Do the presence of filamentous cyanobacteria and an elevated temperature favor small-bodied Daphnia in interspecific competitive interactions?

According to the Size-Efficiency Hypothesis, when the intensity of fish predation is low, large-bodied zooplankton species are competitively superior as they are more efficient filter feeders than small-bodied species and they have relatively reduced metabolic demands per unit mass. The presence of filamentous cyanobacteria can reduce the competitive superiority of large-bodied cladocerans, because they are less resistant than small-bodied ones to interference with food collection by filaments. The viscosity of water decreases with increasing temperature which may alter the filtering mechanisms of cladocerans, making the large-bodied species even more susceptible to interference by cyanobacterial filaments. The aims of this study were (1) to test if the presence of the filamentous non-toxic cyanobacterium Cylindrospermopsis raciborskii shifts competitive dominance from a large-bodied cladoceran species towards a small-bodied one and (2) to determine whether the competitive superiority of a small-bodied species in the presence of cyanobacterial filaments is temperature-dependent. A laboratory competition experiment was conducted with large-bodied Daphnia pulicaria and small-bodied D. longispina, in the presence or absence of cyanobacteria in two different thermal regimes -18 and 25 degrees C. Unexpectedly, D. pulicaria was dominant over D. longispina, irrespective of the food and thermal treatments applied. The large-bodied Daphnia species was competitively superior even in the presence of filamentous cyanobacteria and at an elevated temperature.