Growth rate versus biomass accumulation: Different roles of food quality and quantity for consumers

Population growth and biomass accumulation of Daphnia magna was studied in a two-stage continuous-culture system with light- and phosphorus-limited green algae (Selenastrum capricornutum) as food source. Selenastrum grown at saturating light conditions (70 μmol quanta m^-2 s^-1) had on average 68% higher biomass yield and 103% higher C : P than algae grown under reduced light (10 or 36 μmol quanta m^-2 s^-1). Initial Daphnia population response was affected by food quality, showing faster growth in the low-light treatments where food C : P was low and food quality was correspondingly high. When herbivore biomass became sufficiently high to deplete phytoplankton biomass to the threshold for net positive growth, the Daphnia carrying capacity appeared nevertheless to be determined primarily by the quantity of food. Thus, the high-light treatments, which yielded the highest algal biomass production, also gave twice as high asymptotic Daphnia biomasses as those with low light. This study demonstrates how short-term growth rate and long-term sustainability of populations may be affected differently by the quantity and quality aspects of their food. While high growth rate and reproduction require a diet that is balanced in terms of the grazer's demand for elements and macromolecules, a higher biomass of nongrowing individuals may still be sustained on a sufficiently plentiful supply of nutritionally deficient food.