ECOLOGY OF INTERTIDAL MACROALGAL ASSEMBLAGES ON THE HADRAMOUT COAST OF SOUTHERN YEMEN, AN AREA OF SEASONAL UPWELLING

The species composition and ecology of well-developed macroalgal assem blages that occur on rocky shores of the Hadramout coast of southern Y emen were studied between January 1988 and February 1990. 163 taxa wer e collected. Qualitative data supported by quantitative biomass sample s confirm that there is a very marked seasonal pattern in the growth o f the algal species which also show a clear zonation from upper to low er intertidal. The majority of species show greatest growth during the late summer and autumn (August-September); a much smaller period of g rowth partly involving different species occurs in the spring (Februar y-March), but during midsummer (May-June) no algal growth is apparent. The late summer/autumn period of maximum growth (with algal biomass t ypically 5 to 10 times that during spring) follows the onset of the so uthwest 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 loc al climatological data support the view that the dense autumn growth o f intertidal algae occurs as a result of elevated nutrient levels cons equent upon this upwelling. The smaller peak in growth in spring is al so associated with some elevated nutrient levels, but further study is needed to determine whether this is linked to the onset of the northe ast monsoon or to other seasonal factors.