Vertical structure of small metazoan plankton, especially non-calanoid copepods .1. Deep Arabian Sea

The abundance and vertical distribution of micro-metazoans sampled with fine nets of 0.05 mm mesh size were studied at three stations in the Arabian Sea during the intermonsoon period (April/May 1987) and down to 1850 m depth. In the epipelagic zone (0-100 m), values of biomass and metazoan abundance tended to be higher than those reported for other tropical oceanic areas. In the mesopelagic zone, which is characterized by an extreme oxygen deficiency between 100 and 1000 m depth, the abundance of metazoan taxa and species numbers of non-calanoid copepods were largely reduced. However, intermediate abundance maxima occurred within this zone, which were dominated by specific metazoan taxa (copepods, appendicularians) and species of non-calanoids (Oncaea sp. C). In the bathypelagic zone below 1050 m, the species diversity of the dominant copepod family Oncaeidae increased substantially. Two-thirds of a total of 69 oncaeid species recovered were confined to this layer. As most of them were small in size and occurred in low abundance only, the increase in total oncaeid density and/or plankton biomass was less conspicuous. Dominant Oncaea species in the bathypelagic zone were O. longipes and O.brodskii. The results are compared with published data from the Arabian Sea and other tropical oceanic areas with and without an extreme mesopelagic oxygen minimum zone. Possible causes of the intermediate abundance maxima within the oxygen deficiency zone are discussed.