Global patterns of planktonic diversity are mainly determined by the dispersal of propagules with ocean currents. However, the role that abundance and body size play in determining spatial patterns of diversity remains unclear. Here we analyse spatial community structure - β-diversity - for several planktonic and nektonic organisms from prokaryotes to small mesopelagic fishes collected during the Malaspina 2010 Expedition. β-diversity was compared to surface ocean transit times derived from a global circulation model, revealing a significant negative relationship that is stronger than environmental differences. Estimated dispersal scales for different groups show a negative correlation with body size, where less abundant large-bodied communities have significantly shorter dispersal scales and larger species spatial turnover rates than more abundant small-bodied plankton. Our results confirm that the dispersal scale of planktonic and micro-nektonic organisms is determined by local abundance, which scales with body size, ultimately setting global spatial patterns of diversity.

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Mesozooplankton is a key component of the ocean, regulating global processes such as the carbon pump, and ensuring energy transfer from lower to higher trophic levels. Yet, despite the importance of understanding mesozooplankton diversity, distribution and connectivity at global scale to predict the impact of climate change in marine ecosystems, there is still fragmented knowledge. To fill this gap, we applied DNA metabarcoding to mesozooplankton samples collected during the Malaspina-2010 circumnavigation expedition across temperate and tropical oceans from the surface to bathypelagic depths. By conducting a hidden diversity analysis, we highlight the still scarce knowledge on global mesozooplankton diversity and identify the Indian Ocean and the deep sea as the most understudied areas. By analysing mesozooplankton community spatial distribution, we confirm global biogeographical patterns across the temperate to tropical oceans both in the vertical and horizontal gradients. Additionally, we reveal a consistent increase in mesozooplankton beta-diversity with depth, indicating reduced connectivity at deeper layers, and identify a water mass type-mediated structuring of bathypelagic communities, instead of an oceanic basin-mediated as observed at upper layers. This suggests limited dispersal at deep ocean layers, most likely due to weaker currents and lower mixing of water mass types. Overall, our work supports the neutral theory of biodiversity and thus the importance of oceanic currents and barriers in dispersal in shaping global plankton communities, and provides key knowledge for predicting the impact of climate change in the deep-sea.

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