Three decades of environmental change in European Macaronesia: Long-term trends, vertical gradients, and...

The European Macaronesian region (Azores, Madeira, Canary Islands) represents a key biogeographic transition zone in the Northeast Atlantic. While its large-scale gradients are well recognized, the extent to which long-term variability and vertical structure shape ecosystem dynamics remains poorly quantified. Here, we synthesize nearly three decades of satellite and reanalysis data (1993-2025) to resolve the seasonal, interannual, and vertical variability of physical (sea temperature, currents) and biological (chlorophyll a, phytoplankton, zooplankton, micronekton) variables. Beyond expected climatological patterns, we identify a significant warming trend (0.024 °C yr^-1) characterized by a stronger increase in minimum temperatures, indicating a reduction in seasonal thermal amplitude. This warming coincides with coherent multi-trophic increases in chlorophyll a, plankton carbon biomass and micronekton biomass. Phytoplankton biomass considered in this study, composed of nanophytoplankton and diatoms (nano+dia) only, increased 5.5 times faster than chlorophyll-a, resulting in a declining Chl:C_nano+dia ratio. Vertical analyses reveal a structured ecosystem with a latitudinally varying Deep Chlorophyll Maximum (DCM), shallower and more seasonal in the Azores (∼60 m), deeper and more persistent in the Canary Islands (∼90 m). These results demonstrate that European Macaronesia is not a uniform system defined solely by surface gradients, but by vertically structured and dynamically evolving ecosystems.