Tertiary structure and surface orientation influences on barnacle settlement, contrasted with larval supply

Oyster reef restoration is a common practice in estuaries under stress, where reefs have declined. Reefs provide ecosystem value in the form of habitat, refugia, and hard substrata for sessile attached organisms. Cultch strategy, the material and structural arrangement of common reef restoration efforts, is examined in this study via in situ barnacle settlement experiments. Planktonic larval supply was simultaneously monitored to contrast general supply with substrate accessibility under different cultch arrangements. Cultch constructed from oyster shells was placed in situ in the form of either mats or bags. Bags are a convenient cultch strategy of quick, stackable construction and are used widely in restoration efforts. Mats utilize cultch sparingly, are less stackable, and keep cultch shells spaced out and open to currents. The ivory acorn barnacle (Balanus eburneus) showed substantial settlement, with densities as high as 7 cm^-2. The best settlement correlation with planktonic larval supply was with nauplii (primarily stages II and III) 8 days prior to settlement (R²=0.71). B. eburneus settled at greater densities in mats relative to bags. Orientation of cultch within mats also affected settlement, with highest densities accruing on downward-facing and vertical cultch (p<0.001). Patterns of settlement suggest that excessively high and low flow may hinder settlement, while intermediate flow balances general supply with the opportunity for attachment.