Footprints of global change in Patagonian kelp forests after half a century

Abstract Kelp forests are declining worldwide, but Patagonian giant kelp forests ( Macrocystis pyrifera ) appear to have remained stable over the past half-century. Yet, limited historical baselines and ecological knowledge in this remote area constrain our ability to assess change amid increasing human pressures. Here, we report on extensive surveys spanning a fjord-open ocean gradient in northern Patagonia, including five sites previously surveyed in 1972. Adult giant kelp density did not significantly differ between survey periods, and community patterns across the fjord-ocean gradient closely matched historical descriptions: sea urchin grazing dominated kelp forest dynamics at offshore sites, while abiotic forcing and non-consumptive interactions seemed to become increasingly important toward sheltered inner fjord habitats. Despite this apparent stability, we detected clear footprints of human impact. The invasive sea anemone Metridium senile —absent in 1972—now dominates hard substrata at many sheltered sites (up to 88.4% cover), likely competing with native filter feeders. The commercially exploited sea urchin Loxechinus albus declined by 59–90% in abundance and exhibited smaller maximum body sizes, while the historically widespread Pseudechinus magellanicus has largely disappeared. These findings highlight the need for long-term monitoring to understand how ongoing changes may affect ecosystem functioning and services.