Observations of Biologically Generated Turbulence in a Coastal Inlet

Abstract

Measurements in a coastal inlet revealed turbulence that was three to four orders of magnitude larger during the dusk ascent of a dense acoustic-scattering layer of krill than during the day, elevating daily-averaged mixing in the inlet by a factor of 100. Because vertically migrating layers of swimming organisms are found in much of the ocean, biologically generated turbulence may affect (i) the transport of inorganic nutrients to the often nutrient-depleted surface layer from underlying nutrient-rich stratified waters to affect biological productivity and (ii) the exchange of atmospheric gases such as CO2 with the stratified ocean interior, which has no direct communication with the atmosphere.

Role of vertical migration in biogenic ocean mixing

“…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]. By contrast, recent field measurements by Gregg and Horne [2009] and Lorke and Probst [2010] and numerical simulations by J.-L. Thiffeault and S. Childress (Stirring by swimming bodies, arXiv:0911.5511v1, 2010) all indicate negligible buoyancy flux due to animal swimming.…”

“…[3] A notable difference between the measurements of Kunze et al [2006] and subsequent investigations is that the former experiments were carried out on plankton during vertical migration. By contrast, the field measurements of schooling fish by Gregg and Horne [2009] and Lorke and Probst [2010] and the numerical simulations by Thiffeault and Childress (arXiv:0911.5511v1, 2010) are missing this behavioral component of animal swimming.…”

In situ measurements of turbulence in fish shoals

“…The role of biologically generated turbulence for vertical mixing of the ocean became the issue of a recent debate (Kunze et al 2007;Visser 2007a,b). Although this issue is not really a new one, it was brought into focus by a study of Kunze et al (2006), who observed strongly increased levels of turbulence during the ascent of dense swarms of krill performing diel vertical migration in the coastal ocean.…”

Resuspension by fish facilitates the transport and redistribution of coastal sediments

“…2). We then used the model to examine the spatial distribution of sediment in Saanich Inlet, considering realistic cross-slope advection, u 5 0.77 km d 21 (Yahel et al 2008); vertical eddy diffusivity, e z 5 20 3 10 24 m 22 s 21 , which is in the middle of the range reported for Saanich Inlet (Kunze et al 2006); and settling velocity (w s ). The settling velocities (w s ) for silt and clay were estimated independently according to Stokes's law using particles and seawater density of 2.6 and 1.025 g cm 23 , respectively; viscosity of 0.015 dyne s cm 22 ; and mean particle diameter of 3 and 20 mm for clay and silt, respectively.…”