Changes in kelp forest communities off Washington, USA, during and after the 2014-2016 marine heatwave and sea star wasting syndrome

Abstract

Canopy-forming kelps are foundation species in many coastal ecosystems, but kelp-forest communities are subject to abrupt state changes caused by environmental drivers and trophic dynamics. We examined changes in kelp communities at 5 sites along the Olympic Coast of Washington State, USA, during and following the recent perturbations of anomalous warm-water events and sea star wasting syndrome (SSWS). Anomalously warm water in 2013 and 2014 corresponded with a loss of approximately 50% of Macrocystis pyrifera and Nereocystis luetkeana canopy. However, the canopy quickly recovered, and stipe density increased after 2015. Purple sea urchins Strongylocentrotus purpuratus increased in density 164-fold, largely at one site, but this increase was first observed in 2017 and peaked in 2019, after the warm period. Sea stars did not show recovery from SSWS, with several species including Pycnopodia helianthoides continuing to decline. The majority of variation in assemblage structure occurred at the site level for kelps, macroinvertebrates, and fishes, while year explained most of the variability for juvenile rockfishes Sebastes spp. We did not see strong top-down effects of urchins on kelp, suggesting that top-down impacts were not dominant regionally during this period. In contrast, we found evidence for a bottom-up influence of kelp habitat on juvenile rockfishes, as rockfish recruits occurred with higher probability where kelp stipe density was higher. Our analyses highlight the importance of spatial variation in structuring changes in kelp forest communities associated with disturbance and suggest that it is essential to ensure the protection of a diversity of kelp forests.

Kelp dynamics and environmental drivers in the southern Salish Sea, British Columbia, Canada

“…Another factor, not covered in this research but giving support to this observation, is the relative stability of the community-wide ecosystem. At the south of the Strait of Juan de Fuca (in front of Cluster 1), kelp forests have maintained a relatively stable assemblage of invertebrates, fish, and kelp from 2015-2021 (Tolimieri et al, 2023).…”

Effects of temperature and nutrients on microscopic stages of the bull kelp (Nereocystis luetkeana, Phaeophyceae)

“…In British Columbia, bull kelp declines have occurred primarily along warmer, wave-sheltered coastlines (Starko et al, 2019(Starko et al, , 2022. A marine heatwave in 2013-2014 was followed by a 50% decline in bull kelp canopy cover on the outer coast of Washington, but the bull kelp canopy recovered by the next year (Tolimieri et al, 2023). While kelp recovery occurred on the outer coast, kelp forests are still in decline in the warmer, more inland waters of the Salish Sea.…”

Dynamics of juvenile salmon and forage fishes in nearshore kelp forests

“…Such insights will be a useful base on which to build habitat management conservation and restoration priority actions for these important species and habitats. Anthropogenic pressures, including coastal and shoreline development, invasive species, and ocean warming (Filbee-Dexter & Wernberg, 2018;Verdura et al, 2019;Smale, 2020), are causing major regional declines in nearshore kelp environments, particularly in the southern Salish Sea and north coastal British Columbia (Krumhansl et al, 2016;Filbee-Dexter & Wernberg, 2018;Pfister, Berry & Mumford, 2018;Verdura et al, 2019;Berry et al, 2021;Starko et al, 2022;Tolimieri et al, 2023). Understanding the dynamics of kelp, salmon, and forage fishes is therefore fundamental to defining the extent to which these stressors affect the coastal ecosystem functions of the nearshore kelp environment.…”