Increases in Sea Level Associated with a Marine Heatwave Drive Tidal Marsh Habitat Shifts

Abstract Extreme climate events provide an opportunity to understand how habitats will respond to future long-term climate change. Starting in late 2013, an exceptionally strong marine heatwave, or the “Blob”, which was followed by El Niño in 2015–2016, resulted in an anomalous temporary rise in sea level on the Pacific US coast. We examined the response of tidal marsh plant communities in three California estuaries spanning over 600 km to these elevated water levels. We focused on the seaward ecotone of Salicornia pacifica , the succulent marsh dominant in the region. S. pacifica cover declined rapidly and sharply at all three estuaries in response to increased tidal inundation. In areas with Spartina foliosa (Tijuana River Estuary), this low marsh dominant colonized areas of S. pacifica dieback. In the absence of S. foliosa (Carpinteria Salt Marsh and Elkhorn Slough), areas of low S. pacifica marsh converted to mudflat. We demonstrate the rapid response of these systems to transient sea level changes, and emphasize the value of consistent long-term monitoring using field surveys and remote sensing for detecting tidal marsh responses to sea level rise. These case studies can be used to test model predictions, as we provide empirical evidence of vegetative response to ~ 15 cm of sea-level rise. They also offer concrete examples of marsh habitat loss or type conversion resulting from climate change, and thereby build support for climate adaptation strategies.