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| Titel |
Bio-mixing due to Diel Vertical Migration of Daphnia spp. in a Small Lake |
| VerfasserIn |
Stefano Simoncelli, Danielle Wain, Stephen Thackeray |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250135738
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| Publikation (Nr.) |
 EGU/EGU2016-16637.pdf |
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| Zusammenfassung |
| Bio-turbulence or bio-mixing refers to the contribution of living organisms towards the
mixing of waters in oceans and lakes. Experimental measurements in an unstratified tank by
Wilhelmus & Dabiri (2014) show that zooplankton can trigger fluid instabilities through
collective motions and that energy is imparted to scales bigger than organism’s size of
few mm. Length scales analysis, for low-Reynolds-number organisms in stratified
water by Leshansky & Pismen (2010) and Kunze (2011), estimate eddy diffusivity
up two orders of magnitude larger than the molecular thermal diffusivity. Very
recently, Wand & Ardekani (2015) showed a maximum diffusivity of 10−5 m2/s
for millimetre-sized organisms from numerical simulations in the intermediate
Reynolds number regime. Here we focus our attention on turbulence generated by the
vertical migration of zooplankton in a small lake, mostly populated by Daphnia
spp. This very common species, belonging to Cladocera order, is engaged in a
vertical migration (DVM) at sunset, with many organisms crossing the thermocline
despite the density stratification. During the ascension they may create hydrodynamic
disturbances in the lake interior where the stratification usually suppresses the vertical
diffusion.
We have conducted five turbulence experiments in Vobster Quay, a small (area ∼ 59,000
m2), deep (40m) man-made basin with small wind fetch and steep sides, located in the South
West UK. Turbulence was measured with a temperature microstructure profiler. To asses the
zooplankton vertical concentration we used a 100 μm mesh net, by collecting and analyzing
samples in 8 layers of the lake. A bottom-mounted acoustic Doppler current profiler was also
employed to track their concentration and migration with the measured backscatter
strength.
Measured dissipation rates ɛ during the day showed low turbulence level (<= 10−8
W/Kg) in the thermocline and in the zooplankton layer. Turbulence, during the DVM in two
different days, is highest on the surface, likely due to surface processes. Peaks of 10−6.5
W/kg were measured within the migrating zooplankton layer with respect to profiles before
sunset and estimated eddy diffusivity was as much as 10−5 m2/s. Before and after the time
series there was no wind and penetrative convection associated with night-time cooling
wasn’t active during the experiments. Given the uncertainty in measuring the length scales of
turbulence associated with small zooplankton and the presence of turbulence patches
outside the migrating layer, further datasets are needed for definitive conclusions. |
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