Filamentous cyanobacteria, temperature and Daphnia growth: the role of fluid mechanics.

Viscosity increases significantly with a fall in water temperature, thus temperature change affects not only the metabolic rates of aquatic suspension feeders, but also the physical properties of the surrounding fluid. This mechanistic effect of water temperature change on growth was separated from the effect of metabolism by using culture media with modified viscosity, while the temperature was kept constant. The effect of water viscosity on growth rate and feeding of four Daphnia species (D. magna, D. pulicaria, D. hyalina, D. galeata) was investigated. Increased viscosity decreased the growth rate significantly for three species, with the exception of D. galeata. Changing viscosity also affects growth qualitatively: the filamentous blue-green Cylindrospermopsis raciborskii reduces the growth rate of D. pulicaria at low viscosity, but its negative effect disappears when viscosity is higher. The findings are consistent with the hypothesis that it is the Reynolds number of the filtering appendages that determines the qualitative features of Daphnia filtration. The edibility of C. raciborskii at high water viscosity is most probably caused by lack of interference with filtering combs, and explains the coexistence of D. pulicaria with filamentous blue-green species in the field, and also the observed temperature dependence of growth inhibition of filaments.