Nature封面文章续写达尔文理论

　　

生物通综合：来自加州理工学院生物工程系等处的研究人员利用小型水母进行实验，发现海洋中的小鱼小虾能够将许多的热量、气体和营养物质以水柱的形式混合在一起，就像风和潮汐产生的功效一样。这一研究成果公布在Nature封面。

（Nature封面，一种偏僻海岛湖泊中游动的水母）

物理学家查尔斯•达尔文爵士是进化论创始人查尔斯•达尔文的孙子，他的一项被人遗忘的贡献解决了关于海洋混合的一个长期争论。他在1953年提出，一个物体穿过流体时会随之带走一部分流体。将这一假设应用于海洋，则意味着水中的生物——哪怕是最小和最慢的——运动时所产生的搅拌作用将扰动大量的海水。也就是说游泳的动物也许对海洋中水的混合有相当大贡献。

在这篇文章中，研究人员在南太平洋帕劳群岛的一个泻湖中完成了一系列非常简单的试验，从而验证了这一假设。研究人员在水中放置团状染料，然后观测水母从中游过时染料的分布情况。结果发现，水母游过的路径上出现一长条染料“尾巴”，其中90％都是由于这种机制的作用。研究人员说，一个小型海洋生物只要游出几个身长的距离，就会带动相当于自身4倍体积的液体。由于海洋中存在大量的小型生物，它们通过这种机制而产生的能量非常可观，与风和潮汐的作用在同一个数量级上。

另外据科学时报报道，丹麦哥本哈根大学的海洋学家Gary Shaffer表示，这项研究工作为海洋生物圈如何能够为自己创造一个更有利的条件“给出了一个迷人的可能性”。他解释说，例如，这意味着位于秘鲁和智利海岸边的浮游生物的垂直游动能够将营养物质带到海洋表面，而这些营养物质可供浮游植物食用，而后者则又是浮游动物的食物。

美国塔拉哈西市佛罗里达州立大学的海洋学家William Dewar对此表示赞同。他说：“这项研究表明，即便相对较小的海洋动物也能够在海洋的混合中扮演一个重要的角色。随着许多海洋动物每天达几百米的上升与下潜运动，它们的作用将变得很重要。”

原文摘要：

A viscosity-enhanced mechanism for biogenic ocean mixing

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 together1. 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 mixing2. Ongoing debate regarding biogenic mixing has focused on comparisons between animal wake turbulence and ocean turbulence3, 4. Here, we show that a second, previously neglected mechanism of fluid mixing—first described over 50 years ago by Charles Darwin5— 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 device6. 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.