Changes in stoichiometric constraints on epilithon and benthic macroinvertebrates in response to slight nutrient enrichment of mountain rivers

1. To assess changes in stoichiometric constraints on stream benthos, we measured elemental composition of epilithon and benthic macroinvertebrates in intrinsically P-limited mountain rivers, upstream and downstream of low-level anthropogenic nutrient enrichment by effluents of municipal wastewater treatment plants. 2. While there was a broad range in the elemental composition of epilithon (C : P ratios of 200-16 500, C : N ratios of 8-280, N : P ratios of 8-535) and heptageniid mayfly scrapers (C : P ratios of 125-300, C : N ratios of 5.1-7.2, N : P ratios of 20-60), the average C : P ratio of epilithon was 10-fold lower and the average C : N ratio twofold lower at more nutrient-rich downstream sites. Nutrient ratios in benthic macroinvertebrates were lower than in epilithon and varied little between relatively nutrient-poor and nutrient-rich sites. 3. We modified the existing definition of producer-consumer elemental imbalance to allow for variation in consumer nutrient content. We defined this 'non-homeostatic' imbalance as the perpendicular distance between the producer and consumer C : P, C : N, or N : P ratios, and the 1 : 1 line. 4. At P-limited sites, the estimated mayfly N : P recycling ratio was higher than the N : P ratio in epilithon, suggesting nutrient recycling by consumers could accentuate P-limitation of epilithon. 5. Measuring the degree of producer-consumer nutrient imbalance may be important in predicting the magnitude of effects from nutrient enrichment and can help elucidate the causes and consequences of ecological patterns and processes in rivers.