Spatial and seasonal patterns of carbon cycling through planktonic food webs of the Arabian Sea determined by inverse analysis

We used inverse and network analyses to describe food web dynamics during three seasons (Northeast Monsoon, Spring Intermonsoon and Southwest Monsoon) at three contrasting stations in the Arabian Sea. The goal of this work was to characterize carbon flows and trophic transfers in this highly variable system. The inverse approach allowed us to trace the fate of fixed carbon through a representative food web and to characterize temporal and spatial variability in the role of various food web components in the processing of carbon within, and export of carbon from, the euphotic zone. Zooplankton grazing did not equal net phytoplankton production for most of the stations and seasons examined. The proportion of primary production consumed by the protozoa and microzooplankton was nearly always highest at the coastal station and lowest at the offshore stations. Where large phytoplankton dominated primary production, they were also the primary source of carbon for mesozooplankton. Otherwise, the mesozooplankton were omnivorous, consuming primarily detritus and protozoa. Through direct and indirect routes, picophytoplankton dominated export fluxes at all stations and in most seasons, particularly through the particulate and dissolved carbon export pathways. This is in contrast to their conventional importance in recycling. More frequent measurements of C:Chl ratios, increased availability of depth-dependent rate measurements, and the availability of size-fractionated primary production rates would improve our ability to describe food web processes.