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

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.

冠层型大型褐藻（canopy-forming kelps）是诸多沿海生态系统中的基础物种（foundation species），但海藻林群落易受环境驱动因子与营养级动态作用引发的突发状态转换影响。本研究针对美国华盛顿州奥林匹克海岸的5个站点，在近期异常暖水事件与海星消瘦综合征（sea star wasting syndrome, SSWS）扰动期间及扰动后，调查了海藻林群落的动态变化。2013与2014年的异常暖水事件导致巨藻（Macrocystis pyrifera）与海囊藻（Nereocystis luetkeana）的冠层覆盖度下降约50%。但该冠层随后快速恢复，2015年后褐藻柄状体密度显著上升。紫海胆（Strongylocentrotus purpuratus）的种群密度提升164倍，该增长主要集中在单个站点，但该现象于2017年首次被观测到，并在暖水事件结束后的2019年达到峰值。海星种群未从海星消瘦综合征中恢复，包括向日葵海星（Pycnopodia helianthoides）在内的多个物种种群仍持续衰退。对于大型褐藻、大型无脊椎动物与鱼类而言，群落组成结构的绝大多数变异均发生在站点尺度；而对于幼年岩鱼（Sebastes spp.），年度差异则解释了绝大多数的变异来源。本研究未观测到海胆对大型褐藻存在显著的下行控制效应，表明此时期内区域范围内下行调控作用并非主导因素。与之相反，本研究发现海藻生境对幼年岩鱼存在上行控制影响：当褐藻柄状体密度更高时，岩鱼幼体的出现概率显著提升。本研究分析凸显了空间异质性在调控与扰动相关的海藻林群落变化中的关键作用，并指出保障多样化海藻林的保护工作至关重要。