We examined the relationship between land use in the watershed and nutrient release by mayflies in streams in southcentral Ontario (Canada). Mayfly excretion rates and molar ratios of dissolved organic C (DOC), NH4 , and soluble reactive P (SRP) were measured in 6 streams flowing through watersheds with contrasting agricultural land use. Mass-specific release rates of NH4 and SRP by mayflies and % agricultural land use (% agriculture) in the watershed were strongly positively related (r2 = 0.48, p < 0.01 and r2 = 0.58, p < 0.05, respectively). Changes in excretion rates and ratios probably were caused by lower periphyton C:N and C:P ratios associated with elevated total dissolved N and total P concentrations (r2= 0.30, p < 0.05 and r2= 0.38, p < 0.001, respectively) in streams in watersheds with high % agriculture. Mayfly mass-specific excretion rates and periphyton elemental composition were related to % agriculture, but the relatively constant body C, N, and P content in mayflies across all streams suggests strong homeostasis in these consumers. Watershed land use alters nutrient loading, which influences periphyton elemental composition and, ultimately, stoichiometry of nutrient fluxes through invertebrate consumers.
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