SUMMARY
Extreme weather events are expected to increase in frequency, duration, and severity due to anthropogenic climate change, and they have been implicated in ecosystem phase shifts in terrestrial and marine systems1,2. As these events become more severe, it is critical to understand how they alter ecosystems. Tropical Storm Agnes in 1972 was a “100-year storm” that lowered salinity and increased sedimentation throughout Chesapeake Bay3,4, and was suspected of altering long-term ecosystem dynamics5. Here we show that Tropical Storm Agnes resulted in a phase shift to a low-density state for the soft-shell clam Mya arenaria, which was once a biomass dominant in Chesapeake Bay. The storm caused massive mortality of bivalves, including M. arenaria6. This storm also altered predator-prey dynamics between M. arenaria and the blue crab Callinectes sapidus, shifting from a system controlled from the bottom-up by prey resources, to a system controlled from the top-down by predation pressure on bivalves. Predation by C. sapidus is sufficient to maintain the low-density steady state where M. arenaria densities hover 40 y later. Two species may exhibit nonlinear dynamics that result in phase shifts2, and extreme weather events may serve as a natural pulse stressor, triggering the phase shift7. Considering the increasing frequency of stochastic storm events8 and the preponderance of multispecies interactions exhibiting nonlinear dynamics, phase shifts are likely to become more common in the future. Hence, identification of species that are most at risk to shifts in state under extreme climate events should be a priority for marine ecosystem conservation.