Long-term trends in the abundance and community structure of coastal zooplankton in the southern Benguela system, 1951–1996

Abstract

Long-term trends in the abundance and species composition of zooplankton are described. These trends are based on retrospective analysis of zooplankton samples collected in the St Helena Bay area off the South African west coast since the development of the pelagic fishery in the 1950s. Samples were selected from seven sampling programmes (1951–1967, 1974, 1978, 1983, 1984, 1987 and 1988–1996) that monitored the main commercial fishing grounds during peak recruitment (March–June). Abundance data are presented for size-based taxonomic groups, which include cyclopoid and small, medium and large calanoid copepods, cladocerans, euphausiids, amphipods, chaetognaths and pelagic tunicates. Despite discontinuity in the time-series and the use of different samplers, all taxa showed a significant long-term increase in their abundance by at least one order of magnitude between 1951 and 1996. Total zooplankton abundance increased from 4.2×10³ to 7.2×10⁵ ind·m⁻² over the same period. Multiple regression analysis of the log-transformed data indicates size-based differential rates of increase in population levels among the crustacean size classes, with a maximum rate in cyclopoids (slope = 0.052) and a minimum rate in euphausiids (slope = 0.035). This resulted in a significant shift through time in the crustacean zooplankton community structure, which may reflect differential size-selective predation during periods of anchovy Engraulis capensis or sardine Sardinops sagax dominance. The observed long-term increase in zooplankton abundance could be a response to a long-term decrease in predation pressure following decreases in stocks of pelagic fish, which in turn could be caused by increased predation by top predators. Another mechanism contributing to the long-term increase in zooplankton is the observed long-term intensification of coastal upwelling, which could enhance primary and secondary production, and increase advective input of zooplankton populations into the study area and augment their retention.