Allometric estimates of midwater zooplankton metabolism and vertical flux from image data

The structure of the planktonic food web plays a fundamental role in regulating the export of organic carbon, nitrogen and phosphorous to the deep. The efficiency of this export is, however, modified by the environment and community composition in the midwater (~200-1000 m; the region below the euphotic zone). In this “twilight zone” migratory zooplankton species contribute to the direct transport of flux from surface waters, while stationary resident species assemblages repackage and transform surface derived flux in a process associated with attenuation. Descriptions of the midwater zooplankton community are generally made as biomass and counts of large taxa groups. However, since zooplankton contributions to biogeochemistry are size dependent – being linked to their physiological rates – these standard descriptions have thus far proven insufficient to generate a predictive understanding of the role of zooplankton in biogeochemical cycles. Recent advances in describing zooplankton communities via image analysis are providing the opportunity to redefine how we characterize both resident and migratory zooplankton communities throughout the water column. This study uses image analysis, following modified Zooscan and Ecotaxa pipelines, on multiple MOCNESS tows (0-1000 m) offshore of Bermuda. We demonstrate how these datasets can be used to characterize the depth of vertical migration, the size distribution and relative contribution of the migratory and resident communities, and (using previously established allometric relationships) to calculate the midwater oxygen use by these zooplankton groups.