ASSESSING TRANSIENT STORAGE INFLUENCES ON NUTRIENT SPIRALING: AN UPDATED APPROACH

Studies relating transient storage and nutrient uptake (i.e., spiraling) in streams have been equivocal. In some cases, strong relationships have been found between measures of nutrient spiraling (e.g., uptake length, uptake velocity, uptake rate) and various metrics describing the nature of transient storage. For headwater streams, the magnitude of transient storage is often used to represent the extent of exchange between the channel water and hyporheic zone. At the same time, spiraling metrics are sensitive to the enrichment effect associated with solute injection experiments. Recently, our research group has been involved in multiple projects applying heavy nitrogen isotopes to headwater streams to generate accurate measures of nitrate spiraling. In this presentation, we interpret the results of over 20 solute injection experiments involving heavy nitrate and transient storage modeling to assess spatial and temporal variation in the hydrology and biogeochemistry of headwater streams. Data were gathered from six sites chosen to provide contrasting hyporheic interaction across a gradient of terrestrial-aquatic interaction. Normalized transient storage ranged from 0.11 – 1.92 while the exchange coefficient varied over two orders of magnitude (0.004 – 0.40 min^-1). A number of metrics that emphasize different aspects of storage are used to address its influence on nutrient uptake. Results are also analyzed in light of recent modeling efforts suggesting that it is critical to address the range of values for the transient storage exchange coefficient when addressing how the size of the transient storage zone may influence nitrogen spiraling.