Vertical structure of small metazoan plankton, especially noncalanoid copepods. I. Deep Arabian Sea

The abundance and vertical distribution of micro-metazoans sampledwith fine nets of 0.05 mm mesh size were studied at three stationsin the Arabian Sea during the intermonsoon period (April/May1987) and down to 1850 m depth. In the epipelagic zone (0–100m). values of biomass and metazoan abundance tended to be higherthan those reported for other tropical oceanic areas. In themesopelagic zone, which is characterized by an extreme oxygendeficiency between 100 and 1000 m depth, the abundance of metazoantaxa and species numbers of non-calanoid copepods were largelyreduced. However, intermediate abundance maxima occurred withinthis 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 diversityof the dominant copepod family Oncaeidae increased substantially.Two-thirds of a total of 69 oncaeid species recovered were confinedto this layer. As most of them were small in size and occurredin low abundance only, the increase in total oncaeid densityand/or plankton biomass was less conspicuous. Dominant Oncaeaspecies in the bathypelagic zone were O.longipes and O.brodskii.The results are compared with published data from the ArabianSea and other tropical oceanic areas with and without an extrememesopelagic oxygen minimum zone Possible causes of the intermediateabundance maxima within the oxygen deficiency zone are discussed.