In this project we aim to examine nutrient retention in streams receiving high nutrient loads from the adjacent catchment. We will also compare nutrient retention responses from these streams to those from less polluted streams. It would be very valuable to incorporate information about this stream property into the decision of adequate stream management strategies.

     Stream nutrient retention depends on the interaction of several physical (morphology and hydrology), chemical and biological mechanisms. However, under any given conditions, some of these factors will be favoured over others.

      At the reach scale, streambed permeability favours water exchange between the surface and hyporheic sediments, thus causing a physical delay in nutrient transport. This physical delay, coupled with biological activity within the sediments, suggests that the stream surface-subsurface hydrological linkage may be an important factor enhancing stream nutrient retention, at least in non-polluted streams.

    Therefore, alterations in reach morphology or sediment permeability may indirectly have an effect on stream nutrient retention. Thus, enhanced primary production due to the increase of N and P loads is thought to accelerate clogging of interfacial sediments and thereby reduce surface/subsurface water exchange and mass transfer towards deeper sediment layers.

     In this workpackage we will examine the relationships between geomorphologic and hydrologic factors and stream nutrient retention in polluted streams and how stream surface-subsurface hydrological linkages can be affected by high nutrient loads. We propose to measure the nutrient (ammonium, nitrate and phosphate) uptake length by conducting short-term nutrient additions in the study reaches. Uptake length is an indicator of the nutrient retention efficiency of a given reach. Short distances indicate greater retention efficiency than long distances. This parameter is preferable to the budget calculation approach because the latter method is scale dependent, whereas the uptake length is independent of the length of the study reach. Based on this parameter we will also calculate the nutrient uptake rate and the mass transfer coefficient. 

     While uptake length is affected by variations in stream discharge, these two other parameters are not, and thus, they allow inter-site comparisons. Using these parameters, we will examine the effect of human alterations (in terms of nutrient inputs) on stream nutrient retention at the reach scale by: a) comparing results from reaches affected by nutrient inputs from point sources (e.g., located downstream the effluent of a sewage treatment plant) to upstream reaches that are not affected by this nutrient input within each study site; and b) comparing results from human altered streams obtained in this project to those published from more pristine locations (e.g., sites in the LINX project -- Lotic Intersite Nitrogen eXperiment). In both cases, we will also compare uptake lengths measured for phosphorus (phosphate) and nitrogen (nitrate and ammonium) to examine the extent to which nitrogen and phosphorus dynamics are coupled in each study reach.

     We also propose to estimate the size of the transient storage zone and the surface-subsurface hydrologic linkages within each study reach by conducting additions of conservative tracers (Br, Cl or both) and measuring the vertical hydraulic gradient. This parameter indicates the direction of surface-subsurface linkages (i.e., upwelling, downwelling or non-welling zones). Moreover, we want to analyse not only on the size of the transient storage zone, but also the quality of it (i.e., which are the stream compartments that mostly contribute to delay the water transport). Because each compartment may influence nutrient retention in a particular way depending on the biological processes occurring within it. That is, nutrient processing in the hyporheic zone differs from that occurring along the riparian stream banks, despite that both compartments contribute to delay stream water.

     To test the effect of geomorphologic features and hydrological linkages on stream nutrient retention, we will examine the relationship between these parameters and results from nutrient retention measurements within each study site and among study sites. On the other hand, to test how high nutrient loads affect hydrologic linkages, within each study site we will compare results from reaches affected by nutrient inputs from point sources (e.g., located downstream the effluent of a sewage treatment plant) to upstream reaches that are not affected.

     All these relationships will be then used as the empirical basis for the knowledge base of the Expert System.