Using a Long-term Landsat Timeseries to Understand the Effect of Marine Heatwaves on Unprecedented Declines in Northern California Bull Kelp

Kelp forests are threatened worldwide by the impacts of climate change, overfishing of ecologically relevant species, and resulting ecological disruptions. Within California, giant (Macrocystis pyrifera) and bull (Nereocystis luetkeana) kelp are foundation species that form the base of a productive and diverse ecosystem, which support economically important fisheries, ecotourism, aquaculture, and a commercial harvesting industry. Of the two, life cycle characteristics and physiological temperature thresholds for bull kelp make them particularly sensitive to marine heatwaves, often associated with climate change. This is evidenced by an unprecedented and large-scale loss of bull kelp forests in northern California from 2013 to present. Using a record of Landsat imagery for Sonoma and Mendocino Counties, we were able to infer broader temporal impacts of environmental patterns on kelp canopy area not captured solely by in situ measurements. Landsat imagery was collected between 1985 and 2019 during peak canopy conditions (August through mid-October) for Mendocino and Sonoma Counties. We used multiple endmember spectral mixture analysis (MESMA) to determine kelp canopy area. Partial least-squares regression (PLSR) analysis, which is particularly robust when predictor variables are not independent from one another, revealed the response of kelp to large- and local-scale oceanographic and biological processes. Environmental variables included El Niño/Southern Oscillation (ENSO), North Pacific Gyre Oscillation (NPGO), seasonal patterns in upwelling (SST and surface NO₃ concentration) and significant wave height (H_s), and biological factors such as purple sea urchin (Strongylocentrotus purpuratus; kelp consumer) and sunflower sea star (Pycnopodia helianthoides; urchin predator) densities. Results indicated that (1) anomalously high summer-time temperatures and NPGO had the strongest influence on northern California kelp canopy dynamics across the 34-year timeseries, and (2) the 2013 to 2016 North Pacific marine heatwave (‘the blob’) and strong El Niño event initiated widespread ecological disruptions that systematically altered the stable state condition of northern California kelp forests.