## 4 Discussion
### Residence Time  
Our experimental manipulations provided support for our hypothesis that higher rates of gross primary production and ecosystem respiration would be observed in longer HRT treatments. Field studies in streams have found that GPP and ER rates increase at lower discharge, likely because of greater accumulation and development of autotrophic and heterotrophic communities at low flows compared to high flow that may disturb biological controls on metabolism [76; 77]. All flow rates were extremely low so it is more likely that higher rates of metabolism would be observed with greater assimilatory capacity at lower flows.  
However, the highest rates of net nutrient uptake were observed in the 2 h HRT treatment which was contrary to expectations. In the MAP, storm events often create a combination of high nutrient loads and high flow rates can lower concentrations and allow a significant portion of the load to escape the uptake capacity of different processes in streams [78; 79]. In contrast, within this study there were no rainfall events, and we supplied equal nutrient loads to all streams and varied discharge to manipulate HRT but not load. This should have caused a discharge \(\times\) nutrient concentration relationship where inflow nutrient concentrations were greater in ditches with higher HRTs. However, examination of inflow NO\({}_{3}\)-N concentrations across HRT treatments revealed this was not the case (2 h = 2.43 \(\pm\) 0.46 mg L\({}^{-1}\); 4 h = 2.2 \(\pm\) 0.47; 6 h = 2.41 \(\pm\) 0.53) and highlights the difficulty of managing experimental dosing regimens across extremely low flows which are prone to error. However, all inflow NO\({}_{3}\)'-N concentrations were above values previously reportedas associated with maximum NO\({}_{3}\)'-N retention (1 mg L\({}^{-1}\)) in sediment cores vegetated with _L. oryzae_s [37], so observed differences in HRT are likely driven by some other factor than potential discharge effects on uptake efficiency. A study of vegetated ditches found lower flows to physically limit N removal [80]; here, the highest flows in the 2 h HRT could experience the highest mixing activity due to increased turbulence associated with slightly higher velocities at the sediment water interface thus enhancing N uptake. Epiphytic algae, while visibly negligible, was not captured in these rates and may account for lower observed uptake rates with higher respiration rates in lower HRT treatments. Longer residence times may also create preferential flow paths and actually decrease contact between reactive sites and nutrients as longitudinal dispersion becomes dominated by inefficient exchange in dense canopies with shallow submergence [81]. Finally, these results are somewhat contrary to the literature suggesting that manipulating for longer HRTs through water control structures in agricultural ditches may enhance nutrient processing and retention [19; 20]; our results are more like the result of an unexpected experimental artifact or a potential HRT range that exceeds thresholds for positive impacts on nutrient processing. Unfortunately, facilitating shorter residence times, in order to examine a wider range of HRT, while still maintaining sensitivity in our uptake measurements will likely require longer experimental reaches. More research on nutrient uptake patterns across a wider gradient of residence time in agricultural ditches is needed.