Toward a transport-based analysis of nutrient spiraling and uptake in streams

Nutrient addition experiments are designed to study the cycling of nutrients in stream ecosystems where hydrologic and nonhydrologic processes determine nutrient fate. Because of the importance of hydrologic processes in stream ecosystems, a conceptual model known as nutrient spiraling is frequently employed. A central part of the nutrient spiraling approach is the determination of uptake length (S-w), the average distance traveled by dissolved nutrients in the water column before uptake. Although the nutrient spiraling concept has been an invaluable tool in stream ecology, the current practice of estimating uptake length from steady-state nutrient data using linear regression (called here the S-w approach) presents a number of limitations. These limitations are identified by comparing the exponential S-w equation with analytical solutions of a stream solute transport model. This comparison indicates that ( 1) S-w is an aggregate measure of uptake that does not distinguish between main channel and storage zone processes, ( 2) S-w is an integrated measure of numerous hydrologic and nonhydrologic processes - this process integration may lead to difficulties in interpretation when comparing estimates of S-w, and ( 3) estimates of uptake velocity and areal uptake rate (v(f) and U) based on S-w are not independent of system hydrology. Given these findings, a transport-based approach to nutrient spiraling is presented for steady-state and time-series data sets. The transport-based approach for time-series data sets is suggested for future research on nutrient uptake as it provides a number of benefits, including the ability to ( 1) separately quantify main channel and storage zone uptake, ( 2) quantify specific hydrologic and nonhydrologic processes using various model parameters ( process separation), ( 3) estimate uptake velocities and areal uptake rates that are independent of hydrologic effects, and ( 4) use short-term, non-plateau nutrient additions such that the effects of regeneration and mineralization are minimized. In summary, the transport-based, time-series approach provides a means of estimating traditional measures of nutrient uptake (S-w, v(f), U) while providing additional information on the location and magnitude of uptake ( main channel versus storage zone). Application of the transport-based approach to time-series data from Green Creek, Antarctica, indicates that the bulk of nitrate uptake (similar to 74% to 100%) occurred within the main channel where benthic uptake by algal mats is a likely process. Substantial uptake (similar to 26%) also occurred in the storage zone of one reach, where uptake is attributed to the microbial community.