Shortfalls in the protection of persistent bull kelp forests in the USA

Abstract

Kelp forests are one of the most productive ecosystems on earth, providing critical ecosystem services. Despite their global importance, their persistence in the face of human pressure and climate change is uncertain. We present a 38-year quarterly time series of satellite imagery that maps the distribution and persistence of surface canopy-forming kelp (dominated by the bull kelp, Nereocystis leutkeana) forests along eleven degrees of latitude in the western Pacific of the USA. We estimate kelp persistence as the fraction of years occupied by kelp canopy in the time series and evaluate the representation of kelp in marine protected areas (MPAs). While 3.6 % of kelp habitat is fully protected and 10.1 % is partially protected, only 0.7 % of the highly persistent kelp which may be indicative of climate refugia are fully protected. Regionally, the amount of kelp fully protected inside MPAs decreases from Central Northern California (8.0 %) to Oregon (5.9 %), Northern California (1.7 %), and Washington (0 %). Five years after the 2014–2016 marine heatwaves, kelp forests did not recover in California (∼90 % loss for both regions), while Oregon and Washington remained near pre-heatwave values. The low amount of protection in Northern California is concerning and likely exacerbates the vulnerability of kelp and associated species to marine heatwaves. Meeting a target of protecting 10 % of existing kelp habitat will require a 2.5-fold increase in kelp representation in MPAs. Moreover, we propose protecting highly persistent kelp is a cost-effective approach to increase representation and efficacy of MPAs to support climate resilience of kelp forest ecosystems.