Spatial variability in the resistance and resilience of giant kelp in southern and Baja California to a multiyear heatwave

Kyle C Cavanaugh, University of California Los Angeles, Department of Geography, Los Angeles, CA, United States, Daniel Reed, University of California Santa Barbara, Santa Barbara, CA, United States, Tom William Bell, University of California, Santa Barbara, Earth Research Institute, Santa Barbara, CA, United States, Max Castorani, University of Virginia, Department of Environmental Sciences, Charlottesville, United States and Rodrigo Beas-Luna, Autonomous University of Baja California, Facultad de Ciencias Marinas, Ensenada, BJ, Mexico
Abstract:
In 2014-2016 the west coast of North America experienced a marine heatwave that was unprecedented in the historical record in terms of its duration and intensity. This event was expected to have a devastating impact on populations of giant kelp, an important coastal foundation species found in cool, nutrient rich waters. To evaluate this expectation, we used a time series of satellite imagery to examine giant kelp canopy biomass before, during, and after this heatwave across more than 7 degrees of latitude in southern and Baja California. We examined spatial patterns in resistance, i.e. the initial response of kelp, and resilience, i.e. the abundance of kelp two years after the heatwave ended. The heatwave had a large and immediate negative impact on giant kelp near its southern range limit in Baja. In contrast, the impacts of the heatwave were delayed throughout much of the central portion of our study area, while the northern portions of our study area exhibited high levels of resistance and resilience to the warming, despite large positive temperature anomalies. Giant kelp resistance throughout the entire region was most strongly correlated with the mean temperature of the warmest month of the heatwave, indicating that the loss of canopy was more sensitive to exceeding an absolute temperature threshold than to the magnitude of relative changes in temperature. Resilience was spatially variable and not significantly related to SST metrics or to resistance, indicating that local scale environmental and biotic processes played a larger role in determining the recovery of kelp from this extreme warming event. Our results highlight the resilient nature of giant kelp, but also point to absolute temperature thresholds that are associated with rapid loss of kelp forests.
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