Regeneration of nutrients in the northern Benguela upwelling and the Angola-Benguela Front areas

Nutrient and dissolved organic carbon (DOC) distributions were determined in July 1999 in the northern Benguela upwelling and Angola-Benguela Front areas. Highest silicate and phosphate surface concentrations of up to 30 muM and 3.7 muM, respectively, were determined in recently upwelled waters between 19 degreesS and 21 degreesS off Namibia. Nitrate, on the other hand, exhibited there a local minimum, which indicates an advanced bloom of non-siliceous phytoplankton. Nitrate and DOC concentrations increased with distance from the upwelling centre (up to 15 muM and 720 muM, respectively), probably due to mineralization of phytoplankton-derived organic compounds, whereas silicate strongly decreased. Growth of siliceous phytoplankton, which covered their nitrogen requirements by nutrient recycling within the photic layer, probably caused this pattern in aged waters surrounding the upwelling. In contrast to primary production in the upwelling centre, this phytoplankton growth was therefore not 'new production'. Primary production was presumably limited by nitrate in recently upwelled waters and by silicate in aged waters. Phosphate was probably not limiting, indicated by low N/P ratios in surface waters (< 10) and low surface depletion. Regeneration of silicate and phosphate was evident in source waters of upwelling in similar to 100 m depth. Silicate increased exponentially from off- to inshore by a factor of similar to 10, phosphate increased by similar to 30%. Regenerated silicate was similar to 25 muM, phosphate similar to0.5 muM. Nitrate was not regenerated and oscillated apparently randomly between 11 muM and 24 muM at 100 m depth. Ammonium and nitrite increased exponentially from off- to onshore, indicating mineralization of nitrogenous compounds, but contributed only 3% to dissolved inorganic nitrogen on average. In the front area no evidence for nutrient trapping was found. The lack of nitrogen regeneration and strongly decreasing N/P and N/Si ratios shoreward are evidence for considerable nitrogen losses off Namibia. Denitrification, which is favoured by the oxygen deficit in source waters, is the probable reason for these losses. trice denitrification was disregarded in the past, the productivity of the northern Benguela and its role as a carbon sink have presumably been overestimated.