Climate change is increasing the frequency, intensity, and duration of extreme events like marine heatwaves (MHWs). Bull kelp (*Nereocystis luetkeana*) forests in northern California experienced unprecedented losses from 2014-2019. Using in situ and satellite data, this study shows that this ecosystem shift, initiated by a multi-year MHW, was preceded by keystone predator declines. Historically resilient to environmental fluctuations, the kelp forests were unable to recover from the coupled environmental and biological shifts of 2014-2016. The study recommends ecosystem-based management strategies including monitoring key attributes (kelp, urchins, predators), developing management responses to threshold levels, and creating restoration indices.

Meredith L. McPherson, Dennis J. I. Finger, Henry F. Houskeeper, Tom W. Bell, Mark H. Carr, Laura Rogers-Bennett, Raphael M. Kudela