Pico-Phytoplankton Dynamics during Contrasting Seasonal Conditions in Two Regions of Gulf of Mexico

The ecological dynamics of pico-phytoplankton was investigated in Perdido (PER) and Coatzacoalcos (COAT) regions of the Gulf of Mexico (GoM) during March (MMF-01, cold) and September 2016 (MMF-02, warm). Pico-phytoplankton populations of Prochlorococcus (PRO), Synechococcus (SYN) and PicoEukaryotes (PEU) were assessed by flow cytometry, and total phytoplankton community (TChla) by the non-acidification fluorometric technique. Populations (carbon-based) and community (TChla-based) rates of growth, grazing, production (PP) and consumption (G) were estimated by the dilution technique in 24-h experimental incubations of seawater collected at the mixed layer (down to ~100-m in MMF-01 and <50-m in MMF-02). During the cold period, mean values of TChla and pico-autotrophic biomass were about 50 and 20% lower than those in the warm period. PP and G rates of community and populations follow the same temporal pattern. Up to 4-fold higher community-PP and a significant contribution of SYN+PEU (>60%) to daily pico-phytoplankton-PP was observed in MMF-01, in contrast to MMF-02, when it was mostly attributed to PRO (>80%). The daily protistan impact (G:PP) was lower in MMF-02 (<80% of PP for pico-autotrophs and <60% of PP for TChla) than in MMF-01 (>100% of PP), hence the microbial trophic transference was efficient during the cold conditions. In addition to this seasonal pattern, the influence of mesoscale structures in determining regional differences in the relative importance of pico-phytoplankton and its daily rates is evaluated. We found that convective winter mixing and the presence of a cyclonic eddy in the PER region during MMF-01, could have favored the input of new nutrients into the mixed layer, hence promoting the growing of phytoplankton community, mostly PEU components. In contrast, during MMF-02, anticyclonic structures were mostly present in GoM waters. The Poseidon Eddy, detached from the Loop Current in April 2016, influenced the PER region as is reflected in the low community-PP mostly attributed to PRO populations, which seem have accumulated biomass (impact<68% of PP) in these nutrient-depleted waters. This study highlights the importance of seasonal and regional physical processes on the dynamics of pico-phytoplankton populations, which sustain significantly the oligotrophic GoM ecosystem.