Full-depths zooplankton composition at two deep sites in the western and central Arabian Sea

Abstract

<smarttagtype namespaceuri="urn:schemas-microsoft-com:office:smarttags" name="place"> The temporal distribution and faunistic composition of zooplankton was studied by means of fine stratified oblique tows with a 1 m² MOCNESS at one station in the western and central Arabian Sea, each. The 333 µm net samples covered the whole water column down to ca. 4000 m, commencing 50 m above bottom, during three monsoon periods: the fall intermonsoon in October 1995, the spring intermonsoon in April 1997 and the NE monsoon in February 1998. Copepods were most abundant at both stations, ostracods ranked next, followed by malacostraceans, and chaetognaths. Total numerical standing stocks varied at the western site between 35 500 and 93 100 no./m² and at the central site between 27 300 and 65 800 no./m². In the epipelagic zone (0-150 m), only calanoid copepods, chaetognaths and gelatinous zooplankton showed an in-phase coupling with the seasonal pattern of primary production, with lowest concentrations during the spring intermonsoon; for all other groups the coupling is not clear. In the mesopelagic zone (150-1050 m), characterized by a pronounced oxygen minimum zone (OMZ), distinct changes in the faunistic composition were found: In the core of the OMZ (O₂ below 0.15 mg/l), the relative abundance of calanoid copepods rose up to 95%, whereas non-calanoid copepods and chaetognaths nearly disappeared. The vertical distributions of selected copepod taxa are discussed in relation to the oxygen profiles. Below the core of the OMZ several groups showed a subsurface peak in abundance. In the bathypelagic zone, below 1050 m, 4-11% of the water column zooplankton standing stock was found. </smarttagtype>