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Abstract

Plankton are essential in marine ecosystems. However, our knowledge of overall community structure is sparse due to inconsistent sampling across their very large organismal size range. Here we use diverse imaging methods during the Tara Ocean cruise to establish complete plankton inventories of organisms across 5 orders of magnitude in size (15 in biovolume). Plankton community size and trophic structure variation validate a long-held theoretical link between organism size-spectra and ecosystem trophic structures. Unexpectedly, bottom- heavy ecosystems (the norm on land) appear to be rare in the ocean. Rather, we found that predator/grazer biomass and biovolume typically exceed that of primary producers at most (55%) locations, likely due to our better quantification of gelatinous organisms. Collectively, gelatinous organisms represent 30% of the total biovolume (8-9% of carbon) of marine plankton communities from tropical to polar ecosystems. Communities can be parsed into three main types: diatom/copepod-dominated in eutrophic blooms, rhizarian/chaetognath- dominated in oligotrophic tropical oceans, and gelatinous-dominated elsewhere. While plankton taxonomic composition changes with latitude, functional and trophic structures mostly depend on the amount of prey available for each trophic level. Given future projections of tropicalization and oligotrophication of marine ecosystems, our findings suggest that rhizarian and gelatinous organisms will increasingly dominate the apex position of planktonic ecosystems, and lead to significant alterations in the ocean’s carbon cycle.