0%) relative to flows above bare sediment (51.7 ± 2.2%), unexpectedly strong cross-shore currents were observed nearshore in the dense canopy (VNS), with magnitudes that were nearly twice as large as those measured in the main channel (VCH; VNS/VCH¯ = 1.81 ± 0.08). These results highlight the importance of flow steering and acceleration for within- and across-canopy transport, especially at the scale of individual vegetation patches, with important implications for nutrient and sediment fluxes. Importantly, this work represents one of the first hydrodynamic studies of shoal grass fringes in shallow coastal estuaries, as well as one of the only reports of turbulent mixing within H. wrightii canopies.
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- PAR ID:
- 10249118
- Date Published:
- Journal Name:
- Geosciences
- Volume:
- 11
- Issue:
- 3
- ISSN:
- 2076-3263
- Page Range / eLocation ID:
- 115
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.3390/geosciences11030115
