Composition of macrozooplankton assemblages associated with the formation and decay of pulses within an upwelling plume in greater cook strait, New Zealand

Abstract

The structure of macrozooplankton assemblages associated with an upwelling plume was studied at an active upwelling site located in greater Cook Strait, New Zealand. Zooplankton within the plume divided into five geographically coherent assemblages, generally reflecting temporal changes in the zooplankton community as the upwelled water of the plume advected from west to east into greater Cook Strait—as indicated by the movement of three separate pulses of upwelled water. Near the source of the upwelling plume, inshore zooplankton biomass decreased as did the numbers of several coastal species, and oceanic species were introduced into nearshore waters. The capacity of copepods and the euphausiid Nyctiphanes australis to reproduce was reduced, and the diversity indices and proportion of herbivorous copepods decreased, relative to water south of the plume. Downstream from the plume source, in the eastern part of the upwelling plume, many zooplankton species were distributed in a manner reflecting the physical characteristics of the plume, whereas oceanic forms were entrained along the offshore border of the plume. The proportion of omnivorous copepods was reduced and the capacity of herbivorous copepods and Nyctiphanes australis to reproduce increased. The abundance of selected coastal copepods was apparently related to their vulnerability to offshore transport on an upwelling coast. A comparison of the Kahurangi and Mauritanian upwelling plumes suggests that the upwelled new nitrogen is converted to zooplankton biomass more efficiently in greater Cook Strait. © The Royal Society of New Zealand 1993.