Department of Ecology and Evolutionary Biology, University of California Los Angeles, 621 Charles E. Young Drive South, Los Angeles, CA 90095-1606, USA.
Southern California Coastal Water Research Project, 3535 Harbor Boulevard, Costa Mesa, CA 92626, USA.
Mar Environ Res. 2018 Sep;140:1-9. doi: 10.1016/j.marenvres.2018.05.018. Epub 2018 May 21.
Recent evidence suggests macroalgal blooms may play a role in the worldwide decline in seagrass, but the shape of the functional relationship between seagrass health and dominant bloom-forming macroalgae is poorly characterized. We tested whether the impact of varying abundances of two cosmopolitan bloom-forming macroalgal genera caused linear/quasi-linear or sudden threshold changes in measures of eelgrass, Zostera marina, meadow health. We conducted two caging experiments in a shallow Z. marina bed (∼1 m depth) in Bodega Harbor, California, USA where we maintained six densities within the range of natural abundances of macroalgae, Ulva (0-4.0 kg m) and Gracilariopsis (0-2.0 kg m), as well as uncaged controls over a 10-week period. Shoot density, blade growth, and epiphyte load were measured every two weeks and algal treatments reset. We did not find support for threshold transitions between algal abundance and measures of seagrass bed health using sigmoidal and broken-stick regression analyses for each data set; these models are commonly used to identify threshold patterns in ecological shifts. Instead, final measurements of shoot density and epiphyte load were best modelled as linear or slightly non-linear declines with increasing Ulva abundance. A negative linear relationship also existed between shoot density and Gracilariopsis abundance and a trend towards linear negative effects on epiphyte load. The similar shape of these functional relationships across different types of algae suggests the relationship may be generalizable. At algal abundances that are commonly observed, we found smooth and predictable negative impacts to Z. marina by decline in shoot density and potential impacts to food webs by loss of epiphytes rather than sudden threshold shifts or "ecological surprises". Our work contrasts with the growing body of literature suggesting highly non-linear shifts in response to human impact; thus, it is important to broaden understanding of shifts to more than just pattern but to the processes that drive different patterns of shifts.
最近的证据表明，大型藻类的爆发可能在世界范围内海草减少中发挥了作用，但海草健康与优势形成藻类之间的功能关系的形状特征描述较差。我们测试了两种世界性的形成藻华的大型藻类属的丰度变化是否会导致鳗草（Zostera marina）草地健康的测量值出现线性/准线性或突然的阈值变化。我们在美国加利福尼亚州博德加港（Bodega Harbor）的一个浅层鳗草床（约 1 米深）中进行了两次笼养实验，在自然藻类丰度范围内（0-4.0 kg m 的 Ulva 和 0-2.0 kg m 的 Gracilariopsis）维持了六个密度，以及在 10 周的时间内进行了无笼对照。每隔两周测量一次芽密度、叶片生长和附生植物负荷，并重置藻类处理。我们没有发现支持在藻类丰度和海草床健康测量值之间的阈值转换的证据，使用了每个数据集的 sigmoidal 和 broken-stick 回归分析；这些模型常用于识别生态变化中的阈值模式。相反，芽密度和附生植物负荷的最终测量值最好用线性或略非线性的方式来建模，随着 Ulva 丰度的增加而下降。芽密度与 Gracilariopsis 丰度之间也存在负线性关系，并且对附生植物负荷的影响呈线性负趋势。这些不同类型藻类之间的功能关系的相似形状表明，这种关系可能具有普遍性。在通常观察到的藻类丰度下，我们发现鳗草的芽密度下降和附生植物的潜在损失对食物网的负面影响是可预测的，而不是突然的阈值变化或“生态惊喜”。我们的工作与越来越多的文献形成对比，这些文献表明对人类影响的反应高度非线性；因此，重要的是不仅要拓宽对模式的理解，还要拓宽对驱动不同模式转变的过程的理解。
