Large-scale shift in the structure of a kelp forest ecosystem co-occurs with an epizootic and marine heatwave

Abstract

Climate change is responsible for increased frequency, intensity, and duration of extreme events, such as marine heatwaves (MHWs). Within eastern boundary current systems, MHWs have profound impacts on temperature-nutrient dynamics that drive primary productivity. Bull kelp (Nereocystis luetkeana) forests, a vital nearshore habitat, experienced unprecedented losses along 350 km of coastline in northern California beginning in 2014 and continuing through 2019. These losses have had devastating consequences to northern California communities, economies, and fisheries. Using a suite of in situ and satellite-derived data, we demonstrate that the abrupt ecosystem shift initiated by a multi-year MHW was preceded by declines in keystone predator population densities. We show strong evidence that northern California kelp forests, while temporally dynamic, were historically resilient to fluctuating environmental conditions, even in the absence of key top predators, but that a series of coupled environmental and biological shifts between 2014 and 2016 resulted in the formation of a persistent, altered ecosystem state with low primary productivity. Based on our findings, we recommend the implementation of ecosystem-based and adaptive management strategies, such as (1) monitoring the status of key ecosystem attributes: kelp distribution and abundance, and densities of sea urchins and their predators, (2) developing management responses to threshold levels of these attributes, and (3) creating quantitative restoration suitability indices for informing kelp restoration efforts.

Survivors of Climate Driven Abalone Mass Mortality Exhibit Declines in Health and Reproduction Following Kelp Forest Collapse

“…Recent satellite imagery confirms that surface coverage of the kelp forests in northern California are still at less than 5% of good years (McPherson et al, 2021), indicating that starvation conditions continue to exist for the vast majority of the red abalone stocks. Being at the southern border of Nereocystis spp., this region and species of kelp may be particularly susceptible to warming (Smale et al, 2019).…”

“…However, the intense MHW in 2014-2016 in the Northeast Pacific (Di Lorenzo and Mantua, 2016) impacted this region and there were massive northern shifts in populations (Sanford et al, 2019), reductions in nutrients and productivity (Leising et al, 2015) and changes to the local kelp forest ecosystems. Following the MHW, purple sea urchin (Strongylocentrotus purpuratus) populations exploded and contributed to the collapse of the bull kelp forest across more than 300 km of coastline in northern California McPherson et al, 2021). This region's kelp forest was the epicenter of the recreational red abalone (Haliotis rufescens) fishery, which was the last abalone fishery remaining open in California.…”

Impacts of Marine Heatwaves on Algal Structure and Carbon Sequestration in Conjunction With Ocean Warming and Acidification

“…MHWs in Western Australia led to the lack of seasonality in growth rates and high rates of mortality for crustose coralline algae (Short et al, 2015). Bull kelp (Nereocystis luetkeana) forests, historically resilient to fluctuating environmental conditions, experienced unprecedented losses along 350 km of coastline in northern California due to the extreme marine heatwaves between 2014 and 2016, and the once extensive giant kelp forests have not recovered till now (McPherson et al, 2021). Therefore, the multiple-year MHWs lead to a persistently altered ecosystem state with low primary productivity.…”

A More Comprehensive Climate Vulnerability Assessment Framework for Fisheries Social-Ecological Systems

“…These two pathways impacted the behavior and abundance of the red sea urchin's primary competitor, the purple urchin. Exposure to reduced availability of prey, combined with reduced predator abundance led to a shift in purple sea urchins' foraging behavior, from a passive detritivore restricted to cracks and crevices to an active grazer, denuding the reef of almost all macroalgae (Rogers-Bennett and Catton, 2019;McPherson et al, 2021). This behavioral response was also accompanied by a numerical response: overgrazing by purple sea urchins led to shifts in the macroalgal assemblage from one dominated by kelps and foliose algae to one dominated by grazing-resistant crustose (and erect) coralline algae (CCA).…”