One-Dimensional Turbulence-Ecosystem Model Reveals the Triggers of the Spring Bloom in Mesoscale Eddies

A lower trophic ecosystem model coupled with a one-dimensional physical turbulence closure model was applied to study phytoplankton dynamics and spring bloom initiation in mesoscale anticyclonic eddies (AEs) and cyclonic eddies (CEs). The model simulated ecosystem dynamics between nutrient-phytoplankton-zooplankton-detritus in AEs and CEs, while the physical model provided the seasonal cycle of convective turbulent mixing. The study was motivated by earlier work based on satellite and ship observations, which showed earlier initiation of the spring blooms in CEs with shallow mixed-layer depths than in AEs with deeper mixed-layer depths. The model results supported the hypothesis that mixed-layer depths in eddies play an important role in the dynamics of the spring bloom initiation. Model results revealed that in AEs convective mixing causes light limitation for phytoplankton growth due to deep winter mixing, and the bloom initiation is delayed until relaxation of turbulent convective mixing. Conversely, in the shallow mixed-layer CEs, blooms initiate before the end of convective mixing due to early improvement in light conditions following the increase in solar radiation. Furthermore, the model showed that the relaxation in zooplankton grazing for the deep mixing contributed to weak winter phytoplankton accumulation in AE, while winter phytoplankton accumulation was faster in the shallow mixed-layer CE. Overall, the initiation mechanism and the dynamics of the spring phytoplankton blooms are different for AEs and CEs. Therefore, we suggest that, in many parts of the global ocean, eddies play an important role regulating the dynamics of phytoplankton blooms. Mesoscale eddies are energetic swirling features, on the order of 100km and lifespans of weeks to months, found almost everywhere in the ocean with strong impact on biogeochemical processes and ecosystems dynamics. This study employed a physical-biological coupled model to understand the influence of mesoscale anticyclonic and cyclonic eddies on the initiation of spring phytoplankton blooms. The results showed distinct bloom initiation timings and grazing rates within the interior of the eddies. In the case of anticyclonic eddies with deeper mixed layers, blooms are triggered by suppression of turbulent mixing, which allows for increased phytoplankton light exposure. In contrast, cyclonic eddies with shallower mixed layers, blooms are triggered by increase in surface light that improves phytoplankton light exposure prior to the end of convective mixing. These findings are pertinent to understanding physical-biological interactions and their consequent role in ecosystem dynamics and the biological carbon pump in the ocean under climate change.