Body-size related survival of cladocerans in a trophic gradient: an enclosure study

Populations of Ceriodaphnia reticulata and six species of Daphnia, which represent a wide range of body sizes, were allowed to grow and compete with each other in eight 1.7 m3 enclosures for a period of 100 days. Enclosures contained lake water of different trophic state (3 oligotrophic, 3 hypertrophic, and 2 intermediate). Each of the 48 populations was started with cohorts of 50 two-day old and 50 six-day old juveniles from clonal cultures. Large-bodied species were expected to monopolize resources under oligotrophic conditions, while small-bodied species were expected to win under hypertrophic conditions. In the beginning of the experiment, the seven species exhibited similar population growth rates in all enclosures. Growth rates dropped due to overexploitation of food in the oligotrophic and intermediate enclosures and due to predation by cyclopoid copepods (Acanthocylops), which had been introduced incidentally, in the hypertrophic enclosures. Large-bodied species became dominant in all enclosures, but for different reasons. They profited from competitive superiority (lower food threshold concentration) under oligo-, meso- and eutrophic conditions. In a hypertrophic situation, however, they were successful because of lower vulnerability to copepod predation