Zooplankton vertical migration and the active transport of dissolved organic and inorganic carbon in the Sargasso Sea

The least known component of the `biological pumpa is the active transport of carbon and nutrients by diel vertical migration of zooplankton. We measured CO 2 respiration and dissolved organic carbon (DOC) excretion by individual species of common vertically migrating zooplankton at the US JGOFS Bermuda Atlantic Time-series Study (BATS) station. The inclusion of DOC excretion in this study builds on published research on active transport by respiration of inorganic carbon and allows a direct assessment of the role of zooplankton in the production of dissolved organic matter used in midwater microbial processes. On average, excretion of DOC makes up 24% (range"5}42%) of the total C metabolized (excreted# respired) and could represent a signi"cant augmentation to the vertical #ux that has already been documented for respiratory CO 2 #ux by migrant zooplankton. Migratory #uxes were compared to other transport processes at BATS. Estimates of combined active transport of CO 2 and DOC by migrators at BATS averaged 7.8% and reached 38.6% of mean sinking POC #ux at 150 m, and reached 71.4% of mean sinking POC #ux at 300 m. DOC export by migrator excretion averaged 1.9% and reached 13.3% of annual DOC export by physical mixing at this site. During most of the year when deep mixing does not occur, diel migration by zooplankton could provide a supply of DOC to the deeper layers that is available for use by the microbial community. A carbon budget comparing migrant zooplankton transport to the balance of 0967-0637/99/$ see front matter ( 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 0 6 3 7 ( 9 9 ) 0 0 0 5 2 7 #uxes in the 300}600 m depth strata at BATS shows on average that the total migrant #ux supplies 37% of the organic carbon remineralized in this layer, and that migrant DOC #ux is more than 3 times the DOC #ux gradient by diapycnal mixing. New estimates of active transport of both organic and inorganic carbon by migrants may help resolve observed imbalances in the C budget at BATS, but the magnitude is highly dependent on the biomass of the migrating community. ( 1999 Elsevier Science Ltd. All rights reserved.