Nitrogen and phosphorus retention in a human altered stream.

A whole stream study was designed to determine the nutrient retention capacity of Fosso Bagnatore, a third-order stream in the Mediterranean region. Nitrogen (NH4-N, NO3-N) and phosphorus (PO4-P) retentions were assessed using controlled nutrient addition experiments. The experimental reaches were selected with reference to a waste water treatment plant effluent and characterized from a physical, hydrological, chemical, and biological standpoint on each nutrient addition date. Nutrient retention was thus compared between control and impacted reaches to detect the possible effluent effects on the stream retention capacity. The impacted reach showed on average a higher concentration of NH4-N, NO2-N, DOC, a larger standing crop of benthic organic matter, a lower gross primary production, and a higher respiration rate. Nutrient retention of NH4-N and PO4-P was negatively correlated with discharge, but differences between reaches were significant only for PO4-P retention, being higher in the impacted reach. No differences were observed for NO3-N retention. We conclude that discharge is the main factor influencing nutrient retention of Fosso Bagnatore, but in summer months, at lower discharge levels, nutrient retention is also influenced by nutrient background concentrations and microbial activity associated with the benthic organic matter.