Ammonium and phosphate retention in a Mediterranean stream: hydrological versus temperature control

The role of discharge and temperature changes in controlling the ammonium (NH4-N) and phosphate (PO4-P) uptake length (S-w) was studied for 9 months in a second-order stream affected by riparian tree logging in northern Spain. Over the study period, PO4-P S-w values ranged from 110 to 2793 m, and NH4-N S-w ranged from 67.5 to 693 m. Changes in discharge (for PO4-P) and discharge plus temperature (for NH4-N) explained about 98% of the variance in nutrient retention efficiency. Before logging, NH4-N and PO4-P S-w did not show any seasonal trend, and discharge explained only 31 and 37% of total variance, respectively. After logging, NH4-N and PO4-P retention efficiency was lower than before logging when discharge increased above the basal level (46 L.s(-1)). The results suggest that riparian trees and associated detrital compartments may buffer the influence of seasonal changes in discharge and physical conditions on the ability of the stream to retain nutrients. Results from the intersite comparison of nutrient S-w compiled from the literature indicate that PO4-P retention has a good fit with a hydrodynamic model that states that S-w increases approximately as a square root of discharge. This suggests a strong link between PO4-P S-w and processes related to discharge. On the other hand, NH4-N S-w does not fit well with this model, suggesting that NH4-N S-w is less sensitive to discharge changes and that other mechanisms (such as biotic processes linked with seasonal variation, or abiotic adsorption) may explain the NH4-N S-w results.