Mesozooplankton biomass in the upper 1000 m in the Arabian Sea: overall seasonal and geographic patterns, and relationship to oxygen gradients

Abstract

Mesozooplankton biomass distributions in the upper 1000 m of the Arabian Sea were studied as part of the US JGOFS Arabian Sea Process Study. Samples were collected in vertically stratified MOCNESS tows during four seasons in 1995 at six stations spanning the Arabian Sea. This paper describes results from the size-fractionated dry weight and wet weight biomass profiles and their relationship to the seasonal monsoon cycle and the pronounced oxygen minimum zone (OMZ). The total mesozooplankton biomass and most size fractions exhibited a significant onshore/offshore biomass gradient during most seasons and at most depths. The gradient was strongest in August during the late Southwest Monsoon and weakest in March during the Spring Intermonsoon, when maxima of some size-fractions occurred offshore. Seasonal changes in zooplankton biomass were small. The late Southwest Monsoon was the time of higher biomasses for nearshore stations, while the Spring Intermonsoon was the time of higher biomasses for offshore stations. Most of the zooplankton biomass present in the upper 1000 m of the water column occurred in the upper 200 m, but there was substantial diel vertical migration, especially in the large size class, down to 300–400 m, well within the suboxic water of the OMZ. Subsurface biomass increased in the lower OMZ within the depth range of the 0.05–0.1 ml/l oxygen gradient (about 500–600 m). The enhanced zooplankton biomass, higher potential food levels, and possible short food chains suggested active biological modification of the sinking flux in this depth zone. Biomass depth profiles showed close relationships to oxygen profiles. The shapes of the zooplankton biomass and oxygen profiles varied with geographic location across the Arabian Sea but were remarkably consistent over the year at each station. Geographic differences in the shapes of the profiles were dependent on the vertical extent of the suboxic zone and may correspond to broad regional differences in the processing of the export flux in midwater. However, the consistency of the shapes over time implies long-term stability in the structure and function of the midwater ecosystem at each location.