Microscopy and DNA-based characterization of sinking particles at the Bermuda Atlantic Time-series Study station point to zooplankton mediation of particle flux

Plankton-derived, microscopic, and macroscopic sinking aggregates constitute most of the particulate organic carbon (POC) flux in the oceans. While the flux of particulate organic matter and associated elements has been quantified at the Bermuda Atlantic Time-series Study (BATS) station for several decades, we lack an understanding of the source and composition of sinking particles, as well as the fate of predominant phytoplankton taxa. We determined the composition of individual sinking particles and their microbial communities in the upper 300 m depth at the BATS station in fall 2017 and spring 2018 by image analysis and V4 amplicon sequencing of the 16S and 18S rRNA genes. The sinking particles were primarily composed of phytodetrital aggregates, fecal aggregates, and fecal pellets. In the fall, phytodetrital aggregates were numerically dominant and drove the majority of the POC flux; however, in the spring, particle flux of all particle categories declined below 150 m, and the POC flux at 200 m shifted to one driven by fecal aggregates. The relative composition of the microbial communities associated with phytodetrital and fecal aggregates were statistically indistinguishable in both seasons, and prokaryotic taxa known to be associated with the gut microbiomes of zooplankton were indicators of the sinking particles. Our results point to the utilization and modification of sinking particles by resident midwater zooplankton populations, and to fecal pellets as the predominant mechanism transporting picophytoplankton to depth.