Can small zooplankton mix lakes?

Abstract

The idea that living organisms may contribute to turbulence and mixing in lakes and oceans (biomixing) dates to the 1960s, but has attracted increasing attention in recent years. Recent modeling and experimental studies suggest that marine organisms can enhance turbulence as much as winds and tides in oceans, with an impact on mixing. However, other studies show opposite and contradictory results, precluding definitive conclusions regarding the potential importance of biomixing. For lakes, only models and lab studies are available. These generally indicate that small zooplankton or passive bodies generate turbulence but different levels of mixing depending on their abundance. Nevertheless, biogenic mixing is a complex problem, which needs to be explored in the field, to overcome limitations arising from numerical models and lab studies, and without altering the behavior of the animals under study.

Convection in Lakes

“…This implies that the products L w of the eddy sizes L and velocities w , generated by aquatic organisms, need to overcome those geophysical diffusivities (Kunze et al 2006). From all known observations so far, it appears that biogenic turbulence does not significantly contribute to mixing or stratification changes (Katija 2012, Wang & Ardekani 2015, Simoncelli et al 2017. Although Noss & Lorke (2012) showed that swimming zooplankton can significantly enhance TKE dissipation in direct proximity by creating eddies in their wakes and by dragging water along, the generated L are too short to have a macroscopic effect.…”

On biogenic turbulence production and mixing from vertically migrating zooplankton in lakes

“…2003;Rinke et al 2007). However, it is not certain how C DVM increases with respect to C b in other lakes, because its value depends on the presence of other migrating species, predators and light conditions (Ringelberg 2010;Simoncelli et al 2017). The experiment by Houghton et al (2018) showed that A. salina ( l O = 5-8 mm), swimming in tank-averaged concentration C b > 46 org.…”

“…2). However, the result may depend on the organisms' concentration and how the migration was triggered in artificial laboratory conditions (Simoncelli et al 2017). Experimental measurements in a stratified tank by Houghton et al (2018) indicate instead that collective motions, arising from small zooplankton swimming at high concentration, can trigger fluid instabilities bigger than the size of the individual organism and lead to irreversible mixing.…”

“…A full review of previous studies of small zooplankton can be found in Simoncelli et al (2017). To date, observational studies on zooplankton biomixing have been limited to oceanic species only and no field studies exist on the turbulence production and mixing due to small zooplankton in freshwaters.…”

Functional responses of Daphnia magna to zero-mean flow turbulence

“…In freshwater ecosystems the so-called pusher swimmers 20,21 like the freshwater D. magna individuals, produce mean dissipation rates from 0.034 to 0.018 cm 2 s −3 , which are associated to viscous trail dissipation 22 , although when forming schools Daphnia might produce maximum dissipations of ε = 2.8 cm 2 s −3 21 . Therefore, like many zooplankton species, D. magna might potentially be important for vertical mixing in weak mixing ambient flows 21,23 . In contrast, in aquatic ecosystems, turbulence plays an additional role as a stressor by modifying the swimming capacity and survival of organisms 4 , to shifting community structure 24 or producing active cyclomorphosis 25 .…”