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  1. Free, publicly-accessible full text available August 1, 2023
  2. Wernberg, T. (Ed.)
    Free, publicly-accessible full text available August 1, 2023
  3. Free, publicly-accessible full text available February 1, 2023
  4. 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 managementmore »responses to threshold levels of these attributes, and (3) creating quantitative restoration suitability indices for informing kelp restoration efforts.« less
  5. The prevalence of disease-driven mass mortality events is increasing, but our understanding of spatial variation in their magnitude, timing and triggers are often poorly resolved. Here, we use a novel range-wide dataset comprised 48 810 surveys to quantify how sea star wasting disease affected Pycnopodia helianthoides , the sunflower sea star, across its range from Baja California, Mexico to the Aleutian Islands, USA. We found that the outbreak occurred more rapidly, killed a greater percentage of the population and left fewer survivors in the southern half of the species's range. Pycnopodia now appears to be functionally extinct (greater than 99.2% declines) from Baja California, Mexico to Cape Flattery, Washington, USA and exhibited severe declines (greater than 87.8%) from the Salish Sea to the Gulf of Alaska. The importance of temperature in predicting Pycnopodia distribution rose more than fourfold after the outbreak, suggesting latitudinal variation in outbreak severity may stem from an interaction between disease severity and warmer waters. We found no evidence of population recovery in the years since the outbreak. Natural recovery in the southern half of the range is unlikely over the short term. Thus, assisted recovery will probably be required to restore the functional role of thismore »predator on ecologically relevant time scales.« less
  6. Marine ecosystems are vulnerable to climate driven events such as marine heatwaves yet we have a poor understanding of whether they will collapse or recover. Kelp forests are known to be susceptible, and there has been a rise in sea urchin barrens around the world. When temperatures increase so do physiological demands while food resources decline, tightening metabolic constraints. In this case study, we examine red abalone ( Haliotis rufescens ) looking at sublethal impacts and their prospects for recovery within kelp forests that have shifted to sea urchin barrens. Abalone are a recreationally fished species that once thrived in northern California’s bull kelp forests but have recently suffered mass mortalities since the 2014–2016 marine heatwave. Quantitative data exist on the health and reproduction of abalone both prior to and after the collapse. The survivors of the mass mortality show a 2-year lag in body and gonad condition indices. After the lag, body and gonad indexes decreased substantially, as did the relationship between shell length and body weight. Production of mature eggs per female declined by 99% ( p < 0.001), and the number of eggs per gram of female body weight (2,984/g) declined to near zero (9/g). The numbermore »of males with sperm was reduced by 33%, and the sperm abundance score was reduced by 28% ( p = 0.414). We observed that these reductions were for mature eggs and sperm while immature eggs and spermatids were still present in large numbers. In the lab, after reintroduction of kelp, weight gains were quickly lost following a second starvation period. This example illustrates how climate-driven declines in foundation species can suppress recovery of the system by impacting body condition and future reproduction of surviving individuals. Given the poor reproductive potential of the remaining abalone in northern California, coupled with ongoing mortality and low kelp abundances, we discuss the need to maintain the fishing moratorium and implement active abalone restoration measures. For fished species, such as abalone, this additional hurdle to recovery imposed by changes in climate is critical to understand and incorporate into resource management and restoration.« less