Document: THE-3-78-7

Community structure of southern African sandy beaches.

DONN, JR., T.E.* ¹, A.MCLACHLAN ² and P.H.DUTOIT ³

Tetra Tech, Inc., Lafayette, CA 94549 USA ¹
Sultan Qaboos University, Muscat, Oman ²
University of Port Elizabeth, Port Elizabeth, South Africa ³

Abstract:
Southern Africa has a coastline of approximately 4250 km of which 43% is composed of sandy beaches and a further 31% of mixed shores. Three biogeographic regions are recognized: 1) The West Coast, cold temperate province dominated by the upwelling Benguela Current; 2) The South Coast, a warm temperate province; and 3) The sub-tropical Natal province dominated by the south-flowing Aghulas Current. The purpose of this study was to examine the intertidal community structure of sandy beaches along the Namibian and South African coastlines with an aim to further clarifying their biogeography and to provide insights on the influence of physical factors on community structure. Physical measurements including grain size, TOC, beach slope, and beach state were taken at the 103 beaches visited during this survey. The intertidal macrofaunal community on 45 beaches was sampled using transects composed of triplicate 0.1 m² quadrats at 10 levels from the drift line to just below the low water line. A total of 59 taxa were collected. The number of species per beach averaged 11 and ranged from 4 to 23. Peracarid crustaceans were the most abundant and diverse of the major groups. Biomass was typically dominated by bivalves along the west and south coasts. Three types of distributional patterns were observed. Three genera, the molluscs Donax and Bullia and the mysid Gastrosaccus, exhibited a replacement of one species by another along the coast. Ubiquitous species (Eurydice longicornis) and species with clear distributional limits (Glycera natalensis) were also identified. Cluster analysis yielded five clusters of sites: Namibian site cluster; West Coast cluster; South Coast cluster; Transitional cluster; and a Natal cluster. Ordination analyses confirm the presence of five biogeographic zones. Mean annual water temperature was highly correlated with the first ordination axis. Other physical factors showed only weak correlations. The Namibian sites clearly separate from the West Coast stations along the second ordination axis, providing evidence for a unique beach community in northern Namibia. These results indicate that there are five distinct biogeographic zones along the southern African coast, largely defined by temperature.

Keywords: Sand beach, biogeography

This abstract is being presented at: 3:30 PM in session:
Poster Session #17: Vegetative Analysis.