Body P content increases over ontogeny in hemimetabolous macroinvertebrates in a Mediterranean high mountain stream

Differences in elemental stoichiometry among consumers can regulate fundamental ecological processes such as animal production or nutrient cycling within ecosystems. Research to date has established that consumer stoichiometry differs within and across taxa, but the degree to which organismal stoichiometry ranges ontogenetically across life stages or body sizes is poorly understood. To understand patterns of variation in organismal C:N:P stoichiometry, we analyzed the elemental carbon (C), nitrogen (N) and phosphorus (P) composition of 436 aquatic macroinvertebrates across the ontogeny of 13 taxa representing different life history strategies and developmental modes in a high mountain stream. Our results showed that C:N:P stoichiometry varied among taxonomic groups, macroinvertebrate taxa, but not among functional feeding groups. However, stoichiometric variability within taxa was comparable or exceeded those among taxa and was primarily caused by changes in P content. We found undocumented positive relationships between P content and body mass in hemimetabolous taxa, with %P increments ranging from five to eightfold across ontogeny. Overall, findings lend evidence to the idea that phylogenetic relationships may be an important but secondary determinant of C:N:P stoichiometry relative to ontogeny. Elucidating the mechanistic basis for the positive relationship between body mass and P content in hemimetabolous taxa may reveal fundamental differences in the ecology and evolutionary pressure behind hemi- versus holometabolous developmental modes in nature.