Biotic control of in-stream nutrient retention in nitrogen-rich springs (Po Valley, Northern Italy)

Agriculture is a major contributor of nutrients to surface waters in large parts of the world and affects hydro-morphological conditions of rivers eventually altering their ecological status. Increasing attention has been given to those aspects of river functioning which refer to the processes able to modulate the concentration of nutrients exported downstream. This study aimed to evaluate the nutrient retention in agricultural groundwater-fed lowland streams, also known as "fontanili", contaminated by elevated nitrate loadings. We used the nutrient spiraling method to assess ammonium (NH4) and phosphate (PO4) retention in two fontanili reaches, that significantly differed in presence/absence of aquatic macrophytes and tree canopy in the riparian zone. The results indicate that the overall nutrient retention was higher in the macrophyte-rich than in the unvegetated stream channel. The metric better reflecting the biological removal (i.e. the uptake velocity-V-f) was 9.8-fold and 4.4-fold higher, respectively for NH4 and PO4, in the vegetated than in the unvegetated reach. Temperature was an important driver of nutrient retention in vegetated condition, while discharge explained most variation of the uptake length of both nutrients in the reach where macrophytes were lacking. The role of the autotrophic compartment was further highlighted by the positive association between the uptake velocity (V-f-NH4 and V-f-PO4) and the gross primary production. The relationships observed between some retention metrics and the relative nutrient availability suggest that primary producers strongly controlled NH4 retention despite the extremely high N:P water ratio. The median NH4 uptake rate at the macrophyte-rich spring (30.5 mu g m(-2) min(-1)) was 5.5 fold higher than that at the unvegetated one, and was in the range reported for pristine streams. A few simple management actions are suggested that might be helpful to mitigate nitrogen and phosphorous pollution in the hydraulic networks of the Po Plain.