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Detailed studies of stream N uptake were conducted in a prairie reach and gallery forest reach of Kings Creek on the Konza Prairie Biological Station. Nutrient uptake rates were measured with multiple short-term enrichments of NO 3 Ϫ and NH 4 ϩ at constant addition rates in the spring and summer of 1998. NH 4 ϩ uptake was also measured with 15 N-NH 4 ϩ tracer additions and short-term unlabeled NH 4 ϩ additions at 12 stream sites across North America. Concurrent addition of a conservative tracer was used to account for dilution in all experiments. NH 4 ϩ uptake rate per unit area (U t) was positively correlated to nutrient concentration across all sites (r 2 ϭ 0.41, log-log relationship). Relationships between concentration and U t were used to determine whether the uptake was nonlinear (i.e., kinetic uptake primarily limited by the biotic capacity of microorganisms to accumulate nutrients) or linear (e.g., limited by mass transport into stream biofilms). In all systems, U t was lower at ambient concentrations than at elevated concentrations. Extrapolation from uptake measured from a series of increasing enrichments could be used to estimate ambient U t. Linear extrapolation of U t assuming the relationship passes through the origin and rates measured at 1 elevated nutrient concentration underestimated ambient U t by ϳ3-fold. Uptake rates were saturated under some but not all conditions of enrichment; in some cases there was no saturation up to 50 mol/L. The absolute concentration at which U t was saturated in Kings Creek varied among reaches and nutrients. Uptake rates of NH 4 ϩ at ambient concentrations in all streams were higher than would be expected, assuming U t does not saturate with increasing concentrations. At ambient nutrient concentrations in unpolluted streams, U t is probably limited to some degree by the kinetic uptake capacity of stream biota. Mass transfer velocity from the water column is generally greater than would be expected given typical diffusion rates, underscoring the importance of advective transport. Given the short-term spikes in nutrient concentrations that can occur in streams (e.g., in response to storm events), U t may not saturate, even at high concentrations.

Nutrient uptake length is an important parnmeter tor quantifying nutrient cycling in streams. Although nutrient tracer additions are the preierred method for measuring uptake length under ambient nutrient concentrations, short-term nutrient addition experiments have more irequently been used to estimate uptake length in streams. Theoretical analysis of the relationship between ~tptake length determined by nutrient addition experiments (S, , ') and uptake length determined by tracer additions (Sv) predicted that L.' should be consistentls longer than 5,", and that the overestimate of uptake length by S, V(should be related to the level ot nutrient addition above ambient concentrations and the degree of nutrient limitation. To test these predictions, we used data irom an interbiorne study ot NH,uptake length in whi& "NH,tracer and short-term NH,addition e~periments were perivnntrd i n 10 streams using a uniform experimental approach. The experimental results largely contirmed the theoretical predictions: s,.' was consistently longer than &, and S,,':S, ratios were directly related to the level of NH,addition and to indicatvrs of N limitation. The experimentally derived 5, : : S! , ratios were used with the theoretical results to infer !he N limitation st'itus of each stream. Together, the theoretical and experimental results sl.t~'i%mi that tracer experiments should be used whenever possible to determine nutrient uptake length in streams. S~ttrient dddition experin~ents may be useful tor comparing uptake lengths between different stredm5 or cliiferent times in the same ztream. howre\.er. provided that nutrient additions are kept as lo\%, as piw~ble dnd of similar niagmtude.

Hydraulic characteristics were measured in artificial streams and in 1st-to 5th-order streams in the Appalachian and Cascade mountains. Appalachian Mountain stream sites at Coweeta Hydrologic Laboratory, North Carolina, were on six Ist-order streams and a 1st-through 4th-order gradient of Ball Creek-Coweeta Creek. Cascade Mountain sites were located on constrained and unconstrained reaches of Lookout Creek, a 5th-order stream in H. J. Andrews Experimental Forest, Oregon. At each site, a tracer solution (chloride or rhodamine WT) was released for 30-180 min and then discontinued. At the downstream end of the release site, the resulting rise and fall of the tracer concentration was measured. These data, along with upstream concentration and measured widths and depths, were used in a computer model to estimate several hydraulic parameters including transient storage and lateral inflow. Estimated transient storage zone size (A,) ranged from near zero in artificial streams to 2.0 m 2 in 5th-order streams. A, was largest relative to surface crosssectional area (A) at Ist-drder sites where it averaged 1.2 x A, compared with 0.6 x A and 0.1 x A in unconstrained and constrained 5th-order sites, respectively. Where measured, lateral discharge inputs per metre of stream length ranged from 1.9% of instream discharge in Ist-order streams to 0.05% of instream discharge at 5th-order sites. Our results show that surface water exchange with storage zones is rapid and extensive in steep headwater streams and less extensive but still significant at 3rd-through 5th-order sites. An understanding of relationships between stream morphology, storage zone size, and extent of interactions between surface and subsurface waters will assist comparisons of solute dynamics in physically diverse streams.

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