Nutrient retention efficiency of small regulated streams during the season of low-flow regime in Central Lithuanian lowland

Small streams are polluted with nutrients when crude or partially treated wastewater flows from settlements. The aim of the study was to determine the efficiency of small streams to self-purify themselves of nutrients during the season of low-flow regime. The study was carried out in five regulated streams with catchments of up to 30 km² in the Central Lithuanian lowland. The retention of nitrogen (N) and phosphorus (P) compounds was studied up to 5.2 km downstream from the settlements. Owing to the pollution load from settlements, the concentrations of total N increased from 2.0–3.0 above the settlements to 6.75 ± 2.07 mg L⁻¹ below; the concentrations of total P increased from 0.03–0.07 to 0.56 ± 0.30 mg L⁻¹, with the stream's ecological state deteriorating to ‘poor’ and ‘bad’. For the calculation of total N retention, with respect to the pollution load and the distance from the settlement, an empirical equation was derived. Calculations show that with a pollution load of 2.0 g N m⁻³ at 5.0 km distance, around 90% of total N can be retained. However, when the load is threefold greater, at the same distance only 65% of total N is retained. As a result of self-purification processes only about 50% of total P may be retained.