Temporal dynamics of mesopelagic fishes within a mesoscale eddy: A Lagrangian perspective

Abstract Mesoscale eddies are physically dynamic environments, yet biological responses within them are often treated as static, with eddy polarity (anticyclones vs. cyclones) serving as the dominant conceptual framework. Temporal dynamics of animals within eddies—particularly at mid‐trophic levels—remain largely unresolved. We tracked a long‐lived anticyclonic eddy in the Northeast Atlantic for nearly a month using a Lagrangian framework, generating one of the few continuous time series of mesopelagic fish distribution within an eddy. Coupled physical–biological observations revealed marked vertical changes following a major wind storm: compact fish aggregations dispersed as phytoplankton distributions expanded and zooplankton deepened, despite comparable integrated fish biomass before and after. These results show that aggregation dynamics of mesopelagic fishes are modulated by bottom‐up variability on weekly to monthly timescales. By moving beyond static snapshots, our study demonstrates that eddies function as dynamic ecosystems whose ecological roles in pelagic food webs evolve through time.