Nutrients and oxygen in the upwelling area off NW-Africa : Expedition "Auftrieb '72"

Abstract

Biological, hydrographical and chemical parameters were analysed from surface survey maps, drogue experiments and tracks in the region between Cap Blanc and Cap Timiris off NW Africa between January and March 1972 during the "Meteor"­ Expedition "Auftrieb '72". The chemical data measured included the nutrients: nitrate, nitrite, ammonia, silicate and phosphate as well as oxygen, particulate organic carbon and particulate organic nitrogen. In the area under investigation three different watermasses could be defined by nutrient content and salinity: namely "South Atlantic Central Water" (SACW), "Old Surface Water" and water from the Banc d' Arguin ("Bankwater"). The nutrient content and salinity of the fresh upwelled water at the surface showed that it consisted mainly of SACW that had always been mixed with one of the other watermasses. Mixing between Bankwater and SACW probably takes place close to the seabed on the shelf edge where cells of Bankwater were found several times. The surface survey maps show active upwelling only on the onshore ends of shelfedge canyons. This spatial heterogenity together with the discontinuities in the upwelling processes might explain the fact that no processes with time could be followed during buoyed parachute drogue experiments. The atomic ratio of nutrient uptake was found to be about N: Si: P = 15: 12 : 1 for the first few days of phytoplankton uptake in fresh upwelled water. From a comparison of the original concentration of the nutrients in fresh upwelled water and the calculated uptake, it follows that silicate might pose as a limiting factor after some days. Two sets of data from drogue experiments indicate that biological oxygen production as estimated from nitrate uptake, is sufficient to explain the oxygen input to fresh upwelled water. Thus physical solution of oxygen from the air appears to be slight