The effect of light and nutrients on algal food quality and their consequent effect on grazer growth in subtropical streams

Recent investigators have emphasized that food quality has greater importance than food quantity for herbivore somatic growth. Herbivore growth is thought to be regulated by light and nutrients through changes in food elemental ratios, but our knowledge of the mechanisms driving grazer growth in stream ecosystems is scarce. We manipulated light (through shading) and nutrients (fertilizer addition) in 3 headwater streams in southeastern Queensland, Australia, and measured the responses of natural grazer communities. The growth responses of Austrophlebioides and Helicopsyche to light and nutrients differed with their body size and periphyton food quality. Large larvae were more sensitive to increased light and nutrient availability than were small larvae. Light induced a significant negative effect on Austrophlebioides growth, and this impact increased with increasing nutrient addition. Nutrients had a pronounced positive effect on Helicopsyche growth regardless of light intensity. These responses reflected changes in food quality. Periphyton food quality in terms of C:N played a more decisive role in grazer growth than algal food quantity (as chlorophyll a). The growth of large larvae was significantly greater under low light intensity and nutrient-enriched conditions where high-quality food was more abundant than under high-light and low-nutrient conditions where periphyton food quality was low. Our results suggest that changes in riparian vegetation or nutrient inputs can significantly influence grazer growth through changes in periphyton C:N. We recommend that future investigators of foodweb responses to riparian canopy change or nutrient enrichment on stream ecosystems pay more attention to the effects of algal food quality rather than quantity.