Influence of animals on turbulence in the sea

Abstract

Analysis of data on the hydrodynamics of swimming by 100 species, ranging in body mass (M ) from bacteria to blue whales, leads to a model of animal-induced turbulence in the ocean. Swimming speeds and Reynolds number (Re) are strongly correlated with body mass, both at typical cruising speeds and at escape speeds associated with predator-prey interactions. We find that animals operating at Re > 1000 typically form schools that are concentrated by many orders of magnitude above their average abundance. We calculate the rate of kinetic energy production by 11 representative species of schooling animals ranging in size from euphausiids to whales, and find it to be of the order of 10 -5 W kg -1, regardless of animal size. Animal-induced turbulence is comparable in magnitude to rates of turbulent energy dissipation (ε) that result from major storms. The horizontal length scale (10 to 1000 m) of energy production rate by animal schools is comparable to the observed fine-scale variability in ε. We present detailed case studies of 4 species -Atlantic bluefin tuna, Norwegian herring, northern anchovy and Antarctic krill -all of which have schooling behavior that places them within the zone of maximum seasonal stratification where their energy production rate would be 3 to 4 orders of magnitude greater than the background average rate of turbulent energy dissipation. We conclude that schooling animals are an important source of fine-scale turbulent mixing in the ocean, especially in coastal regions during summer. KEY WORDS: Turbulence · Animal swimming · EpsilonResale or republication not permitted without written consent of the publisher

In situ measurements of turbulence in fish shoals

“…TL M , WW M , % M , N SG are the corresponding mean values of length, weight, and relative and absolute abundances used in the following calculations. U 0 is the observed swimming speed and U c is the cruising speed determined from WW M using an empirical relationship provided by Huntley and Zhou (2004). Re is the Reynolds number (Eq.…”

“…Our measurements fit surprisingly well into this analysis, as exemplified by the close agreement between the cruising speed U c , estimated solely from body mass, and observed swimming speed U 0 (Table 3). Following Huntley and Zhou (2004), the production rate of mechanical energy P Drag by swimming fish with an abundance N (m 23 ) is the product of swimming speed and the work each fish does to overcome the drag force D:…”

“…Huntley and Zhou (2004) provided a set of empirical formulas relating Reynolds number of animal swimming to body mass, size, and swimming speed by analyzing data from 100 marine species. Our measurements fit surprisingly well into this analysis, as exemplified by the close agreement between the cruising speed U c , estimated solely from body mass, and observed swimming speed U 0 (Table 3).…”

Role of vertical migration in biogenic ocean mixing

“…[2] The proposition that swimming animals can influence large-scale ocean mixing and circulation was introduced in jest by Munk [1966Munk [ , also personal communication, 2007, but has received more serious attention in recent years [Huntley and Zhou, 2004;Dewar et al, 2006]. Kunze et al [2006] observed significantly elevated kinetic energy dissipation rates in the vicinity of aggregations of krill, and subsequently noted shear fluctuations at length scales up to one meter, significantly larger than the individual animals [Kunze et al, 2007].…”

Active and passive vertical motion of zooplankton in a lake

“…In lakes, the latter scales are in the horizontal dimensions associated with variations of depth (e.g., littoral-pelagic gradients) and hence with basin scales. For zooplankton in lakes, which is most abundant in size classes around and smaller than one millimeter, the swimming speed can be estimated to be within a few body lengths per second (Huntley and Zhou 2004). Hence, in horizontal dimensions, with typical current velocities of several centimeters per second (Wü est and Lorke 2003), zooplankton meet the criteria of passive plankton organisms.…”