Patterns of caddisfly stoichiometry within riverine networks: The relative importance of homeostasis, food stoichiometry, and stream size

The carbon (C), nitrogen (N), and phosphorus (P) content of invertebrates is sometimes linked with their competitive ability and role in ecosystem function. Recent studies of freshwater invertebrates demonstrate considerable stoichiometric variability among some species. Yet, few studies have examined intra-specific patterns in animal stoichiometry, which is an important issue because theory assumes this to be invariant. We examined how the body C:N:P of two generalist caddisflies (Lepidostoma sp. and Psychoglypha sp.) varied within a river network. In northern California streams (Mendocino County, CA), these caddisflies are abundant in streams spanning 3 orders of magnitude in watershed size. Previous research indicates that these generalists consume the dominant food resource: leaf detritus in headwater streams and algae in mid-size rivers. Thus, within this riverine network, Lepidostoma and Psychoglypha experience a wide range of food stoichiometry. We measured the C:N:P of caddisflies species along with their food in ten streams of the Eel River watershed. Stable isotopes of nitrogen and carbon were used to identify diet sources. Individuals of Lepidostoma and Psychoglypha varied in body C, N, and P concentrations. Caddisfly P contents scaled negatively with body size in both species. Similar-sized Lepidostoma varied little in body P content across sites and were considered homeostatic. In contrast, the P content of similar-sized Psychoglypha varied among streams. Our results indicate that allometry and network position interact with animal homeostasis to shape caddisfly stoichiometry.