Picophytoplankton Phenology in the Global Ocean Addressed by Quantitative Niche Models

Understanding seasonal cycles is key to evaluate the impact of future climates. Lineages within picophytoplankton were projected to increase or remain unchanged in future climates, although seasonal variation remains unclear. Our objective was to describe the global abundance and distribution seasonal cycle for the main lineages of picophytoplankton: Prochlorococcus (PRO), Synechococcus (SYN) and picoeukaryotic phytoplankton (PEUK). Abundance was calculated using quantitative niche models for each lineage and environmental data on PAR (Ocean Color), temperature and nitrate (World Ocean Atlas). Cells/ml were projected globally in a 1-degree grid from surface to 200m depth with a monthly time step. For each lineage we defined high abundance as the cell concentration >90th percentile, 15.5, 3.1 and 1.3 x10⁴ cells/ml for PRO, SYN and PEUK respectively. We observed that seasonal variation was higher for PEUK>SYN>PRO. The peak with maximum abundance did not coincide corresponding to May, March and September for PRO, SYN and PEUK, respectively. For PRO, areas with high abundance showed an extended region of high prevalence (>7 months) in the band 25 N-S. For SYN, regions with high abundance showed a similar area with high and low (<6 months) prevalence, around the Equator, 40N and 40S. Finally, PEUK high abundance was similar to SYN but displaced poleward. Thus, PRO regional variation was low, while SYN and PEUK showed expansion and shifts of high abundance waters along seasons. High cell abundance also varied seasonally in the water column. The three lineages showed average meridional depths in summer shallower than in winter, with SYN showing the largest seasonal variation. Our results suggested that future projections far exceed seasonal variation for PRO, were similar for SYN, and were smaller for PEUK. Thus, projected future climates will present novel conditions not seen in the present.