Long-term and multilevel impact assessment of the 2015–2016 El Niño on a sandy beach of the southwestern Atlantic

As a land-sea interface, the fingerprints of climate perturbations may be immediately and profoundly felt in sandy beaches and the macroinvertebrates they harbour. In particular, extreme climatological events can result in long-lasting or irreversible ecological changes, and therefore, it has become critical to understand how these ecosystems respond to strong pulse perturbations. This study assessed the main impacts prompted by the 2015–2016 El Niño on a Southwestern Atlantic sandy beach ecosystem. A long-term (1982–2019) analysis was carried out, attending historical climate components and multilevel indicators of change across levels of ecological organization. The trophic networks of four ecosystem states were compared, and the macroinvertebrate community structure was analysed in terms of species richness, abundance and biomass and deconstructed by taxonomy, beach zone occupied, feeding, and development modes. The potential recovery pathway of the system was also assessed. Climatic effects were reflected in a marked increase in sea surface temperature anomalies, rainfall, and in the discharge of the widest estuary of the world (Río de la Plata). An abrupt disruption of ecological attributes due to El Niño effects was evidenced. After the event, the ecosystem shifted to a higher organization of the flow structure (Ascendency), a lower adaptive potential (Overhead), and a marked increase in efficiency (Robustness), reflecting a more vulnerable state to absorb disturbances. The decrease in species abundance and biomass was particularly noticeable in molluscs, filter feeders, and low intertidal/subtidal groups. By contrast, polychaetes/deposit feeders were favoured, triggering a transitional community state dominated by opportunistic species. The results highlight how extreme climatic events could prevent the recovery of a sandy beach ecosystem, as pulses may induce lag and legacy effects.