Effects of Cyanobacteria on Competitive Interactions Between Different-Sized Cladoceran Species

Cyanobacteria negatively affect zooplankton through several mechanisms including mechanical interference, toxicity, and poor food quality due to a shortage of essential lipids. To understand the nature of each of these mechanisms, they should be examined independently. The goal of our study was to assess the influence of cyanobacteria food quality on the competitive outcomes between the small-bodied Daphnia longispina and the large-bodied Daphnia magna. We conducted life-table experiments to assess R* (population threshold food concentration), competition experiments to determine the outcome of competition, and computer simulation experiments at high levels of food supply, which are difficult to realize in laboratory conditions. We used two types of food: the high-quality green algae Chlamydomonas klinobasis (GREEN) and the cyanobacterium Synechococcus elongatus (CYANO), which contains low levels of essential lipids, but is non-toxic and unicellular. We found that the small-bodied D. longispina was a superior competitor in GREEN, while the large-bodied D. magna was more abundant in CYANO. We established that the species ratio in GREEN was dependent on competitive interaction, while abundances of daphnids in CYANO were controlled by poor food quality. Since cyanobacteria act as a powerful force for structuring cladoceran communities, the role of competition for food between these two Daphnia species greatly declined under their effects.