Distribution of nutrients, chlorophyll and phytoplankton primary production in relation to hydrographic structures bordering the Benguela-Angolan frontal region

Abstract

The aim of this study was to determine the characteristic rates of phytoplankton production in water bodies bordering the Benguela-Angolan frontal region and differing in their hydrographic, hydrochemical and biological properties. Two cruises were undertaken in the vicinity of the Cape Frio upwelling cell and the Angola-Benguela front in April–May 1997 and August–September 2000. In all, seven bodies of water were identified in terms of their temperature, salinity, primary production, mixing depth, euphotic depth, Secchi depth, and concentrations of nitrate, phosphate, silicate, oxygen and chlorophyll a. The distribution and properties of these bodies of water were similar for the two seasons investigated. The Cape Frio upwelling cell was the most productive area, with a primary production rate of 4 145mgC m⁻² day⁻¹ and a chlorophyll a content in the euphotic zone of 114mg m⁻². Upwelling off southern Angola was less intense and the Angola-Benguela frontal zone separated matured Benguela water from subtropical surface water. The ratio of mixed layer depth to euphotic depth was >1 south of the front and <1 north of the front, leading to the presence of deep chlorophyll and primary production maxima in the region of the subtropical surface water. Knowledge of these deep chlorophyll maxima is important in deriving estimates of primary production from satellite imagery. Estimates of the annual primary production for each of the water bodies were calculated. The Cape Frio upwelling cell, including the nearshore matured water, accounted for an annual net production of 84 × 10⁶ tonsC.

Keywords:

• ANGOLA-BENGUELA FRONT
• CHLOROPHYLL
• EUPHOTIC ZONE
• MIXED LAYER
• NUTRIENTS
• PRIMARY PRODUCTION
• UPWELLING