Further studies of plankton ecosystems in the eastern Indian Ocean. V. Ecology of the Copepoda

Abstract

Seasonal changes in 82 species of epiplanktonic copepods along a longitudinal section in the eastern Indian Ocean (meridian 110°) were studied so that the main structural features of the pelagic ecosystem could be identified. Counts were made of 46 species of Eucalanidae, Euchaetidae, and Sapphirinidae, and the presence or absence of the remainder was recorded. This mixture of qualitative and quantitative data was used to identify the major niche complexes ('biocenoses') and habitat complexes (geocenoses') in the study area.

Many species were ubiquitous. The greatest proportion of endemic species occurred in a narrow fringe to the south of Java; these included neritic species such as Acartia erythraea and Eucalanus dentatus, upwelling species such as Calanoides carinatus, and other species of less certain origin (e.g. Eucalanus crassus, Euchaeta concinna, and Candacia catula). The best indicator of tropical water was Candacia pachydactyla, and the best indicator of its mixtures with subtropical water was Euchneta wolfendeni.

Whereas presence-absence data were sufficient to group many tropical species which had a limited range, numerical data were needed to classify subtropical species such as Eucalanus subtenuis, Euchaeta longicornis, and Copilia mediterranea. Diurnally separate, as well as seasonally separate, biocenoses could be recognized; these were characterized, in particular, by species of the genus Pleuromamma.

The agglomerative program MULTCLAS, using quantitative as well as qualitative data, defined plankton geocenoses more clearly than did the simple divisive program DIVINF using qualitative (presence-absence) data alone. Six systems could be recognized. Their latitudinal and seasonal distribution, and their temperature-salinity properties showed that two were tropical, two were subtropical, and two were tropical-subtropical mixtures.

The tropical geocenoses were early and late phases of the 'Java Dome', a south-east monsoon upwelling system. The subtropical geocenoses corresponded to the central water mass and the west wind drift transition zone. The tropical-subtropical mixtures were seasonal phases, the south-east monsoon phase being generally richer than its successor, due probably to lateral advection from the north, possibly from coastal upwelling off the north-west Australian shelf. The west wind drift transition zone had unusual biological properties due, apparently, to its characteristic turbulence and deep mixed layer.

Using the scaled exponent of the Shannon-Wiener entropy function H, a diversity maximum was located at about 20ºS. in the tropical convergence. Eucalanus subtenuis was responsible for diversity minima in the subtropics. A variety of species was responsible for diversity minima in the tropics, in particular Rhincalanus cornutus and Euchaeta russelli, the latter swarming in the upper layers in midsummer and reducing diversity to practically zero.