Spatio-temporal patterns of C : N : P ratios in the northern Benguela upwelling system

[摘要] On a global scale the ratio of fixed nitrogen (N) and phosphate (P) ischaracterized by a deficit of N with regard to the classical Redfield ratioof N : P = 16 : 1 reflecting the impact of N loss occurring in the oceanicoxygen minimum zones. The northern Benguela upwelling system (NBUS) is knownfor losses of N and the accumulation of P in sub- and anoxic bottom watersand sediments of the Namibian shelf resulting in low N : P ratios in thewater column. To study the impact of the N : P anomalies on the regionalcarbon cycle and their consequences for the export of nutrients from the NBUSinto the oligotrophic subtropical gyre of the South Atlantic, we measureddissolved inorganic carbon (C_T), total alkalinity (A_T), oxygen(O₂) and nutrient concentrations in February 2011. The results indicateincreased P concentrations over the Namibian shelf due to P efflux fromsediments resulting in a C : N : P : -O₂ ratio of 106 : 16 : 1.6 : 138.N reduction further increase C : N and reduce N : P ratios in those regionswhere O₂ concentrations in bottom waters are < 20 μmol kg⁻¹.However, off the shelf along the continental margin, the mean C : N : P : -O₂ ratio is again close to the Redfield stoichiometry.Additional nutrient data measured during two cruises in 2008 and 2009 implythat the amount of excess P, which is created in the bottom waters on theshelf, and its export into the subtropical gyre after upwelling variesthrough time. The results further reveal an inter-annual variability ofexcess N within the South Atlantic Central Water (SACW) that flows from thenorth into the NBUS, with highest N values observed in 2008. It is postulatedthat the N excess in SACW occurred due to the impact of remineralizedorganic matter produced by N₂ fixation and that the magnitude of excessP formation and its export is governed by inputs of excess N along with SACWflowing into the NBUS. Factors controlling N₂ fixation north of the BUSneed to be addressed in future studies to better understand the role of theNBUS as a P source and N sink in the coupled C : N : P cycles.

[学科分类] 地球化学与岩石