Hikkaduwa rocky reef brown seaweed zonation: Sargassum ilicifolium holdfast attachments?
What wave hydrodynamic forces govern the intertidal zonation of brown macroalgae (Sargassum and Turbinaria) on exposed granite reef shelves in southern Sri Lanka?
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7 Answers from travellers
Intertidal rocky shelves at Hikkaduwa and Galle exhibit sharp macroalgal zonation shaped by wave exposure and tidal desiccation. The high-energy surf zone is dominated by brown seaweeds: Sargassum ilicifolium and Turbinaria ornata. These algae anchor to basalt and granite reef platforms using disc-like, cellular holdfasts that secrete phenolic bio-adhesives capable of withstanding hydraulic drag forces exceeding 25 knots of crashing surf. Gas-filled air bladders (pneumatocysts) keep algal fronds buoyant at high tide to maximize sunlight capture for photosynthesis.
Dense Sargassum beds serve as essential grazing grounds for herbivorous reef fish like marbled spinefoot (Siganus rivulatus) and juvenile green sea turtles.
Coastal communities harvest red macroalgae (Gracilaria edulis / Ceylon Moss) in calm lagoons to extract food-grade agar for cooling summer puddings.
At low spring tides, Turbinaria thalli resist desiccation through thick, pyramid-shaped fleshy blades that trap water within cellular tissues.
Commercial harvesting of marine algae inside declared national park sanctuary borders requires research clearance from the Department of Wildlife Conservation.
Walking across wet seaweed-covered granite shelves is hazardous; wet Sargassum forms a slick biological film that makes rubber shoes slip.
Seasonal monsoonal storm waves tear drifting mats of Sargassum from reef shelves, depositing rich organic wrack along sandy beaches.
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