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Citation: García-Reyes M, Thompson SA, Rogers-Bennett L, Sydeman WJ (2022) Winter oceanographic conditions predict summer bull kelp canopy cover in northern California. PLoS ONE 17(5): e0267737. https://doi.org/10.1371/journal.pone.0267737
Bull kelp, Nereocystis luetkeana, is an iconic kelp-forest species from the Northeast Pacific that provides a wide range of ecosystem services and benefits to coastal marine species and society. In northern California, U.S.A., Nereocystis abundance declined sharply in 2014, and has yet to recover as of 2020. While extreme abiotic and biotic stressors were present prior to 2014, the population collapse highlights the need for a better understanding of how environmental conditions impact Nereocystis canopy cover. In this study, we used a newly-developed, satellite-based dataset of bull kelp canopy area over 20 years to investigate how seasonal environmental conditions may impact its extent. We found that for the period before the decline, 1991–2013, wintertime ocean conditions, synthesized in a Multivariate Ocean Climate Indicator (MOCI), were a good predictor of summer Nereocystis canopy cover. However, for Nereocystis south of Point Arena, California, winter ocean conditions had lower explanatory power than north of Point Arena, as spring upwelling was also significant and combined with winter temperature indicators in a multivariate model show higher predictability than winter alone. These results indicates that the gametophytes and early sporophytes stages of Nereocystis are highly sensitive to oceanographic conditions. Further, we found that unfavorable environmental conditions in winter 2014 could have been used to predict the Nereocystis collapse in summer 2014, and for kelp north of Point Arena, a further decline in 2015. Importantly, the models do a poor job of predicting changes in kelp after the decline in 2015, suggesting that other factors were suppressing kelp recovery, most likely herbivory from a sea urchin population boom. We conclude that winter oceanographic conditions are useful for predicting summer bull kelp forest canopy extent and will be useful in interpreting and adapting restoration and conservation actions.
Link to paper: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0267737
* Corresponding author: Marisol García-Reyes, marisolgr@faralloninstitute.org