AMMONIUM AND PHOSPHATE RETENTION IN A MEDITERRANEAN STREAM - HYDROLOGICAL VERSUS TEMPERATURE CONTROL

The role of discharge and temperature changes in controlling the ammon ium (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 1 10 to 2793 m, and NH4-N S-w ranged from 67.5 to 693 m. Changes in disc harge (for PO4-P) and discharge plus temperature (for NH4-N) explained about 98% of the variance in nutrient retention efficiency. Before lo gging, NH4-N and PO4-P S-w did not show any seasonal trend, and discha rge explained only 31 and 37% of total variance, respectively. After l ogging, NH4-N and PO4-P retention efficiency was lower than before log ging when discharge increased above the basal level (46 L.s(-1)). The results suggest that riparian trees and associated detrital compartmen ts may buffer the influence of seasonal changes in discharge and physi cal conditions on the ability of the stream to retain nutrients. Resul ts from the intersite comparison of nutrient S-w compiled from the lit erature indicate that PO4-P retention has a good fit with a hydrodynam ic 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 proce sses related to discharge. On the other hand, NH4-N S-w does not fit w ell with this model, suggesting that NH4-N S-w is less sensitive to di scharge changes and that other mechanisms (such as biotic processes li nked with seasonal variation, or abiotic adsorption) may explain the N H4-N S-w results.