Humans simplified trophic structures in island lake ecosystems

Miguel Graca Matias , Vítor Gonçalves, Sergi Pla-Rabes, Armand Hernández, Manuel Mendoza, Miguel B. Araújo, Jacqueline van Leeuwen, Sandra Nogué, Nora Richter, Martin Souto, Ricardo Trigo, Roberto Bao, Alberto Sáez, Santiago Giralt, Pedro M. Raposeiro

Lakes on oceanic islands are exceptionally vulnerable ecosystems and serve as natural archives of long-term environmental change, yet long-term reconstructions spanning multiple trophic levels remain rare. We reconstruct 2,000 years of ecological change across nine Azorean lakes by analysing sediment records of community composition and trophic-guild richness for aquatic producers and consumers, interpreted alongside catchment vegetation and hydroclimatic variability. We identified an archipelago-wide trajectory of trophic restructuring and simplification, consistent with a shift toward plankton-dominated structures. Widespread catchment vegetation change was followed by major reorganisation of aquatic compositional turnover, with producers typically responding earlier than consumers. This multi-trophic, multi-lake perspective reveals that human land use initiated a long-term trajectory of ecosystem restructuring, with the initial reorganisation phase occurring between approximately 1050 and 1450 CE. Land-use effects persisted thereafter, while climatic variability increasingly interacted with already modified catchments after approximately 1600 CE. These findings indicate that ecological baselines are not fixed states but dynamic outcomes of long-term interactions among humans, climate, and ecosystem resilience.