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

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Nutrient and dissolved organic carbon (DOC) distributions were determined in July 1999 in the northern Benguela upwelling and Angola-Benguela front area. Highest silicate and phosphate surface concentrations of up to 30 µM and 3.7 µM, respectively, were determined in recently upwelled waters between 19°S and 21°S 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 µM and 720 µM, respectively), probably due to mineralisation 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 surround the upwelling. Contrary 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 ~100 m depth. Silicate increased exponentially from off- to onshore by the factor ~10, phosphate increased by ~30%. Regenerated silicate was ~25 µM, phosphate ~0.5 µM. Nitrate was not regenerated and oscillated apparently randomly between 11 µM and 24 µM at 100 m depth. Ammonium and nitrite increased exponentially from off- to onshore, indicating mineralisation 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 evidences for considerable nitrogen losses off Namibia. Denitrification, which is favoured by the oxygen deficit in source waters, is the probable reason for these losses. Since denitrification was disregarded in the past, the productivity of northern Benguela and its role as carbon sink has presumably been overestimated.

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Dittmar, T. and Birkicht, M. (2001): Regeneration of nutrients in the northern Benguela upwelling and the Angola-Benguela Front areas , South african journal of science, 97, 2001 , pp. 239-246 .

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