Microbial enzyme activity, nutrient uptake and nutrient limitation in forested streams

Summary 1. We measured NH 4 + and PO 4 −3 uptake length ( S w ), uptake velocity ( V f ), uptake rate ( U ), biofilm respiration and enzyme activity and channel geomorphology in streams draining forested catchments in the northwestern (Northern California Coast Range and Cascade Mountains) and southeastern (Appalachian and Ouachita mountains) regions of the United States. Our goal was to use measures of biofilm enzyme activity and nutrient uptake to assess nutrient limitation in forested streams across broad regional scales. 2. Geomorphological attributes, biofilm enzyme activity and NH 4 + uptake were significantly different among streams in the four study units. There was no study unit effect on PO 4 −3 uptake. The proportion of the stream channel in pools, % woody debris, % canopy closure, median substrate size ( d 50 ), stream width ( w ), stream velocity ( v ), discharge ( Q ), dispersion coefficient ( D ) and transient storage (A s /A) were correlated with biofilm enzyme activity and nutrient uptake in some study units. 3. Canonical correlation analyses across study units revealed significant correlations of NH 4 ‐ V f and PO 4 ‐ V f with geomorphological attributes ( w , d 50 , D , % woody debris, channel slope and % pools) and biofilm phosphatase activity. 4. The results did not support our expectation that carbon processing rates by biofilm microbial assemblages would be governed by stream nutrient availability or that resulting biofilm enzyme activity would be an indicator of nutrient uptake. However, the relative abundances of peptidases, phosphatase and glycosidases did yield insight into potential N‐, P‐ and C‐limitation of stream biofilm assemblages, and our use of biofilm enzyme activity represents a novel application for understanding nutrient limitations in forested streams. 5. Regressions of V f and U against ambient NH 4 + and PO 4 −3 indicated that none of our study streams was either NH 4 + or PO 4 −3 saturated. The Appalachian, Ouachita and Coastal streams showed evidence of NH 4 + limitation; the Ouachita and Coastal streams were PO 4 −3 limited. As a correlate of nutrient limitation and saturation in streams, ratios of total aminopeptidase and phosphatase activities and the ratio of NH 4 ‐ U to PO 4 ‐ U indicate these forested streams are predominantly N‐limited, with only the streams draining Ouachita and Coastal catchments demonstrating appreciable levels of P‐limitation. 6. Our results comparing the stoichiometry of microbial enzyme activity with nutrient uptake ratios and with the molar ratios N and P in stream waters suggest that biological limitations are not strictly the result of stream chemistry and that the assessments of nutrient limitations in stream ecosystems should not be based on chemistry alone. 7. Our present study, along with previous work in streams, rivers and wetlands, suggests that microbial enzyme activities, especially the ratios of total peptidases to phosphatase, are useful indicators of nutrient limitations in aquatic ecosystems.