Daphnia Magna responses to zero-mean flow turbulence

Abstract

Daphnia magna is a key zooplankton organism found in many freshwater aquatic systems. Its feeding usually includes phytoplankton as well as bacteria, but Daphnia are also able to feed on wastewater particles, which is why they have recently been considered as potential organisms for tertiary biologically based wastewater treatment. Even though D. magna is a well-studied species, little is known about the possible effect of the flow environment on its fitness. This study focuses on analysing the effect of zero mean turbulence on the filtration capacity, the swimming velocity and the 24h-survival of D. magna individuals with three different body lengths (L = 1.2, 1.5 and 1.8 mm). Results show a clear difference between the response to low or high turbulent shear rates. At low shear rates, D. magna individuals were able to swim more or less freely, especially the bigger ones, and the filtration rate increased, due to the greater Daphnia-particle encountering frequency. Hereafter, at low shears the swimming velocity of D. magna depended positively on the shear rate and negatively on the body length. On the other hand, at high shear rates D. magna individuals were not able to overcome the flow, thus presenting similar swimming speeds independent of the body length, and the filtration capacity was completely inhibited. The survival of the D. magna individuals followed the same trend, that is, at low shear rates individuals presented lower mortality than at high shear rates. However, smaller individuals showed to be more sensitive to the flow environment, as they presented higher mortality at low shear rates than the bigger individuals. In contrast, bigger D. magna individuals presented higher mortality at high shear rates, proving to be more vulnerable to high shears