Data_Sheet_1_Extreme Marine Heatwaves Alter Kelp Forest Community Near Its Equatorward Distribution Limit.docx

Climate change is increasing the frequency and severity of marine heatwaves. A recent extreme warming event (2014–2016) of unprecedented magnitude and duration in the California Current System allowed us to evaluate the response of the kelp forest community near its southern (warm) distribution limit. We obtained sea surface temperatures for the northern Pacific of Baja California, Mexico, and collected kelp forest community data at three islands, before and after the warming event. The warming was the most intense and persistent event observed to date, with low-pass anomalies 1°C warmer than the previous extremes during the 1982–1984 and 1997–1998 El Niños. The period between 2014 and 2017 accounted for ∼50% of marine heatwaves days in the past 37 years, with the highest maximum temperature intensities peaking at 5.9°C above average temperatures for the period. We found significant declines in the number of Macrocystis pyrifera individuals, except at the northernmost island, and corresponding declines in the number of fronds per kelp individual. We also found significant changes in the community structure associated with the kelp beds: half of the fish and invertebrate species disappeared after the marine heatwaves, species with warmer affinities appeared or increased their abundance, and introduced algae, previously absent, appeared at all islands. Changes in subcanopy and understory algal assemblages were also evident; however, the response varied among islands. These results suggest that the effect of global warming can be more apparent in sensitive species, such as sessile invertebrates, and that warming-related impacts have the potential to facilitate the establishment of tropical and invasive species.

气候变化正加剧海洋热浪的发生频次与强度。加利福尼亚洋流系统（California Current System）在2014至2016年间发生了一场强度与持续时间均前所未有的极端暖化事件，使我们得以评估巨藻林群落于其南部（暖性）分布极限附近的响应情况。我们于该暖化事件前后，在墨西哥下加利福尼亚半岛北部太平洋海域获取了海表温度数据，并在三座岛屿处采集了巨藻林群落的相关数据。此次暖化事件是迄今观测到的强度最高、持续时间最久的海洋暖化事件，其低通海温异常较1982至1984年及1997至1998年厄尔尼诺（El Niño）期间的极端暖化事件高出1℃。2014至2017年间的海洋热浪天数占过去37年总天数的约50%，其中最高温峰值较该时段平均温度高出5.9℃。我们发现，除最北部岛屿外，巨藻（Macrocystis pyrifera）的个体数量均出现显著下降，且每株巨藻的叶片数也随之减少。我们还观测到巨藻床相关群落结构发生了显著变化：海洋热浪过后，近半数鱼类与无脊椎动物物种消失；暖性偏好物种出现或其种群丰度上升；此前从未出现的外来藻类在三座岛屿均有发现。亚冠层与冠下层藻类群落的组成也出现了明显变化，但不同岛屿的响应存在差异。上述结果表明，全球变暖的影响在敏感物种（如固着无脊椎动物）中表现得更为显著，且暖化相关的环境变化可能会促进热带物种与入侵物种的定植。