Winter picoplankton composition, abundance, and vertical distribution in the Midriff islands and central regions of the Gulf of California

This study provides the first quantitative comparison of the composition and distribution of picoplankton components in the Gulf of California central region, including the Midriff islands Archipelago, in February 2014. Autotrophic picoplankton and heterotrophic bacterial abundances, estimated using flow cytometry were quantified at 5 m depth, at the peak of maximum fluorescence (FMax), and 30 m below FMax. Temperature and dissolved oxygen concentrations were lower in the northern Midriff islands Archipelago (MiA) than in the southern deep central gulf with a bathymetric sill toward the San Pedro Martir basin, where a more stratified water column prevailed (up to 40 m of the mixed layer). Gulf of California Water in the upper 150–200 m​ and Subtropical Subsurface Water below 200 m depth were observed. Heterotrophic bacteria (mean 3.59 × 10⁵ cells mL⁻¹) dominated numerically the picoplankton composition, followed by Synechococcus (mean 1.36 × 10⁴ cells mL⁻¹), Prochlorococcus (mean 7.15 × 10³ cells mL⁻¹), picoeukaryotes (mean 6.11 × 10³ cells mL⁻¹) and phycoerythrin-containing eukaryotes (mean 3.21 × 10² cells mL⁻¹). Heterotrophic bacteria contributed 70.6 ± 16.7% to the total picoplankton carbon biomass. Total picoeukaryotes averaged 24 ± 2.85%, and Synechococcus accounted for 5.10 ± 4.83%. Prochlorococcus showed the lowest biomass of all picoplankton taxonomic groups (0.29 ± 0.50%). The canonical correspondence analysis showed that temperature was the main driver of Prochlorococcus and picoeukaryotes abundances, whereas picoeukaryotes were related to sigma-Θ and Synechococcus to dissolved oxygen concentration. The sharp differences in the spatial distribution of the picoplankton community in the surface layer (0–60 m) were closely associated with the oceanographic dynamics of the MiA and the central region of the Gulf of California. They provided evidence that heterotrophic bacteria and picoeukaryotes dominate the picoplankton biomass during the cold period of the 2014-year with a typically high nutrient input.