Physical heterogeneity, organic matter quantity, and nutrient uptake in a forested headwater stream

We examined the role of physical heterogeneity in stream nutrient uptake by comparing ammonium and phosphorus uptake velocities between a bedrock and boulder reach.  Physical heterogeneity has been correlated with increased water residence time, contact time, transient storage zone potential and increased gross primary productivity and organic matter retention.  We conducted short-term nutrient addition experiments in two reaches of a headwater stream in the Shawangunk Mountains, NY during both summer and fall. Hydraulic uptake lengths were longer in the bedrock reach.  Respiration was significantly higher in the boulder reach, but there was no difference in GPP. Ammonium and SRP uptake velocities were two times higher in the boulder reach, a significant difference.  However, ammonium and SRP uptake velocities did not correlate with R.   Likewise, measures of transient storage zone did not explain much of the variation in nutrient uptake.  Within a season there was little variation in either R or uptake velocity.  Instead, we saw major seasonal shifts in uptake velocity, that coincided with large inputs of low N:P organic matter.  Thus, we conclude that metabolism and hydrology are influenced by physical heterogeneity, but their  relationship to nutrient uptake may be mediated by organic matter  quality.