Daphnia magna filtration efficiency and mobility in laminar to turbulent flows

Abstract

Daphnia are filter feeder organisms that prey on small particles suspended in the water column. Since Daphnia individuals can feed on wastewater particles, they have been recently proposed as potential organisms for tertiary wastewater treatment. However, analysing the effects of hydrodynamics on Daphnia individuals has scarcely been studied. This study focuses then, on quantifying the filtration and swimming velocities of D. magna individuals under different hydrodynamic conditions. Both D. magna filtration and movement responded differently if the flow was laminar or if it was turbulent. In a laminar-dominated flow regime Daphnia filtration was enhanced up to 2.6 times that of a steady flow, but in the turbulent-dominated flow regime D. magna filtration was inhibited. In the laminar flow regime D. magna individuals moved freely in all directions, whereas in the turbulent flow regime they were driven by the streamlines of the flow. A model based on Daphnia-particle encountering revealed that the filtration efficiency in the laminar regime was driven by the length of the D. magna individuals and the shear rate imposed by the system.

Functional responses of Daphnia magna to zero-mean flow turbulence

“…The rectilinear oscillating motion of the grid was ensured by constraining the movement of the connecting grid shafts so that they travelled along a guide rail using precision bearings 33 . In addition, to avoid secondary circulation in the tank 4 the grid was designed by deeming that the distance between the end of the grid and the wall of the cylinder was equal to half a square of the grid.…”

“…The camera recorded 25 frames per second and the D. magna trails were recorded for 1 min for each case, giving a total of 1500 frames. These frames were analysed with ImageJ software using the mTrack plug-in 4,12,49 . The mean size of the D. magna individuals was also obtained from the ImageJ software video recording of the trajectories made by 25 individuals during each set of experiments.…”

“…Daphnia (a genus from the order of Cladocera) are known to be ubiquitous in many aquatic ecosystems. They are filter feeders that not only feed on small bacteria and phytoplankton 1 , but also on small wastewater sludge particles 2–4 . The Daphnia genus serve as food for fish and invertebrates and are key organisms in terms of the water quality in lakes and ponds.…”

“…Swimming behaviour 11 , survival 12 , growth and reproduction 13 are, among others, the key characteristics of Daphnia that determine to what degree biotic effects such as metals 14 , pharmaceuticals 15 and pesticides will impact on them. Besides biotic parameters, Daphnia are intolerant of and stressed by abiotic conditions such as temperature 14,16–18 , hypoxia, salinity 19 and hydrodynamics 4 which have been found to alter their behavior. So far, no study has documented the effects zero-mean flow turbulence has on D. magna individuals.…”

“…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 . The ingestion rate of weak planktonic protozoa swimmers rises sigmoidally with increasing turbulence that is characterised by shears of 1–10 s −1 which correspond to turbulent kinetic energy dissipations in the order of 1 to 0.01 cm 2 s −3 26 .…”