Importance of sedimentation for survival of canopy-forming fucoid algae in urchin barrens

Sedimentation can provide indirect benefits to the survival of macroalgae in areas with potentially high grazing pressure. Field studies were performed in an embayment with extensive urchin barrens but also with locally persistent fucoid beds, on the coast of Kagoshima, south-western Japan, to elucidate the physical and biological processes responsible for the maintenance of the beds. Rocky subtidal reefs devoid of fine sediment were almost completely barren and dominated by sea urchins (primarily Echinometra sp. A), while fucoid algae (predominantly Sargassum duplicatum) densely populated cobble sites overlaid with a thin layer of fine sediment (medium grain size: 0.15–0.25 mm). Quadrat samplings in areas intermixed with urchin barrens and sand flats as well as experimental addition of sediment suggested that Echinometra sp. was readily excluded from hard substratum overlaid with even only a thin layer of fine sediment. Quadrat surveys and a transplant experiment conducted at the border area between a cobble bed with a thin cover of fine sediment and a sediment-free boulder one indicated that sea urchins (mainly composed of Echinometra sp. and Diadema spp.) rarely invaded the sediment-covered bed to graze. Wave measurements at the entrance (8 m deep) of the embayment over a 3.5-year period showed that the study area had long-term extremely calm conditions (84% of significant wave heights < 0.1 m) and seasonal or transient moderate disturbances due to relatively high waves (significant wave height: 0.8–1.3 m). However, the 2-year time series of root-mean-square wave orbital velocity estimates at different sites consistently suggested that the wave-action intensities at urchin barrens were still too high for deposition of fine sediments which occurred in more wave-sheltered persistent fucoid area. Nearly 2-year investigations on sediment level change and on cobble substrates together with overlying sediment in the fucoid area suggested that the absence of fatal sediment inundation and maintenance of the thin overlying sediment layer (mean thickness < ca 2 mm) throughout the year allowed settlement and growth of sand-tolerant fucoids by preventing urchin grazing.