Seawater was sampled from five depths in the euphotic zone corresponding to 100 % (ca. 3 m depth), 50 %, 20 %, 7 % and 1 % of incident Photosynthetically Active Radiation (PAR). For each depth, four 72 mL polystyrene bottles (three clear bottles and one dark bottle) were filled with unfiltered seawater, inoculated with 10 - 20 µCi NaH¹⁴CO₃ and incubated on-deck from dawn to dusk. Temperature and irradiance in the incubators simulated the water temperature and the incident irradiance at the corresponding depth of each sample by using a combination of neutral density and blue filters (Mist Blue, ref. 061, Lee Filters ®). After incubation, samples from three of the five depths (100 %, 20 % and 1 % PAR) were sequentially filtered through 20, 2 and 0.2 µm polycarbonate filters while the other depths (50 % and 7 % PAR) were directly filtered by 0.2 µm. Immediately after filtering, filters were then exposed to concentrated HCl fumes at least 12 h to remove the non-fixed inorganic ¹⁴C. Filters were placed in scintillation vials to which 5 mL of liquid scintillation cocktail was added. The radioactivity on each filter (disintegrations per minute, DPM) was determined using a Wallac scintillation counter. To compute the rate of photosynthetic carbon fixation, the dark-bottle DPM was subtracted from the light-bottle DPM values. A constant value of 24,720 µg L-1 (or 2,060 µmol L-1) was assumed for the concentration of dissolved inorganic carbon for surface waters in tropical ocean (Key et al., 2004). A correction factor of 1.05 was applied to this constant value for discrimination isotopic. Total primary production was calculated as the sum of the primary production on each size class.

Supplement to: Regaudie-de-Gioux, Aurore; Huete-Ortega, Maria; Sobrino, Cristina; López-Sandoval, Daffne C; González, N; Fernández-Carrera, Ana; Vidal, Montserrat; Marañón, Emilio; Cermeño, Pedro; Latasa, Mikel; Agustí, Susana; Duarte, Carlos Manuel (2019): Multi-model remote sensing assessment of primary production in the subtropical gyres. Journal of Marine Systems, 196, 97-106