A viscosity-enhanced mechanism for biogenic ocean mixing
A viscosity-enhanced mechanism for biogenic ocean mixing
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DOI：
10.1038/nature08207
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发表时间：
2009-07-30
期刊：
影响因子：
64.8
通讯作者：
Dabiri, John O.
中科院分区：
文献类型：
--
作者：
Katija, Kakani;Dabiri, John O.
最近对海洋中生物产生的湍流的观察导致了关于动物游泳对海洋混合的贡献的重要性的相互矛盾的结论。测量结果表明，在大量浮游动物一起游泳的附近，湍流消散水平升高，与风和潮汐引起的水平相当（1）。然而，也有人指出，湍流耗散升高本身并不足以证明大量的生物混合，因为小动物的大部分湍流动能都注入到奥兹米多夫浮力长度尺度以下，在那里，它主要是通过流体粘度在影响海洋混合之前以热量的形式耗散的（2）。关于生物混合的持续争论集中在动物尾流湍流和海洋湍流之间的比较上（3，4）。在这里，我们表明，第二个，以前被忽视的流体混合机制-首次描述超过50年前由查尔斯达尔文（5）-是游泳动物混合的主要机制。达尔文机制的混合效率取决于动物形状而不是流体长度尺度，并且与湍流尾流混合不同，流体粘度会增强混合效率。因此，它提供了一种在小型浮游动物和大型哺乳动物中同样有效的生物混合手段。达尔文混合和湍流尾流混合的相对贡献的理论模型被创建，并通过使用新开发的基于水肺的激光测速设备（6）对游泳水母进行现场测量来验证。这些结果外推到其他动物是直接给定的知识的动物形状和方向在垂直迁移。计算的基础上，广泛的水生动物物种，我们得出结论，生物混合通过达尔文的机制可以是一个重要的贡献者海洋混合和营养输送。
Recent observations of biologically generated turbulence in the ocean have led to conflicting conclusions regarding the significance of the contribution of animal swimming to ocean mixing. Measurements indicate elevated turbulent dissipation-comparable with levels caused by winds and tides-in the vicinity of large populations of planktonic animals swimming together(1). However, it has also been noted that elevated turbulent dissipation is by itself insufficient proof of substantial biogenic mixing, because much of the turbulent kinetic energy of small animals is injected below the Ozmidov buoyancy length scale, where it is primarily dissipated as heat by the fluid viscosity before it can affect ocean mixing(2). Ongoing debate regarding biogenic mixing has focused on comparisons between animal wake turbulence and ocean turbulence(3,4). Here, we show that a second, previously neglected mechanism of fluid mixing-first described over 50 years ago by Charles Darwin(5)-is the dominant mechanism of mixing by swimming animals. The efficiency of mixing by Darwin's mechanism is dependent on animal shape rather than fluid length scale and, unlike turbulent wake mixing, is enhanced by fluid viscosity. Therefore, it provides a means of biogenic mixing that can be equally effective in small zooplankton and large mammals. A theoretical model for the relative contributions of Darwinian mixing and turbulent wake mixing is created and validated by in situ field measurements of swimming jellyfish using a newly developed scuba-based laser velocimetry device(6). Extrapolation of these results to other animals is straightforward given knowledge of the animal shape and orientation during vertical migration. On the basis of calculations of a broad range of aquatic animal species, we conclude that biogenic mixing via Darwin's mechanism can be a significant contributor to ocean mixing and nutrient transport.