| As part of the U.S. Joint Global Ocean Flux Study, zooplankton were collected day and night in vertically stratified MOCNESS net tows at six stations, during four seasonal cruises in the Arabian Sea. Organisms comprising the large size fraction, primarily fish, shrimp, amphipods, and euphausiids (macrozooplankton and micronekton), were identified and enumerated, and their abundance and distributions from 300--1000 m were examined in relation to environmental factors including the OMZ and the monsoon cycle. Contributions to carbon cycling were also estimated for several taxa. Few animals showed significant seasonal differences in abundance. Abundances and distributions of many taxa varied geographically, with some of these showing decreasing abundance with increasing distance from shore. Oxygen concentrations and the strength of the OMZ were important in determining geographical and vertical distributions. Diversity of macrozooplankton and micronekton differed between three different oxygen concentration categories. Highest diversity occurred in the most oxygenated water at stations with a strong OMZ. The lower OMZ interface, where oxygen concentrations rose from 0.15 to 0.1 ml l-1 , was characterized by high abundance of a few taxa, especially the fish Cyclothone spp. and the decapod Gennadas sordidus . Diversity in the lowest oxygen waters (<0.14 ml O2 l-1) varied between day and night. Diversity in this zone was strongly influenced by diel vertical migrators, found in the core of the OMZ (∼300--500 m) during the day. This resulted in dominance by a few taxa during the day. Contributions to vertical carbon flux by diel vertical migrators reached 0--34% of the POC flux. Recycling by permanent residents of the OMZ was equivalent to 5--126% of the POC flux. One decapod, Gennadas sordidus, was studied in greater detail to determine the effects of the OMZ and seasonal cycle on life history traits. G. sordidus , an indicator species of the biomass peak at the lower OMZ interface, reproduced year-round with a slight peak in ovigerous females during the Northeast Monsoon. Oxygen concentration was a major factor in determining distributions of G. sordidus. Overall, this research shows that macrozooplankton and micronekton are significant contributors to carbon cycling within the Arabian Sea OMZ. |
