On the Seasonal Cycle of Phytoplankton Bio‐Optical Properties Inside a Warm Core Ring in the Gulf of Mexico

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, 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 profiling 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 idea of an increase in phytoplankton biomass during winter, consistent with nutrient entrainment into the euphotic zone. The changes in fluorescence emission per chlorophyll‐a unit and their 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 or photoacclimation. Although the glider data set is convolved by temporal and spatial variability, and chlorophyll‐a fluorescence is affected by multiple 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 vertical mixing in winter 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. Plain Language Summary Recent technological advancements have revolutionized our ability to monitor changes in the marine primary producers. Specialized robots are nowadays capable of measuring bio‐optical properties, such as chlorophyll‐a fluorescence and the particle backscattering coefficient even during severe climate conditions. However, interpreting chlorophyll‐a fluorescence measurements can be challenging, as they are influenced by multiple factors, including phytoplankton community shifts, and nutritional status. Here, we used data acquired by remotely controlled platforms to track a large coherent rotating oceanic eddy as it propagated westward through the Gulf of Mexico. Near the center of the eddy, the upper 170 m of the water colu