Abstract
Zooplankton diel vertical migration (DVM) is an ecologically important process, affecting nutrient transport and trophic interactions. Available measurements of zooplankton displacement velocity during the DVM in the field are rare; therefore, it is not known which factors are key in driving this velocity. We measured the velocity of the migrating layer at sunset (upward bulk velocity) and sunrise (downwards velocity) in summer 2015 and 2016 in a lake using the backscatter strength (VBS) from an acoustic Doppler current profiler. We collected time series of temperature, relative change in light intensity chlorophyll-a concentration and zooplankton concentration. Our data show that upward velocities increased during the summer and were not enhanced by food, light intensity or by VBS, which is a proxy for zooplankton concentration and size. Upward velocities were strongly correlated with the water temperature in the migrating layer, suggesting that temperature could be a key factor controlling swimming activity. Downward velocities were constant, likely because Daphnia passively sink at sunrise, as suggested by our model of Daphnia sinking rate. Zooplankton migrations mediate trophic interactions and web food structure in pelagic ecosystems. An understanding of the potential environmental determinants of this behaviour is therefore essential to our knowledge of ecosystem functioning.
Highlights
Zooplankton diel vertical migration (DVM) is an ecologically important process, driven by internal and external drivers, and affecting nutrient transport and trophic interactions in lakes and oceans (Hays, 2003; Ringelberg, 2010)
Since Daphnia always sank during the observational period, our results indicate that these zooplankton may favour passive sinking over active swimming during the reverse DVM to preserve energy and avoid generating hydrodynamic disturbances detectable by predators
Chlorophyll concentration, relative change of light intensity and zooplankton concentration and size during the DVM did not seem to play an important role in affecting vup
Summary
Zooplankton diel vertical migration (DVM) is an ecologically important process, driven by internal and external drivers, and affecting nutrient transport and trophic interactions in lakes and oceans (Hays, 2003; Ringelberg, 2010). Avoidance of visually hunting juvenile fish and food requirements are generally considered the most significant factors (Ringelberg, 1999; Hays, 2003; Van Gool & Ringelberg, 2003; Williamson et al, 2011) According to this reasoning, zooplankton hide during the daytime in the deep and dark layers of the lake in response to chemical cues released by fish (kairomones) (Dodson, 1988; Neill, 1990; Loose & Dawidowicz, 1994; Lass & Spaak, 2003; Boeing et al, 2003; Beklioglu et al, 2008; Cohen & Forward, 2009). DVM can still take place in very transparent and fish-less lakes where zooplankton hide to prevent damage from surface UV radiation (Williamson et al, 2011; Tiberti & Iacobuzio, 2013; Leach et al, 2014)
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