Decadal <scp>ENSO</scp> ‐ and <scp>PDO</scp> ‐Driven Shifts in Zooplankton Communities Off Central Chile (2003–2020)

ABSTRACT Aim To investigate how the El Niño‐Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) influence zooplankton diversity and community structure in the central Chile coastal upwelling system, focusing on long‐term changes in species composition and abundance under shifting climatic conditions. Location Eastern South Pacific off central Chile, covering an area of approximately 38,500 km 2 . Time Period 2003–2020. Major Taxa Studied Zooplankton, with emphasis on dominant groups such as copepods, jellyfish, radiolarians, siphonophores and cladocerans. Methods We analysed an 18‐year time series of zooplankton abundance and composition. Partial Canonical Correspondence Analysis (pCCA) examined relationships between zooplankton groups, the PDO and environmental variables (sea surface temperature, dissolved oxygen, wind). Threshold Indicator Taxa Analysis (TITAN) identified taxa characteristic of warm (PDO+) or cold (PDO–) phases, clarifying their roles as either sensitive or tolerant indicators of climatic shifts. Results Warm phases (PDO+) were dominated by jellyfish, siphonophores and pteropods, whereas cold phases (PDO–) favoured radiolarians and cladocerans. Copepods, the most abundant group, exhibited notable fluctuations across climate phases. Zooplankton diversity showed an ‘in‐phase’ response to climatic oscillations, indicating strong links between decadal‐scale climate regimes and ecosystem dynamics in this productive upwelling region. Main Conclusions Our findings underscore the sensitivity of marine zooplankton communities to climate variability, with distinct PDO phases exerting significant influence on zooplankton composition and diversity. Recognising key indicator taxa in warm versus cold periods is crucial for ecosystem management and developing indicator‐based monitoring programs. Continued long‐term surveys will be essential to anticipate future shifts in marine biodiversity under ongoing climate change.