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

Data on temporal and spatial changes in phytoplankton biomass and distribut ion 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 chloroph yll changed markedly during these seasonal periods, reflecting changes in h ydrographic structure and in nutrient availability. During late winter, chl orophyll was evenly distributed in the deep, upper-mixed layer (>40 m) acro ss the shelf. A clump-forming Thalassiosira sp. contributed to the moderate ly 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 chl.m(-3)) spring bloom, typical of the temperate zone. Thi s was terminated in November by an influx across the shelf of warm, nutrien t-impoverished water. Upwelling was sporadic and weak in spring. Summer was characterized by intense, episodic upwelling inshore, with pronounced cros s-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, sustaine d upwelling, quiescence and downwelling. The upwelling productive zone, bou nded by the 20 degrees C isotherm, varied from <12 to 50 miles across the s helf. Chlorophyll was low in newly upwelled water (<0.5 mg.m(-3)) and attai ned peak concentrations in mature upwelled water (5-25 mg.m(-3)). Outside t he productive zone, mean chlorophyll levels were low (<0.5 mg.m(-3)). Enhan ced phytoplankton growth during the spring bloom in September and/or Octobe r may be an important factor contributing to spawning success of pelagic fi sh in the southern Benguela.