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

1. We measured NH4+ and PO4- 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 NH4+ uptake were significantly different among streams in the four study units. There was no study unit effect on PO4-3 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 NH4-V-f and PO4-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 NH4+ and PO4-3 indicated that none of our study streams was either NH4+ or PO4-3 saturated. The Appalachian, Ouachita and Coastal streams showed evidence of NH4+ limitation; the Ouachita and Coastal streams were PO4-3 limited. As a correlate of nutrient limitation and saturation in streams, ratios of total aminopeptidase and phosphatase activities and the ratio of NH4-u to PO4-u indicate these forested streams are predominantly N-limited, with only the streams draining Ouachita and Coastal catchments demonstrating appreciable levels of P-limitation.