Response of biofilm extracellular enzymes along a stream nutrient enrichment gradient in an agricultural region of north central Pennsylvania, USA

Anthropogenic nitrogen and phosphorus loading to streams has the potential to shift ecosystems from inorganic nutrient limitation toward increased carbon flux. We tested the hypothesis that relative patterns in the activities of carbon-, phosphorus- and nitrogen-acquiring extracellular enzymes associated with stream microbial communities could potentially indicate a transition from nutrient limitation to increased carbon flux. Biofilm extracellular enzyme activities (beta-glucosidase, beta-xylosidase, phosphatase, and leucine-aminopeptidase) were measured in 23 streams along an agricultural nutrient gradient in north central Pennsylvania. We also measured water column total nitrogen and total phosphorus along with biofilm chlorophyll a, ash free dry mass, and phosphorus concentrations. Although most absolute enzyme activities tended to be positively correlated with phosphorus concentrations, relative activities between carbon- and phosphorus-acquiring enzymes indicated a shift away from phosphorus limitation within a relatively narrow and low range of phosphorus concentrations. These results have potential implications for establishing nutrient guidelines for streams because they indicate that nutrients such as phosphorus might elicit a significant ecosystem-level response at relatively low concentrations of phosphorus before more apparent changes in community structure or biomass are observed.