Controls on spatial and temporal variation of nutrient uptake in three Michigan headwater streams

Abstract

We measured whole‐stream uptake of ammonium (NH 4 + ), nitrate (NO 3 − ), and phosphate (as soluble reactive phosphorus [SRP]) in two reaches of three forested headwater streams eight times from May 2003 to April 2004 (n = 46 measurements per nutrient type). We also measured factors that could affect uptake, including ambient nutrient concentrations, whole‐stream metabolism, and organic matter standing stocks. In all three streams, we measured the highest rates of NH 4 + and NO 3 − uptake velocity (V f ) during the spring. Low ambient NH 4 + concentrations limited NH 4 + uptake (U) in two streams. In one stream, when ambient NO 3 − concentrations increased during summer, NO 3 − Vf decreased. Temporal patterns of SRP V f varied among streams, but were unrelated to variability in ambient SRP concentration. However, in all three streams, seasonal variation in SRP V f was strongly influenced by heterotrophic metabolism (as measured by community respiration; State Creek r = 0.81, p = 0.03; Shane Creek r = 0.89, p r = 0.91, p

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