Further studies of plankton ecosystems in the eastern Indian Ocean. III. Numerical abundance and biomass

Abstract

The numerical abundance of 13 zooplankton taxa in the eastern Indian Ocean (meridian 110ºE.) was examined in relation to some factors likely to control their distribution. Regression analysis showed that season, latitude (and their interaction), and time of day were frequently significant sources of variance. Decapoda, Amphipoda, fish eggs and larvae, Coelenterata and, sometimes, Copepoda and Euphausiacea were more abundant by night than by day. Numbers were generally high in late winter (June-September) and low in early summer, with a secondary peak, in some cases, in early autumn (March). Numbers were generally high at low latitudes (9-15ºS.) and low at high latitudes (25-32ºS.), one exception being fish eggs and larvae whose centre of abundance lay at 24-25ºS.

Seasonal periodicity was frequently in phase along the study section, numerical abundance reaching a peak 1-2 months after a general phytoplankton bloom at the onset of the south-east monsoon. There is no ready explanation why the subtropical regime should be in synchrony with that prevailing in the tropics, no subtropical source of nutrient enrichment being known which could match, for example, the Java Dome. An explanation for the observed tropical-subtropical synchrony was therefore sought in terms of interzonal advection.

Data from a variety of sources showed that between May and August there was considerable enrichment not only at the Java Dome but also on the north-west Australian shelf. The area between North-West Cape and Port Hedland is the place where (plankton-feeding) humpback whales are known to gather. Sperm whales, on the other hand, congregate 1100-1300 km further to the south- west where the waters are rich in micronekton. The mass transport of upper waters during the south- east monsoon season suggests that these phenomena constitute a trophic sequence.

The second zooplankton peak, in March, is the result of a summer algal bloom generated, perhaps, by remineralization of organic matter produced during the previous south-east monsoon season.