El Niño and marine heatwaves: Ecological impacts on Oregon rocky intertidal kelp communities at local to regional scales

El Niños and marine heatwaves are predicted to increase in frequency under
greenhouse warming. The impact of climate oscillations like El
Niño-Southern Oscillation on coastal environments in the short-term likely
mimics those of climate change in the long-term; therefore, El Niños may
serve as a short-term proxy for possible long-term ecological responses to
an increasingly variable climate. Understanding and prediction of
ecosystem responses requires elucidating the mechanisms underlying
different organizational scales (organism, space, and time). We analyzed
spatiotemporal variation in the effect of the 2015-16 El Niño and the
overlapping 2014-2016 East Pacific marine heatwave on three intertidal
kelps (Hedophyllum sessile, Egregia menziesii, and Postelsia palmaeformis)
at 7 sites across 300 km of the Oregon coast and over three years post El
Niño. We measured percent cover, density, maximum length, growth, and
Carbon:Nitrogen (C:N) ratios monthly in spring/summer at each site from
2016 through 2018. Results revealed a complex interplay between spatial,
temporal, and biological factors that modified the effects of these
thermal effects on Oregon intertidal kelp populations. Our findings
generally agree with prior literature showing detrimental effects of El
Niño on kelp. However, El Niño and possibly marine heatwave effects can be
mitigated or amplified by environmental processes and kelp life history
strategies. In our study, coastal upwelling provided regional relief for
the kelp populations with respect to their growth needs and mitigated the
adverse effects of warming. On the other hand, we also found that coastal
upwelling amplified, or compounded, detrimental effects of El Niño by
increasing phytoplankton-induced shading and mollusk grazing on juvenile
and adult kelps, thereby reducing their density. Given the greater
uncertainty associated with warming events and climate change in the
California Current Upwelling System and its biological implications, our
findings reiterate the importance of acquiring better understanding of how
context-specific underlying conditions modify ecosystem processes. More
specifically, understanding how demographic traits and life history stages
of kelp change with biological interactions and environmental forcing over
temporal and spatial scales is crucial to anticipating future climate
change ramifications.

在温室增温背景下，厄尔尼诺（El Niño）事件与海洋热浪的发生频率预计将有所上升。诸如厄尔尼诺-南方涛动（El Niño-Southern Oscillation）这类气候振荡，其对沿海环境的短期影响大概率与长期气候变化的影响相似；因此，厄尔尼诺事件可作为气候日益多变背景下，生态系统长期响应的短期替代指标。要理解并预测生态系统的响应，需阐明不同组织尺度（个体、空间与时间）下的内在机制。我们针对俄勒冈海岸300公里范围内的7个采样点，在厄尔尼诺事件结束后的三年内，分析了2015-2016年厄尔尼诺事件与2014-2016年东太平洋海洋热浪叠加对3种潮间带褐藻（Hedophyllum sessile、Egregia menziesii及Postelsia palmaeformis）的影响的时空变化。我们于2016年至2018年期间，在每年春夏季每月测定各采样点的褐藻盖度、种群密度、最大体长、生长速率以及碳氮比（C:N）。研究结果显示，空间、时间与生物因子间存在复杂的交互作用，这些因子会调控上述热胁迫事件对俄勒冈州潮间带褐藻种群的影响。本研究结果与此前相关文献的结论基本一致，即厄尔尼诺事件会对褐藻产生不利影响。但环境过程与褐藻的生活史策略，可缓解或加剧厄尔尼诺事件乃至海洋热浪的影响。在本研究中，沿岸上升流可在区域尺度上满足褐藻种群的生长需求，并缓解升温带来的不利影响。但与此同时，我们也发现沿岸上升流会通过增加浮游植物引发的遮光效应以及软体动物对幼体和成体褐藻的啃食，加剧厄尔尼诺事件的不利影响，进而降低褐藻种群密度。鉴于加利福尼亚流上升流系统（California Current Upwelling System）中升温事件与气候变化相关的不确定性更高，且其生物学效应尚不明确，本研究结果再次强调：深入理解特定背景下的内在条件如何调控生态系统过程，具有重要意义。更具体而言，明确褐藻的种群统计特征与生活史阶段如何随生物交互作用和环境驱动因子在时空尺度上发生变化，对预测未来气候变化的影响至关重要。