Understanding the acute and longer-term effects of marine heatwaves on coastal ecosystems is critical for predicting their response to future warming. We used a 23-year time series that included two marine heatwaves (the 2014-2015 "warm blob" and 2018) to investigate their effects on understory macroalgae at nine rocky reefs with giant kelp forests in the Santa Barbara Channel monitored by the Santa Barbara Coastal Long Term Ecological Research program. We observed little change in macroalgal biomass or species richness in response to the heatwaves, but significant changes in community composition, including the loss of the dominant understory kelp Pterygophora californica and shifts towards subtropical and introduced species. Understory communities overall were surprisingly resilient, as community composition largely returned to pre-heatwave conditions within a few years. However, there were exceptions. In particular, subtropical species and geniculate coralline algae increased in abundance in the decade following the heatwaves, suggesting that macroalgal communities are likely to shift in the long term if marine heatwaves increase in frequency and intensity.
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