Résumé
This article reviews the variability of plankton over time scales ranging from mesoscale upwelling events of a few days' duration to decadal scale changes in the northern and southern subsystems in the Benguela Current. It focuses on the plankton that are considered important for fish, particularly the crustacean zooplankton. The southern Benguela is strongly pulsed over periods of 4-12 days with a series of upwelling events modulated by passing cyclonic weather systems. The northern Benguela is less pulsed with short-term variability linked to continental shelf waves. Upwelling is particularly active at seven major sites in the Benguela system. Dense phytoplankton blooms develop in the cool nutrient-rich plumes, which merge and blend with surrounding waters, creating a broad band of phytoplankton-rich water over the shelf. Species succession from small to large diatoms, dinoflagellates and small flagellates occurs as the waters mature after upwelling and generally move offshore, although numerous exceptions occur, with small-celled communities occasionally dominant in nearshore waters. Much regeneration and recycling of nutrients occurs, resulting in lower than expected f-ratios. Frontal zone aggregations provide important feeding opportunities in the transport phase of ichthyoplankton between the Agulhas Bank spawning grounds and the nursery grounds on the South African West Coast. The Angola-Benguela front in the northern Benguela is also an important region for pelagic fish spawning.
Seasonal changes in wind forcing indicate maximum upwelling in spring and autumn throughout the Benguela, with a tendency for a summer maximum in the south. Luderitz (25 degrees S) and Cape Frio (17 degrees S) are particularly active upwelling regions. Phytoplankton biomass, estimated as chlorophyll a, shows a winter maximum in the northern Benguela and a summer maximum in the southern Benguela. The Luderitz area shows perennial phytoplankon minima, possibly due to strong turbulence. The central Namibian shelf and the South African west coast shelf have persistently high phytoplankton biomass. A seasonal intrusion of warm oligotrophic water from Angola in late summer (December to March) results in strong contrasts between winter and summer in the extreme northern Benguela. Zooplankton biomass shows different cycles along the coast, with spring, summer and autumn maxima in the south, and a slight maximum during the second half of the year (July to December) off central Namibia. The dominant fish spawning period is spring-summer throughout the region.
Long-term changes in the southern Benguela include a significant increase in zooplankton over the past five decades, with a decline since 1995. Fish abundance has declined in the northern Benguela but remained reasonably stable in the southern Benguela until 2000, when pelagic fish biomass increased dramatically with concomitant declines in zooplankton biomass.
A range of modelling exercises, including expert systems, statistical models and linked IBM-hydrodynamic models, has been compared to or derived from field data, and has stimulated new observational programmes at improved space and time scales. Observational data at pertinent time and space scales are lacking in the northern Benguela system, which will hamper validation of prognostic and diagnostic models.