FACTORS POTENTIALLY PREVENTING TROPHIC CASCADES - FOOD QUALITY, INVERTEBRATE PREDATION, AND THEIR INTERACTION

Recent laboratory work on food quality constraints on zooplankton grow th and reproduction, as well as several examples of weak effects of fo od-web manipulations on lower trophic levels in lakes with phosphorus- deficient phytoplankton, suggests that food quality effects may have c urrently unappreciated effects on zooplankton success and food-web int eractions under field conditions. We experimentally manipulated two fa ctors that we anticipated might play a role in suppressing Daphnia in P-limited lakes-the quality of phytoplankton food and the presence of the invertebrate predator Chaoborus punctipennis. We used a two-factor design, manipulating food source and presence of Chaoborus, and measu red growth rate, survivorship, and fecundity of Daphnia rosea neonates incubated at fixed food levels in flow-through growth chambers. D. ro sea grew significantly faster and was significantly mon fecund when fe d seston from a high-food quality lake (Lake 979) relative to a treatm ent fed seston from a low-food quality lake (Lake 110). Chaoborus redu ced survivorship of D. rosea but the food source-predator interaction term was not significant, indicating that invertebrate predation and p hytoplankton food quality did not influence Daphnia populations synerg istically in this experiment. A second experiment was conducted to det ermine if variation in Daphnia growth rate and fecundity when fed food of varying quality was caused by a change in feeding rate. Daphnia fe eding rate increased with improved food quality, suggesting that Daphn ia responds to increases in food quality, at least in part, by increas ing feeding rate. We conclude that food quality can strongly affect Da phnia feeding, growth, and reproduction, thereby constraining food-web dynamics in nutrient-deficient lakes.