Anthropogenic nutrient enrichment has altered the relative supply of nitrogen (N) and phosphorus (P) in freshwater ecosystems, with implications for organismal growth, nutrient recycling, and ecosystem functioning. Ecological stoichiometry predicts that alignment between consumer nutrient demand and resource supply should reduce metabolic costs, yet most aquatic macroinvertebrates exhibit strong stoichiometric homeostasis at the individual level. Whether communities of macroinvertebrates can achieve stoichiometric flexibility through species sorting or intraspecific variation remains unclear, particularly at broad spatial scales.
We evaluated community‑level stoichiometric flexibility of stream macroinvertebrates across North America using data from 27 National Ecological Observatory Network (NEON) stream sites spanning wide gradients in basal resource stoichiometry. We combined long‑term macroinvertebrate abundance and biomass data with taxon‑specific body stoichiometry from the STOICH database to estimate community‑weighted C:N:P for three functional feeding groups: herbivores, collector‑gatherers, and collector‑filterers. We quantified apparent stoichiometric flexibility as the relationship between resource stoichiometry (periphyton or seston) and community body stoichiometry using the inverse homeostasis coefficient (H⁻¹). To identify mechanisms and constraints, we compared observed communities with counterfactual scenarios that isolated the effects of intraspecific variation, global trait availability, and local species pools.
Across nearly all nutrient combinations and feeding groups, observed macroinvertebrate communities were effectively homeostatic (H⁻¹ ≈ 0), showing little alignment with resource stoichiometry despite large variation in basal resource C:N:P. Intraspecific variation in body stoichiometry did not increase apparent flexibility. In contrast, theoretical communities assembled from the global species pool exhibited substantial flexibility, indicating that sufficient trait variation exists at the continental scale. However, communities constrained by local species pools generally lacked the stoichiometric range necessary to track resource gradients, demonstrating that environmental filtering and dispersal limitation overwhelm resource‑driven species sorting.
Our results suggest that resource stoichiometry plays a limited role in structuring macroinvertebrate communities across landscapes, leading to persistent mismatches between consumer demand and resource supply. As nutrient imbalances intensify with human activity, the apparent homeostasis of stream communities may have important consequences for nutrient retention, recycling, and downstream ecosystem function.