Four underwater glider missions were carried out to sample the physical and bio-optical properties inside a Loop Current Eddy (LCE) in the Gulf of Mexico (GoM), to investigate whether the winter deepening of the mixed-layer and erosion of the nitracline stimulates phytoplankton growth. Recent coupled physical-biogeochemical numerical models support this mechanism, but observations using Lagrangian floats suggest that there is no seasonal cycle on integrated phytoplankton biomass. Here, data collected by underwater gliders during a full seasonal cycle and inside the LCE Poseidon support the occurrence of a seasonal cycle, which is consistent with nutrient entrainment into the euphotic zone. The changes in fluorescence emission per chlorophyll-a unit and its implications for interpreting bio-optical variability were also assessed. Linear regressions between in vivo chlorophyll-a fluorescence and satellite chlorophyll-a concentration show the largest (smallest) slopes during winter (summer), suggesting a shift in the phytoplankton community along the year. Although the glider dataset is convolved by temporal and spatial variability, and chlorophyll-a fluorescence is affected by several factors, the concomitant enhancement of particle backscattering coefficient and chlorophyll-a observed during winter supports the occurrence of a seasonal cycle in phytoplankton biomass. Deep winter convection inside the core of the LCE, can promote fertilization through vertical diffusion of nutrients. Poseidon was an extraordinary, large, and strong, LCE that prompted phytoplankton blooms in winter highlighting their relevance for primary production and in general for biogeochemical processes.