Impact of high-density suspended oyster culture on benthic sediment characteristics

• Experimental floating rafts holding oysters were moored in shallow water (1–2 m).
• The oysters were stocked at high densities (16 and 24 kg oysters m−2).
• The oysters produced no discernible biodeposition or benthic enrichment signatures.
• Rather, the sediment collected under the rafts had relatively low organic content.
• The rafts possibly impacted local hydrodynamic processes, causing seabed erosion.

Benthic impacts that may ultimately result from further intensification of suspended oyster culture in eastern Canada were proactively investigated. Eastern oysters (Crassostrea virginica) were placed in experimental floating rafts (12.1 m2) designed to hold densities 2–3 times higher than those presently found in floating bag operations. Experimental rafts were moored in shallow water (1–2 m) at fixed positions where no aquaculture had been practiced. A biodeposition model predicted that the majority of feces released by suspended oysters would fall onto the seabed area directly beneath the rafts. However, field measurements over a 132 d period indicated that the fecal deposition from the highly aggregated oysters was not reflected in higher organic sedimentation rates or seabed sulfide levels. Rather, the proportion of organic matter in the top sediment layer was significantly lower in samples collected underneath rafts (5.1 ± 1.5%) than in samples taken at reference sites (10.5 ± 3.2%). This same pattern was observed for control rafts holding shells only. It is suggested that the floating raft impacted local hydrodynamic processes, forcing water to move underneath the structure, thereby amplifying turbulence and resuspending low shear strength particles such as biodeposits.