The Role of Bottom Current and Estuarine Geomorphology on the Sedimentation Processes and Productivity of Wreck Shoal, an Oyster Reef of the James River, Virginia

Abstract

Wreck Shoal is a subtidal oyster reef in the James River, Virginia, Two significantly different types of oyster reefs are found in adjacent areas on Wreck Shoal. The ‘hard-rock’ reefs are characterized by a relatively thick shell layer, higher densities of live oysters, a coarser interstitial sediment, and a negligible sediment cover. In contrast, the ‘mud shell’ reefs are characterized by a very thin oyster shell layer, considerably lower densities of live oysters, a finer interstitial sediment, and a 1–2 cm layer of very fine sediments covering the reef. The ‘hard rock’ oyster reefs are flourishing with respect to oyster productivity and shell deposition, and are non-accretional with respect to fine sediments. In contrast, the ‘mud-shell’ oyster reefs are marginal in oyster productivity and shell deposition, and are accretional with respect to fine sediments.

The results of a mirco-circulation study of Wreck Shoal indicate considerable spatial variation in the strength of the bottom current regime. The magnitude of the bottom current is inversely correlated to mean water depth. Temporal variations in the bottom current are accounted for by a net downstream current and a strong periodic tidal component. The ‘hard-rock’ oyster reefs experience relatively strong bottom currents over a hydraulically rougher bottom, while the ‘mud-shell’ oyster reefs experience relatively weak bottom currents over a hydraulically smoother bottom. The bottom shear stress and therefore sediment transport power of the bottom currents for the two oyster reefs are substantially different and this is reflected in the observed sedimentation process. The contemporary sedimentation processes and the productivity of the oyster reef are correlated to the magnitude of the bottom current and to the geomorphology of the shoal.