Gradients and instability: Macrozoobenthic communities in the Benguela Upwelling System off Namibia

Abstract

This is the most comprehensive study to date on the distribution of macrofaunal assemblages of the Oxygen Minimum Zone (OMZ) off Namibia, one of the least studied benthic marine macrofaunas in the world. OMZs are mid- and bottom-waters at continental margins characterized by dissolved oxygen concentrations lower than 0.5 ml O2 l−1. We present the distribution of macrofaunal assemblages between 17°S and 25°S latitudes and between 25 m and 1523 m water depth with a total of 534 taxa. We performed both abundance-based and biomass-based hierarchical clustering analysis, which differ slightly in the resulting assemblages. This difference is due to the greater importance of molluscs in the biomass-based clustering, while the abundance-based analysis seems to shift the importance to the short-lived crustaceans. The Namibian benthic macrofauna, in general, shows high total biomasses and high representativeness of molluscs compared to OMZs worldwide. Evidences suggest that temperature, depth, diffusive hydrogen sulphide flux, interseasonal dissolved oxygen average and dissolved oxygen seasonal variability as significant forces structuring benthic macrofauna in the Namibian shelf. Multivariate analysis defined 6 faunal assemblages. The deepest and shallowest assemblages were the most diverse, but biomass is low in the first and the highest in the second. The major assemblages within the OMZ core areas are found at an interseasonal average dissolved oxygen below 0.4 ml l−1, all of which co-occur quite frequently with diffusive hydrogen sulphide fluxes. Dead spots are found at an interseasonal average dissolved oxygen level below 0.05 ml l−1.