Bob and Norma Street Environmental Fluid Mechanics Laboratory, Civil and Environmental Engineering, Stanford University, Stanford, CA, USA.
Mechanical Engineering, Stanford University, Stanford, CA, USA.
Nature. 2018 Apr;556(7702):497-500. doi: 10.1038/s41586-018-0044-z. Epub 2018 Apr 18.
Biologically generated turbulence has been proposed as an important contributor to nutrient transport and ocean mixing. However, to produce non-negligible transport and mixing, such turbulence must produce eddies at scales comparable to the length scales of stratification in the ocean. It has previously been argued that biologically generated turbulence is limited to the scale of the individual animals involved , which would make turbulence created by highly abundant centimetre-scale zooplankton such as krill irrelevant to ocean mixing. Their small size notwithstanding, zooplankton form dense aggregations tens of metres in vertical extent as they undergo diurnal vertical migration over hundreds of metres. This behaviour potentially introduces additional length scales-such as the scale of the aggregation-that are of relevance to animal interactions with the surrounding water column. Here we show that the collective vertical migration of centimetre-scale swimmers-as represented by the brine shrimp Artemia salina-generates aggregation-scale eddies that mix a stable density stratification, resulting in an effective turbulent diffusivity up to three orders of magnitude larger than the molecular diffusivity of salt. These observed large-scale mixing eddies are the result of flow in the wakes of the individual organisms coalescing to form a large-scale downward jet during upward swimming, even in the presence of a strong density stratification relative to typical values observed in the ocean. The results illustrate the potential for marine zooplankton to considerably alter the physical and biogeochemical structure of the water column, with potentially widespread effects owing to their high abundance in climatically important regions of the ocean .
生物产生的湍流被认为是营养物质输送和海洋混合的一个重要因素。然而，为了产生不可忽视的输送和混合，这种湍流必须在与海洋分层长度相当的尺度上产生漩涡。先前有人认为，生物产生的湍流仅限于所涉及的单个动物的尺度，这使得由大量厘米级浮游动物（如磷虾）产生的湍流与海洋混合无关。尽管它们体积小，但浮游动物在进行数百米的昼夜垂直迁移时，会在数十米的垂直范围内形成密集的聚集。这种行为可能会引入其他相关长度尺度，如聚集尺度，这与动物与周围水柱的相互作用有关。在这里，我们表明，厘米级游泳生物（如卤虫 Artemia salina）的集体垂直迁移产生了聚集尺度的漩涡，这些漩涡混合了稳定的密度分层，从而导致有效湍流扩散系数比盐的分子扩散系数大三个数量级。这些观察到的大规模混合漩涡是由于单个生物体尾部的流动在向上游动时合并形成一个大规模向下射流的结果，即使在存在与海洋中观察到的典型值相比强烈的密度分层的情况下也是如此。研究结果表明，海洋浮游动物有潜力极大地改变水柱的物理和生物地球化学结构，由于它们在海洋中对气候重要的区域数量众多，因此可能会产生广泛的影响。
