Seasonal changes in phytoplankton biomass on the western Agulhas Bank, South Africa

Abstract

Data on temporal and spatial changes in phytoplankton biomass and distribution on the western Agulhas Bank during the main spawning season of pelagic fish were obtained from monthly cruises conducted between August and March in 1993/94 and September and March in 1994/95. The period was divided into three oceanographic seasons based on different levels of upwelling activity: late winter (August and September), spring (October–December) and summer (January–March). Cross-shelf and vertical distribution patterns of chlorophyll changed markedly during these seasonal periods, reflecting changes in hydrographic structure and in nutrient availability. During late winter, chlorophyll was evenly distributed in the deep, upper-mixed layer (>40 m) across the shelf. A clump-forming Thalassiosira sp. contributed to the moderately high mean chlorophyll concentration (1.9 mg·m⁻³) in the upper 30 m. In October and/or September, warming of surface waters inshore gave rise to a modest (2–5 mg·m⁻³) spring bloom, typical of the temperate zone. This was terminated in November by an influx across the shelf of warm, nutrient-impoverished water. Upwelling was sporadic and weak in spring. Summer was characterized by intense, episodic upwelling inshore, with pronounced cross-shelf thermal gradients, intensified by the presence of water of Agulhas origin along the shelf-edge. During an upwelling cycle, rapid hydrographic and biological changes occur over four phases: onset of upwelling, sustained upwelling, quiescence and downwelling. The upwelling productive zone, bounded by the 20°C isotherm, varied from <12 to 50 miles across the shelf. Chlorophyll was low in newly upwelled water (<0.5 mg·m⁻³) and attained peak concentrations in mature upwelled water (5–25 mg·m⁻³). Outside the productive zone, mean chlorophyll levels were low <0.5 mg·m⁻³). Enhanced phytoplankton growth during the spring bloom in September and/or October may be an important factor contributing to spawning success of pelagic fish in the southern Benguela.