We compiled data on 10 environmental variables that represent key dimensions of phytoplankton ecological niches (4446), which shape species’ distributions via effects on physiology, growth, and species competition (17, 25, 47). These variables served as candidate predictors for SDMs and as single predictors in species richness models. Variables were aggregated at a monthly (n = 12) climatological and globally gridded resolution (1° latitude × 1° longitude), as this was the best available resolution shared among datasets. Sea surface temperature (T; °C), salinity (S), nitrate (NO3−; μM), phosphate (PO43−; μM), and silicic acid [Si(OH)4; μM] were obtained from World Ocean Atlas 2013 (1955–2012) fields (34, 43, 48). MLD (m) was included using the temperature criterion (40). Photosynthetically active radiation (PAR; μmol m−2 s−1) and chlorophyll (Chl; μg liter−1) were derived from the Sea-viewing Wide Field-of-view Sensor, using data from 1997 to 2007 (https://oceancolor.gsfc.nasa.gov). Sea surface wind stress (m s−1) was derived from the Cross-Calibrated Multi-Platform (49) using data from 1987 to 2011 (https://podaac.jpl.nasa.gov). Data on carbon dioxide partial pressure in the surface sea (pCO2; μatm) stem from (50).

We derived further predictors from these 10 variables: Photosynthetically available radiation over the MLD (MLPAR; μmol m−2 s−1) was computed from T, MLD, and Chl (44). We also used the excess concentration of NO3, relative to the Redfield ratio of 16:1 (μM; N-star), computed as [NO3] − 16[PO43−]. The use of N-star, rather than NO3, avoids strong global correlations between NO3 and T (Spearman’s ρ = −0.71). Si-star, the ratio of [Si(OH)4] to [NO3], was included as a predictor particularly relevant for the Bacillariophyceae (45). We also considered the temporal trends of T (dT/dt), NO3 (dNO3/dt), PO43− (dPO43−/dt), Si(OH)4 [dSi(OH)4/dt], and MLD (dMLD/dt), calculated as the centered mean difference of the data of each month with its neighboring months. Logarithmic MLD, Chl, and nutrient levels to the base of 10 were used in addition to their original forms. Sea level height anomaly (m) (https://www.aviso.altimetry.fr/es/data/products/sea-surface-height-products/global/index.html) and nutricline depth (m), defined as the first depth at which nutrient levels exceeded a certain threshold (0.05 μM for NO3 and 0.05:16 μM for PO43−), were also tested, yet these two variables were discarded due to poor skill in single-predictor model tests. Net primary production (NPP; mg C m−2 day−1) was used for correlative analyses only, using data from 1998 to 2007 from the standard vertically generalized production model (http://www.science.oregonstate.edu/ocean.productivity).

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