Population growth rates of two closely related rotifer species: effects of food quantity, particle size, and nutritional quality

SUMMARY. 1. The relationship between population growth rates and the concentrations of several algal species was determined in laboratory experiments with the rotifers Brachionus rubens and B. calyciflorus . 2. The effects of food quantity were well described by a modified Monod model with a threshold for zero population growth. The model parameters depended on particle size and nutritional quality of the food algae. Differences between the rotifer species were significant and reflected their varying food‐size preferences. 3. For each rotifer species, thresholds were lowest for algae in the most readily ingested size range. The lowest thresholds were 0.07–0.09 mgC 1 −1 with algae of about 5 μm equivalent spherical diameter (ESD) for B. rubens , and 0.19 mgC 1 −1 with algae of about 10 μm ESD for B. calyciflorus . 4. Maximal growth rates ( r max ) were slightly below 0.8 day −1 for both rotifers with most algal species. The highest r max values for both rotifers were observed when Cyclotella meneghiniana was provided as food. With this alga, B. calyciflorus had a significantly higher r max (1.02 day −1 ) than B. rubens (0.838 day −1 ). 5. From a comparison of the relationship between growth rates and ingestion rates, Chlamydomonas reinhardii appeared to be of low nutritional quality for B. rubens . 6. Egg ratios were related to growth rate and were not influenced by the algal food used. Egg development times and average mortality rates were estimated from the relationship between egg ratio and growth rate. B. calyciflorus appeared to have a high average mortality rate (0.383 day −1 ) compared to B. rubens (0.083 day −1 ).