Stirring by Swimming Organisms
Abstract
The question of which factors contribute to mixing in the ocean has been the subject of many studies. The idea that organisms could play a role in affecting their environment through something like mixing first appeared in Darwin's last book, called "The Formation of Vegetable Mould Through the Action of Earth Worms, with Observations on Their Habits." At the time, the idea that many small events could accumulate to create something much bigger was quite controversial. It was not until almost a century later that Walter Munk seriously proposed marine organisms as a possible factor for mixing in the ocean. However, Munk discounted the effect as negligible. The idea lay dormant for 40 years, until 2004 when Huntley & Zhou analyzed the swimming of 100 species in a wide range of sizes, and concluded that for 11 representative species the turbulent energy production is approx. 10(exp -5) W kg (exp -1), the total of which would add up to an amount comparable to the energy dissipation by major storms. An experimental study by Kunze et al. showed that the level of turbulent activity in an inlet was elevated by 2 to 3 orders of magnitude during the day, due to swimming krill. In reaction to the papers mentioned above, Visser argues that very small organisms such as zooplankton cannot produce sufficient mixing to overturn a stratified medium. The author points out that the efficiency of mixing is proportional to the ratio of the lengthscale of the organism (L) and a buoyancy lengthscale, defined as B. When L < B, the mixing efficiency quickly drops by several orders of magnitude, rendering biomixing negligible. Katija and Dabiri, however, suggest that it is not the scale of turbulence produced by the marine organisms that is the relevant quantity, but the net displacement of fluid particles they cause. This aspect will be discussed in these notes by constructing a simple model of stirring by swimming organisms, based on drift trajectories due to a moving object.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jun 23, 2010
- Accession Number
- ADA557117
Entities
People
- Jean-Luc Thiffeault
- Renske Gelderloos
- Woosak Moon
Organizations
- Woods Hole Oceanographic Institution