JGOFS (Arabian Sea): Effects of the Oxygen Minimum Zone on Mesozooplankton Distributions and Food Webs

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9310590 Gowing This research, part of the Joint Gobal Ocean Flux Study (JGOFS) of the Arabian Sea, addresses the question of whether the widespread suboxic conditions below the euphotic zone in the Arabian Sea restrict carbon cycling by metazoans primarily to regions above and below the oxygen minimum zone and whether this results in elevated carbon concentrations (in the form of plankton biomass) at depth. This project has two foci: (1) the measurement of mesozooplankton distributions and size-fractionated biomass in vertical strata to the deep sea, with particular attention to the oxygen minimum zone and its upper and lower interfaces, and (2) examination of oxygen minimum zone effects on zooplankton trophic relationships, including the potential zooplankton-bacteria link. Vertically stratified zooplankton samples down to 3500 m will be taken during four seasons with a MOCNESS plankton net system day and night at the four intensive JGOFS stations and at intervals along transects between the stations for the determination of mesozooplankton distributions and size-fractionated biomass. Gut contents of zooplankton will be analyzed by transmission electron microscopy for determination of trophic webs. The upper and lower interfaces of the oxygen minimum zone, which may be locations of high zooplankton and particle abundances and intense processing of organic material, will be specially targetted for studies of distributions and food webs. Low mesozooplankton abundances within the oxygen minimum zone may contribute to enhanced fluxes of organic materials to the sea floor, because the utilizatimn of particulate material in midwater by these detritivores would be reduced. High mesozooplankton abundances and a short food chain (bacteria to zooplankton) at the oxygen minimum zone interfaces would suggest intense processing of organic material localized within a narrow depth window, possibly acting as a filter for sinking particles. This work will help elucidate how organic material may be altered during passage through the water column to the sea floor. It will contribute to understanding how the seasonal monsoon cycle of the Arabian Sea and resultant productivity peaks are coupled to carbon fluxes and deposition and may provide analogies for predicting effects of climate change.