Factors Structuring a Subtidal Encrusting Community in the Southern Gulf of California
Abstract:Subtidal encrusting communities on adjacent horizontal and vertical surfaces of a tropical rock reef in the southern Gulf of California were quantitatively described and experimentally manipulated. The horizontal rock surfaces supported a high cover of algal turf and consolidated sediments, while the vertical rock surfaces supported a high cover of crustose corallines, prostrate Padina, and sessile invertebrates. Shading and grazer-exclusion experiments revealed that light, grazing, and topographic complexity were important determinants of community composition. The horizontal surfaces of the reef were exposed to higher light intensities and grazing pressure than the vertical surfaces. This was due not only to the difference in slope but also to the more rugose nature of the vertical walls, the latter enhancing shading and providing refuges from grazers. Erect filamentous and fleshy algae out-competed crustose algal species and sessile invertebrates for space on the high-light horizontal surfaces. Intense grazing on horizontal surfaces resulted in the maintenance of an algal-turf assemblage. Crustose corallines, prostrate Padina, and sessile invertebrates were favored by the low light intensities of the vertical rock surfaces due to a reduction in competition with erect algal species. Reduced grazer success further enhanced the survival of sessile invertebrates utilizing spatial refuges on the vertical walls. Results suggest that many a-symbiotic sessile-invertebrates may be restricted to cryptic habitats in the tropics owing to competition with benthic algae as well as intense grazing.
Document Type: Research Article
Publication date: May 1, 1999
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