Dittmar, Thorsten and Birkicht, Matthias ORCID: https://orcid.org/0009-0006-0386-0278 (2001) Regeneration of nutrients in the northern Benguela upwelling and the Angola-Benguela Front areas : BENEFIT Marine Science. South African Journal of Science, 97 (5). pp. 239-246.

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Abstract

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 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 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 ~100 m depth. Silicate increased exponentially from off- to inshore by a factor of ~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 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. Since denitrification was disregarded in the past, the productivity of the northern Benguela and its role as a carbon sink have presumably been overestimated.

Document Type:	Article
Programme Area:	PA Not Applicable
Research affiliation:	Infrastructure > Chemistry Laboratory
Refereed:	Yes
Open Access Journal?:	Yes
ISSN:	1996-7489
Date Deposited:	30 Sep 2025 12:41
Last Modified:	30 Sep 2025 12:41
URI:	https://cris.leibniz-zmt.de/id/eprint/5720

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