Hydrodynamics of Migrating Zooplankton in a small lake

Abstract

Zooplankton diel vertical migration (DVM) plays a pivotal role in controlling trophic interactions and nutrient transport in lakes and oceans. Understanding behaviours and responses of diel migrators is therefore essential to knowledge of physical processes and ecosystem functioning. This thesis investigated zooplankton hydrodynamics during the DVM in freshwater bodies through two different research topics. The first research explored the potential of swimming zooplankton hydrodynamics in affecting lake turbulent and biological processes. Past research suggests that zooplankton may be able to inject turbulent kinetic energy (TKE) in the water column when organisms swim. This process, referred to as biomixing, may increase vertical mixing in lakes. Since no field studies exist about biomixing by small zooplankton, turbulence and mixing were sampled in a lake during the dusk DVM of Daphnia. Results indicate that swimming Daphnia did not intensify dissipation rates of TKE and vertical fluxes. This suggests that small zooplankton cannot affect lake mixing even when organisms collectively swim. The second research examined how changes in ecosystem conditions affect zooplankton displacement velocity (DV) during the DVM of Daphnia. Currently, it is not known which environmental factors are key in driving this velocity. DV was measured in the field during the sunset migration (upwards DV) and sunrise migration (downwards DV) along with temperature, relative change in light intensity, chlorophyll-a and zooplankton concentration, as possible velocity drivers. Results show that upwards velocities were strongly correlated with the water temperature in the migrating layer, suggesting that temperature can control swimming activity, metabolic rates and escape reactions from predators. Downwards velocities were instead constant. Modelling this velocity as a sinking rate indicates that buoyancy and gravity are the governing parameters. The model also suggests that zooplankton favour passive sinking over active swimming to preserve energy and generate hydrodynamic disturbances not detectable by predators.

Date of Award	1 Feb 2018
Original language	English
Awarding Institution	University of Bath
Supervisor	Danielle Wain (Supervisor) & Thomas Kjeldsen (Supervisor)