A suggested physical explanation for the existence of a biological boundary at 24°30′S in the Benguela system

The distributions of shoals, commercial catches and larvae of pelagic fish species in the Benguela ecosystem suggest that a well-defined biological boundary exists near 24°30′S, in the vicinity of Meob Bay. Available physical oceanographic data, particularly the seasonal distribution of temperature and cube of the wind speed, are examined and reference is made to two case studies consisting of images derived from NOAA-7 and NIMBUS-7, to demonstrate mesoscale variability in sea surface temperature, near-surface chlorophyll and the surface circulation pattern. Surface distributions of temperature, salinity and chlorophyll revealed no significant longshore gradients in the region that could explain the biological boundary, but the wind data suggest that the rate at which kinetic energy is supplied to the surface layers, and hence the turbulent mixing, decreases substantially north of 24°30′S. The effect is seen in vertical temperature profiles along the coast as appreciably stronger stratification on the equatorward side of 24°30′S. The NIMBUS-7 imagery provides evidence of a complex surface circulation pattern in the same region, a factor which may further increase turbulent mixing. It is therefore suggested that it is the combined effect of changes in circulation and turbulence/stratification which causes the biological discontinuity.