Hydrodynamics Affecting Larval Transport and Settlement onto Intertidal Oyster Reefs

The Eastern Oyster (Crassostrea virginica) once played a pivotal ecological role in Virginia waters and the Chesapeake Bay. However, unregulated over-harvesting, combined with reduced water quality and disease, has caused a drastic decline in oyster populations, such that present day levels are less than 2% pre-harvest populations. Restoration efforts, currently underway to re-establish healthy oyster populations, are focused on rehabilitating benthic habitat to be suitable for natural oyster larval recruitment and growth. The goal of this study was to understand the hydrodynamics involved in fluid and sediment transport over reefs, and how these dynamics may impact larval transport to healthy and restored reef areas.

Velocity and turbulence data was collected off of the Eastern shore of Virginia over multiple benthic surfaces including a mud flat, a healthy reef, and two restoration sites comprised of either fossil oyster or whelk shell. Reynolds stresses, shear velocity, u*, and drag coefficients, CD, were computed and due to the extreme roughness of the reef, mean estimates of u* over a healthy reef were found to be 47% greater than those found over a restoration site. Enhanced shear increased both turbulent mixing and drag above the reef, but within the interstitial areas between individual oysters, mean velocities and turbulent otions were reduced. CD, used as a measure of roughness, also increased with elevation on the healthy reef.

Small-scale hydrodynamic forces were studied in an open-channel, recirculating, water flume along benthic roughnesses of varying height and spacing, used to mimic variability found on the reef. Drag and lift forces within the structure decreased with increasing height and increased with increased spacing. Geometrically similar slate tile structures were deployed in the field over a five month period, and the greatest larval recruitment corresponded closely to locations where drag and lift forces were reduced. The combined field and laboratory data suggests that restoration efforts should consider both elevation and bed roughness similar to those found on healthy oyster reefs, to provide suitable hydrodynamic conditions that promote larval recruitment, prevent burial by sediment, and may provide some refuge from predation.

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