Combined effects of food quality and temperature on competitive interactions between small- and large-bodied cladoceran species

Competition is one of the main drivers of population dynamics of cladoceran species. According to the Size Efficiency Hypothesis, large-bodied cladocerans are superior competitors over small-bodied species because they are more effective filter-feeders and can consume a wider size spectrum of food particles. However, we hypothesized that food quality in terms of phosphorus (P) and/or polyunsaturated fatty acid (PUFA) content can alter competitive interactions, and these potential effects can be modified by temperature. Since large-bodied cladocerans are more vulnerable to poor food quality and are regarded as less adaptive to high temperatures, we hypothesized that large-bodied species would be superior competitors at low temperature and/or high food quality, while small-bodied cladocerans would be competitively superior at enhanced temperature and/or poor food quality. To verify this hypothesis, we conducted laboratory experiments with the small-bodied D. longispina and large-bodied D. magna fed with three different types of food: (1) P-poor and PUFA-rich green algae (low phosphorus or LP-treatment), (2) PUFA-poor-cyanobacteria treatment (CYANO) and (3) P-rich and PUFA-rich green algae (high phosphorus or HP-treatment) at 18°C and 24°C. We found that D. magna had a stronger suppressive competitive effect on D. longispina in all food quality and temperature treatments. Nevertheless, food quality and temperature modified competitive interactions between the small- and large-bodied Daphnia. In HP and CYANO, both Daphnia species were more strongly suppressed by competitor presence at high than low temperatures, whereas in LP mutual suppression was relatively weaker at higher temperatures. This phenomenon was attributed to higher requirement for P at enhanced temperature which created constraints for population growth thus decreasing competitive interactions at higher temperatures. Mutual competitive suppression in CYANO was weaker than in the HP treatment although cyanobacteria had the greatest negative impact on the abundance of both Daphnia species. In general our results show that the outcome of competition between large and small-bodied cladoceran species was not affected by food quality and/or temperature, yet, these factors markedly altered competitive interactions between species.