FOOD LIMITATION OF 2 LOTIC GRAZERS - QUANTITY, QUALITY, AND SIZE-SPECIFICITY

A food augmentation experiment demonstrated that growth and lipid accumulation by grazing snails (Elimia clavaeformis) and caddisflies (Neophylax etnieri) from a snail-dominated stream in eastern Tennessee were strongly food limited. Elimia (two size classes) and third-instar Neophylax from White Oak Creek were given one of four diets in laboratory streams: (1) low-biomass periphyton (from White Oak Creek), (2) high-biomass periphyton (from an unshaded, nutrient-enriched stream that lacked effective grazers), (3) low-biomass periphyton and leaves, and (4) high-biomass periphyton and leaves. Small Elimia grew 2x faster and large Elimia grew 5x faster on high-biomass periphyton than on their normal diet of low-biomass periphyton. Neophylax grew 7x faster on the high-biomass periphyton. The high-biomass periphyton also produced significantly higher ratios of lipid to ash-free dry mass for both grazers. Leaf additions did not increase growth or lipid accumulation, even for Elimia, a feeding generalist. Measurements of grazer growth in situ closely matched those obtained on low-biomass periphyton in the laboratory, indicating few laboratory artifacts. The degree of Elimia's food limitation was size-specific: growth stimulation by high-biomass periphyton was much more pronounced for large snails than for small snails, and large snails had lower lipid reserves after feeding on low-biomass periphyton than did small snails. Food limitation appears to be a chronic condition for grazers in streams like White Oak Creek, where grazer biomass is high and primary production is low.