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13 pages, 7 figures, 3 tables, 2 appendices, supplementary data https://doi.org/10.1016/j.ecolmodel.2017.06.020 ; The effect of biodiversity on ecosystem functioning is one of the major questions of ecology. However, the role of phytoplankton functional diversity in ecosystem productivity and stability under fluctuating (i.e. non-equilibrium) environments remains largely unknown. Here we use a marine ecosystem model to study the effect of phytoplankton functional diversity on both ecosystem productivity and its stability for seasonally variable nutrient supply and temperature. Functional diversity ranges from low to high along these two environmental axes independently. Changes in diversity are obtained by varying the range of uptake strategies and thermal preferences of the species present in the community. Species can range from resource gleaners to opportunists, and from cold to warm thermal preferences. The phytoplankton communities self-assemble as a result of species selection by resource competition (nutrients) and environmental filtering (temperature). Both processes lead to species asynchrony but their effect on productivity and stability differ. We find that the diversity of temperature niches has a strong and direct positive effect on productivity and stability due to species complementarity, while the diversity of uptake strategies has a weak and indirect positive effect due to sampling probability. These results show that more functionally diverse phytoplankton communities lead to higher and more stable ecosystem productivity but the positive effect of biodiversity on ecosystem functioning depends critically on the type of environmental gradient ; This work was funded by national research grants CGL2013-41256-P (MARES) and CTM2014-54926-R (SUAVE) from the Spanish government. S.M.V. and P.C. are supported by «Ramon y Cajal» (RyC) contracts from the Spanish government. S.D. was supported by NSF (grant 1434007) from the United States government. M.L. and J.M.M. are supported by the French Laboratory of Excellence project «TULIP» (ANR-10-LABX-41; ANR-11-IDEX-0002-02). M.L. was also supported by the BIOSTASES Advanced Grant, funded by the European Research Council under the European Union's Horizon 2020 research and innovation programme (grant agreement No 666971) and J.M.M. by the Region Midi- Pyrenees project (CNRS 121090) ; Peer Reviewed

10 pages, 10 figures ; The shape of the productivity-diversity relationship (PDR) for marine phytoplankton has been suggested to be unimodal, that is, diversity peaking at intermediate levels of productivity. However, there are few observations and there has been little attempt to understand the mechanisms that would lead to such a shape for planktonic organisms. Here we use a marine ecosystem model together with the community assembly theory to explain the shape of the unimodal PDR we obtain at the global scale. The positive slope from low to intermediate productivity is due to grazer control with selective feeding, which leads to the predator-mediated coexistence of prey. The negative slope at high productivity is due to seasonal blooms of opportunist species that occur before they are regulated by grazers. The negative side is only unveiled when the temporal scale of the observation captures the transient dynamics, which are especially relevant at highly seasonal latitudes. Thus selective predation explains the positive side while transient competitive exclusion explains the negative side of the unimodal PDR curve. The phytoplankton community composition of the positive and negative sides is mostly dominated by slow-growing nutrient specialists and fast-growing nutrient opportunist species, respectively. © 2014 Macmillan Publishers Limited ; This work was supported by a Marie Curie Fellowship (IOF—FP7) to S.M.V. from the European Union and was performed within the MIT's Darwin Project (http://darwinproject.mit.edu/). M.J.F. and S.D. are grateful for support from the >Gordon and Betty Moore Foundation>. M.L. was supported by the TULIP Laboratory of Excellence (ANR-10-LABX-41). S.M.V., P.C. and J.M.M. are currently supported by >Ramon y Cajal> contracts from the Spanish Ministry of Economy and Competitiveness (MINECO) ; Peer Reviewed