Key link between iron and the size structure of three major mesoplanktonic groups in the upper ocean.

In marine ecosystems, critical services like fish production, carbon export, or the delivery of nutrients through N2-fixation rely heavily on the size spectrum of pelagic organisms, particularly mesoplankton (200-20,000 µm). However, the linkages between environmental factors and mesoplankton spectral biogeography remain largely unresolved, as so far only limited datasets exist to understand the large-scale shifts in mesoplankton size. Using global compilations of Rhizarian, colonial N2-fixer, and Crustacean images, we reveal the role of iron in shaping the size structure and related biogeography of these groups. Our findings underscore the importance of atmospheric sources of iron for N2-fixers and Rhizarians while total iron, accounting for organic and inorganic compounds, appeared to explain a high percentage of the variance in Crustacean size structure via apparent recycling. Using environmental explanatory variables, our models reached high R2 (0.93, 0.88, and 0.79 respectively), providing robust predictions of mesoplankton size structure related to elemental cycling and ecosystem services. Our results suggest that without compensatory mechanisms, future increases in global temperatures could have negative effects on mesoplankton size, possibly limiting carbon export from the productive layers to sequestration depth, that might be offset by expected increases in iron inputs that benefit N2-fixers, Rhizarians, and eventually Crustaceans.