E-Letter responses to:

reports: Eric Kunze, John F. Dower, Ian Beveridge, Richard Dewey, and Kevin P. Bartlett Observations of Biologically Generated Turbulence in a Coastal Inlet Science 2006; 313: 1768-1770 [Abstract] [Full text] [PDF]

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Kunze and Dowers' Response to Tom P. Rippeth et al.

Eric Kunze, John F. Dower   (7 June 2007)

Turbulent Dissipation of Coastal Seas

Tom P. Rippeth, Jo Clare Gascoigne, J. A. Mattias Green, Mark E. Inall, Matthew R. Palmer, John H. Simpson, Philip J. Wiles   (7 June 2007)

Kunze and Dowers' Response to Tom P. Rippeth et al. 7 June 2007
Eric Kunze School of Earth and Ocean Sciences, Department of Physics and Astronomy, University of Victoria, John F. Dower Respond to this E-Letter: Re: Kunze and Dowers' Response to Tom P. Rippeth et al.	Finding no evidence for enhanced turbulence during vertical migration of acoustic backscatterers in 11 summertime turbulent dissipation rate time series, T. P. Rippeth et al. suggest that turbulent mixing by swimming marine organisms does not contribute significantly to mixing in coastal shelf seas. They conclude that our finding of elevated turbulence associated with vertically migrating swarms of krill (1) is not typical of continental shelves. Given the dissipation rates that we reported from Saanich Inlet (10–5 to 10–4 W kg-1), we agree that it is unlikely that Rippeth et al. could have missed significant biologically generated turbulence if it was present as it would have been readily spotted above the average dissipation rates of (0.7 to 5.0) × 10–8 W kg-1 that they report. Although echosounder data were collected during their measurements, the vertically migrating scattering layers were not sampled directly to see which planktonic species may have been present. Instead, Rippeth et al. cite an observation that scattering layers on the European shelf have zooplankton concentrations ranging from 1 to 40 individuals m–3 that they assume are Meganyctiphanes norvegica (2), a large euphausiid common to the eastern North Atlantic, since most zooplankton are too small to contribute significantly to turbulent mixing (3). Although the abundance of Euphausia pacifica where we made our observations is indeed very high (which was why we chose Saanich Inlet to first attempt measurements of biological turbulence), the abundances cited by Rippeth et al. are near the low end of the range reported for many other euphausiids (3, 4), which can exceed 103 to 104 individuals m–3. Since planktonic organisms large enough to generate significant turbulence are likely very rare in the region where Rippeth et al. made their measurements and vertically migrating backscatterers were detected in only 4 of their 11 series (2), their finding no significant enhancement of turbulent mixing is hardly surprising. A stronger test of the hypothesis that swimming marine organisms can produce significant turbulence would be to look for a signal where and when dense swarms of vertically migrating euphausiids (or other densely packed swimming species) are known to occur. Spatial and temporal distributions of euphausiids are highly patchy, seasonally variable, and often tied to topographic features. In the Antarctic (5, 6), Euphausia superba displays a white-noise like distribution of abundance, a log-normal distribution of biomass, and typical spacing of ~4 km between aggregations that range in size from 10 to 1000 m. Assuming that similar numbers apply to other euphausiid species, the likelihood of randomly encountering a dense swarm ranges from 1 in 4 to 1 in 400. Thus, although the observations reported by Rippeth et al. represent a valuable contribution to the current debate, in light of the extreme patchiness of schools and swarms of swimming marine organisms, we contend that accurately quantifying the role of biologically generated turbulence will require many more measurements. As a first step, we recommend that measurements be undertaken in other regions where both (i) dense aggregations of swimming marine organisms are known to occur (3) and (ii) other sources of turbulence such as wind and atmospheric cooling are weak. Furthermore, biological and physical observations should be collected concurrently. It may well turn out that elevated biologically generated turbulence is even more elusive (7, 8) than the long-sought average turbulent diffusivity of 10–4 m2 s–1 called for by large-scale balance for deep-ocean stratification (9, 10). However, given that a mixing event of the magnitude observed in Saanich Inlet (1) need only occur a few times a year to produce the same mixing as deep-ocean internal waves suggests that even highly episodic biologically generated turbulence may play an important mixing role. This will represent a considerable observational challenge for the microstructure community. Eric Kunze School of Earth and Ocean Sciences, Department of Physics and Astronomy, University of Victoria, Victoria, BC, Canada. John F. Dower School of Earth and Ocean Sciences, Department of Biology, University of Victoria, Victoria, BC, Canada. References 1. E. Kunze et al., Science 313, 1768 (2006). 2. T. P. Rippeth, personal communication. 3. M. E. Huntley, M. Zhou, Mar. Ecol. Prog. Ser. 273, 65 (2004). 4. V. Siegel, Can. J. Fish. Aquat. Sci. 57 (Suppl. 3), 151 (2000). 5. D. G. Miller, I. Hampton, Polar Biol. 10, 125 (1989). 6. L. H. Weber et al., Deep-Sea Res. 33, 1327 (1986). 7. J. R. Ledwell, A. J. Watson, C. S. Law, Nature 364, 701 (1993). 8. E. Kunze et al., J. Phys. Oceanogr. 36, 1553 (2006). 9. W. Munk, Deep-Sea Res. 13, 707 (1966). 10. M. C. Gregg, J. Geophys. Res. 92, 5249 (1987).
Turbulent Dissipation of Coastal Seas 7 June 2007
Tom P. Rippeth School of Ocean Sciences, University of Wales, Bangor LL59 5AB, UK, Jo Clare Gascoigne, J. A. Mattias Green, Mark E. Inall, Matthew R. Palmer, John H. Simpson, Philip J. Wiles Respond to this E-Letter: Re: Turbulent Dissipation of Coastal Seas	E. Kunze et al. (22 Sept. 2006, p. 1770) report measurements of turbulent dissipation from a coastal inlet showing a period of much enhanced dissipation within the thermocline. In consequence of this one event, they estimate that the daily averaged mixing rate is raised by a factor ~100. As the enhancement is correlated with the dusk ascent of a dense acoustic scattering layer, they infer that the elevated mixing is due to turbulence generated by ascending zooplankton. On the basis of this single set of measurements, they argue that biologically generated turbulence may contribute significantly to vertical mixing. In order to test this proposal, we have reanalyzed 11 of our own turbulent dissipation time series collected from the stratified coastal seas to the west of the UK (1, 2). Despite having profiled dissipation repeatedly during observed migration periods, we find no statistically significant enhancement of dissipation at these times in any of the data tested. We do find, however, that the daily averaged dissipation rates in the thermocline region, although consistent with background dissipations reported for North American coastal seas (3–5), are substantially larger (7 to 50 x 10-9 W kg-1) than the daytime average (< 10-9 W kg-1 ) reported by Kunze et al. We interpret our results as indicating that the contribution of bioturbulence to turbulent mixing within the thermocline is not significant against the background of physically induced turbulence. This interpretation is further supported by observations of zooplankton concentration within the migrating layer, which are typically in the range of 1 to 40 individuals m-3 for European coastal seas (6, 7), considerably fewer than the exceptionally dense swarm (~10,000 individuals m-3) inferred by Kunze et al. We would therefore argue that the Kunze et al. result is not representative of coastal seas because of (i) unusually low levels of physically induced turbulence and (ii) very high concentrations of zooplankton. Tom P. Rippeth, Jo Clare Gascoigne, J. A. Mattias Green School of Ocean Sciences, University of Wales Bangor, UK. Mark E. Inall Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, Oban, UK. Matthew R. Palmer, John H. Simpson, Philip J. Wiles School of Ocean Sciences, University of Wales Bangor, UK. References 1. T. P. Rippeth, Philos. Trans. R. Soc. London 363, 2837 (2005). 2. M. E. Inall, T. P. Rippeth, Enivron. Fluid Mech. 2, 219 (2002). 3. E. Horne et al., Deep Sea Res. II 43, 1683 (1996). 4. J. A. MacKinnon, M. C. Gregg, J. Phys. Oceangr. 33, 1476 (2003). 5. J. Moum, J. Nash, J. Phys. Oceangr. 30, 2049 (2000). 6. G. A. Tarling, T. Jarvis, S. M. Emsley, J. B. L. Matthews, Mar. Ecol. Prog. Ser. 420, 183 (2002). 7. S. M. Emsley, G. A. Tarling, and M. T. Burrows, Fish Oceanogr. 14, 161 (2005).