SummaryThe ocean is home to myriad small planktonic organisms that underpin the functioning of marine ecosystems. However, their spatial patterns of diversity and the underlying drivers remain poorly known, precluding projections of their responses to global changes. Here we investigate the latitudinal gradients and global predictors of plankton diversity across archaea, bacteria, eukaryotes, and major virus clades using both molecular and imaging data from Tara Oceans. We show a decline of diversity for most planktonic groups toward the poles, mainly driven by decreasing ocean temperatures. Projections into the future suggest that severe warming of the surface ocean by the end of the 21st century could lead to tropicalization of the diversity of most planktonic groups in temperate and polar regions. These changes may have multiple consequences for marine ecosystem functioning and services and are expected to be particularly significant in key areas for carbon sequestration, fisheries, and marine conservation.Video Abstract

The signature of westward propagating mesoscale eddies in sea surface salinity (SSS) is analyzed for the tropical Pacific by collocating 7 years (2010)(2011)(2012)(2013)(2014)(2015)(2016) of Soil Moisture and Ocean Salinity SSS satellite data with coherent mesoscale eddies automatically identified and tracked from altimetry-derived sea level anomalies. First, the main characteristics of the long-lived coherent eddies are inferred from sea level anomalies maps. Then, the mean signature of the mesoscale eddies on SSS is depicted for the whole tropical Pacific before focusing in regions centered around the central and eastern parts of the tropical North Pacific. In these areas, composite analyses based on thousands of eddies reveal regionally dependent eddy impacts with opposite SSS anomalies for cyclonic and anticyclonic eddies. In the central region, where the largest meridional SSS large-scale gradients and smallest eddy amplitudes are observed, results show dipole-like SSS changes with maximum anomalies on the leading edge of the composite eddy. In contrast, in the eastern region, where the largest near-surface vertical salinity gradients and largest eddy amplitudes are observed, the composite eddy shows monopole-like SSS changes with maximum anomalies near the composite eddy center. These distinct dipole/monopole SSS patterns suggest the dominant role of horizontal advection and vertical processes in the central and eastern regions, respectively. Other possible explanations, notably one involving the contrasted eddy amplitudes of the two regions, are discussed.Plain Language Summary Sea surface salinity (SSS) is an Essential Climate Variable needed to improve our knowledge of the Earth's water cycle and climate. SSS has proven to be valuable for improving estimates of evaporation minus precipitation (E − P) budgets, describing and understanding climate variability at seasonal to decadal time scales, testing physical processes, assessing numerical model skills, quantifying the role of salinity on sea level change, improving El Nino prediction lead time, and quantifying the ocean-atmosphere CO 2 exchanges. Very few studies have, however, focused on what we call small-scale (that is mainly eddies of the order of 50-to 300-km radius) SSS changes in the open ocean, mainly due to the lack of high-resolution measurements. Relying on unprecedented satellite measurements of SSS, the present study shows how eddies in the tropical Pacific can modify the spatial distribution of SSS. We suggest that these modifications are likely due (i) to the rotational sense of the eddies, which move SSS horizontally, and (ii) to their capability to move or mix waters up and down while rotating.