Trophic Basis of Production Among Net‐Spinning Caddisflies in a Southern Appalachian Stream

Life histories and annual production were determined for six species of net—spinning caddisflies in a headwater stream of the Tallulah River in north Georgia, USA. Five species in the family Hydropsychidae were univoltine, whereas the sixth, a member of the Philopotamidae, had at least two generations per year. Combined annual production, as determined by the Hynes method, was 1.0 g/m 2 (ash—free dry mass). Seventy—five percent of the production was concentrated in the two largest species, Arctopsyche irrorata and Parapsyche cardis. The remaining production from highest to lowest percent, was contributed by Dolophilodes distinctus, Hydropsyche sparna, Diplectrona modesta, and Hydropsyche macleodi. Analysis of gut contents alone indicated that detritus was the most important food source. However, food preference and food—specific ecological efficiencies were utilized to calculate the amount of production attributable to each major food category. Surprisingly, almost 80% of all caddisfly production was attributed to animal food, 13% to detritus, and 8% to algae. Actual annual consumption required to account for this production was 2.28 g/m 2 animals, 2.54 g/m 2 detritus, and 0.51 g/m 2 algae. We attempt to quantify the role that net—spinning caddisflies play in the @`spiralling" of seston in mountain streams. Our results show that the omnivorous caddisflies are not the major consumer of detritus and algae, and that they produce more detritus in their feces than they consume, thus appearing to lower the food quality of the seston. Net—spinning caddisfly production in this mountain stream appears to be limited by the amount of high quality food available in the seston.