How efficient are pre-dams as reservoir guardians? A long-term study on nutrient retention

Assessing nutrient loading and processing is crucial for water quality management in lakes and reservoirs. Quantifying and reducing external nutrient inputs in these systems remains a significant challenge. The difficulty arises from low monitoring frequencies of the highly dynamic external inputs and the limited availability of measures to reduce diffuse source loading. One option for the latter is the use of pre-dams, i.e. small impoundments at the inflow points into reservoirs, designed to retain nutrients by algal uptake and sedimentation. This study analyzes long-term (ranging from 8 to 22 years) nutrient and discharge time series for nine German pre-dams to assess their retention capacity. For that, we (i) quantified nutrient loading using four different mathematical methods, (ii) derived their retention efficiencies, and (iii) identified environmental factors determining the retention of nitrogen (N), phosphorus (P), and silica (Si). We show that retention of soluble reactive phosphorus (SRP) (43.6 %) and total phosphorus (TP) (39.9 %) is far higher than for nitrate (NO₃) (15.3 %) and Si (15.9 %). The retention efficiency for SRP and TP was higher during the warm seasons because of higher algal nutrient uptake and thus higher nutrient sedimentation. Mixed effects models documented a significant positive effect of the pre-dams' hydraulic residence time (HRT) on retention efficiency. Pre-dams provide substantial service in retaining nutrients and help to protect downstream waterbodies from nutrient inputs. They provide effective measures for trapping nutrients including those originating from non-point sources.