Macrozoobenthos diversity in an oxygen minimum zone off northern Namibia

Abstract

A benthological survey in the Benguela upwell- ing area oV northern Namibia (located at 17.3°S and water depth ranging between 26 and 117 m) showed the concen- tration of dissolved oxygen and the accumulation of organic-rich sediments to control macrozoobenthic com- munity patterns. In contrast to highly biodiverse nearshore areas with well-structured shell deposits of the brachiopod Discinisca tenuis (Sowerby 1847), the benthic community in the oxygen minimum zone (OMZ) decreased strongly in species numbers. Nevertheless, a well-established commu- nity ranging from 13 to 31 species persisted. Species densi- ties (300-3,350 ind m i2 ) and biomass (4-109 g afdw/m 2 ) were surprisingly high for areas with near bottom oxygen concentrations from 0.06 to 0.88 ml l i1 . In contrast to OMZ's of other upwelling areas, where the benthic macro- fauna is generally dominated by small-bodied polychaetes, oV Namibia larger key organisms like the bivalve Nuculana bicuspidata (Gould 1845) and the snail Nassarius vinctus (Marrett 1877) accounted for a large proportion of the mac- rozoobenthos >1 mm. This is supposed to have a distinct eVect on the functional properties of the sediments.