Ecology of intertidal macroalgal assemblages on the Hadramout coast of southern Yemen, an area of seasonal upwelling

Abstract

The species composition and ecology of well-developed macroalgal assemblages that occur on rocky shores of the Hadramout coast of southern Yemen were studied between January 1988 and February 1990. 163 taxa were collected. Qualitative data supported by quantitative biomass samples confirm that there is a very marked seasonal pattern in the growth of the algal species which also show a clear zonation from upper to lower intertidal. The malority of species show greatest growth during the late summer and autumn (August-September); a much smaller period of growth partly involving different species occurs in the spnng (February-March), but during midsummer (May-June) no algal growth is apparent. The late summer/autumn period of maximum growth (with algal biomass typically 5 to 10 times that during spring) follows the onset of the southwest monsoon which is known to generate intense seasonal upwelling of cold nutrient-rich water along the southern Arabian coast, regarded as one of the 5 major upwelling areas within the world ocean system. Measurement of near-shore oceanographic parameters and analysis of local climatological data support the view that the dense autumn growth of intertidal algae occurs as a result of elevated nutrient levels consequent upon this upwelling. The smaller peak in growth in spring is also associated with some elevated nutrient levels, but further study is needed to determine whether this is linked to the onset of the northeast monsoon or to other seasonal factors.