Body size alters consumer response to long-term nutrient enrichment

Nutrient enrichment of aquatic ecosystems can increase total consumer production.  However, this positive response varies among taxa and within functional feed groups (FFGs), which suggests that species-specific traits may modify consumer response to enrichment.  To assess how such traits (e.g., cohort production interval, body size, body elemental composition, and FFG) alter consumers’ enrichment responses, we used a multiple regression model to analyze data from a five-year nutrient enrichment of a headwater stream. We also used random intervention analysis to assess whether consumers of comparable body size, but different taxonomic classifications, responded similarly to nutrient enrichment.  Our multiple regression models suggested that consumer response was weakly correlated with body phosphorus content and FFG identity after two years of enrichment, while body size and FFG partially explained consumer response after five years of enrichment.  The direct body-size comparison showed that most large-bodied primary consumers, but not large-bodied predators, increased with nutrient enrichment.  Thus, our results indicate that consumer body size, phosphorus content, and FFG were relatively important in determining consumer response.  As these traits can be linked to consumer functional roles (e.g., excretion rates, leaf-shredding, predation), nutrient enrichment may indirectly affect stream functions via alterations in consumer assemblages based on particular traits.