One of the major uncertainties for oceanic biogeochemical cycles regards processes happening in the midwater region, also known as the twilight zone, below 200 m depth. For zooplankton (animals drifting in the oceans and other water bodies), studies are scarce due to the difficulty and associated time and costs of sampling deep waters. Since the advent of automated image analyses and molecular methods, our knowledge about surface communities—their diversity, abundances and size distribution—is rapidly growing. Our understanding of the deeper layers of the ocean is, however, still in its infancy, and studies combining these three facets of the zooplankton ecology are very rare. The objective of this project is to leverage already existing samples, obtained from previously NSF-funded research in the North Pacific and North Atlantic, to study how environmental and latitudinal gradients affect the abundances, diversity, and size distribution of zooplankton in the midwater. These factors have profound effects on the role of the zooplankton in biogeochemical cycles; specifically, the capture and export of carbon and nitrogen from surface to deeper waters. Automated image analyses will provide abundance and size data, and molecular analyses will give unprecedented data on the diversity of the midwater community for several biogeographical regions in the North Atlantic and the North Pacific, from subtropical to subarctic environments. This project will provide high quality hands-on training opportunities for at least two undergraduate researchers, and will be used to generate material for undergraduate and graduate courses. A workshop will be co-developed for ten educators on ways to facilitate the classroom use of the NSF-funded Biological and Chemical Oceanography Data Management Office (BCO-DMO) open access oceanographic data.