Characterizing zooplankton vertical distribution in the Sargasso Sea using an Underwater Vision Profiler

Zooplankton are important controllers of nutrient cycling in oligotrophic mid-ocean gyres and are the major contributors to vertical carbon flux in these systems. In order to improve our understanding of the fine-scale vertical distribution of zooplankton in the Sargasso Sea, we used an Underwater Vision Profiler 5 HD (UVP5) to take quantitative, in situ images of meso- and macro-zooplankton in conjunction with the monthly Bermuda Atlantic Time-series Study (BATS) cruises during the summer and fall of 2019. This camera system allowed us to 1) determine individual zooplankter vertical distributions at a fine scale (~10 cm), 2) sample zooplankton to bathypelagic depths (~4,000 m), and 3) collect data on fragile zooplankton that are often damaged in net tows (Rhizaria, Cnidaria, etc.). The UVP5 was attached to the BATS CTD, and concurrent environmental, chemical, and biological data were collected for each UVP cast. We used the EcoTaxa machine-learning application to identify individuals down to the lowest taxonomic level possible in the UVP images and calculate abundance and vertical distribution of a wide variety of zooplankton taxa. These abundance data were combined with the CTD sensor and bottle data to characterize the vertical niches of abundant zooplankton taxa (e.g.Copepoda, Rhizaria, Chaetognatha), and the role of vertical features such as the Oxygen Minimum Zone and Deep Chlorophyll Maximum on zooplankton vertical distribution were also considered. Multiple zooplankton net tows were conducted using a MOCNESS fitted with 150 μm mesh during the June and July BATS cruises to compare abundance estimates between the UVP5 and traditional zooplankton collection methods. The future prospects, advantages, and limitations of using a UVP5 to characterize zooplankton communities in an oligotrophic region are also discussed.