Abstract: Giant kelp and bull kelp serve as important foundation species along the coast of California and provide energy and structure for vibrant kelp forest ecosystems. However, these forests are increasingly at risk from marine heatwave events, herbivore outbreaks, and the loss or alterations in the behavior of key herbivore predators. From 2014 to 2016, persistent marine heatwave conditions were associated with losses in kelp forest canopy area across the state, with larger losses at southern latitudes for each kelp species range. The recovery of kelp forests since the end of the heatwave period has been variable across space. For example, the Big Sur coastline in Central California and much of the Santa Barbara Channel in Southern California have recovered to pre-heatwave conditions while the Northern California coast has shown little recovery. Additionally, local areas such as the Monterey Peninsula in Central California, have displayed a continued decline in kelp canopy after the heatwave period with few signs of recovery despite a lack of heatwave temperatures. Here, we used a long-term satellite imagery derived time series of kelp forest canopy area across the state of California, coupled with remotely sensed and modeled time series of environmental variables to examine the drivers of kelp forest decline and recovery.
Kelp canopy dynamics were statistically modeled as functions of disturbance by large waves, inputs of upwelled nutrients (i.e., nitrate), and ocean temperature. The timing of nutrient delivery was found to be important to the development of the kelp forest canopy, with high winter nutrient levels related to large bull kelp canopies and high spring and summer nutrient concentrations related to large giant kelp canopies for Central and Southern California, respectively. Non-linear relationships between seawater nutrient concentration and kelp canopy area were used to assess the nutrient compensation dynamics across the coast and multi-year periods of negative nutrient compensation were associated with kelp canopy decline and reduced recovery during the post-heatwave period. Additionally, the use of these models identified areas where kelp canopy dynamics were underperforming environmental conditions and may indicate arrested recovery due to other drivers, such as sea urchin herbivory.