A reservoir-scale experiment to simulate recycled water inputs using artificial nutrient addition

Highly treated recycled water (HTRW) can supplement surface water supplies, including as input to water reservoirs. Whilst advanced water treatment technologies remove pathogens and chemicals, trace concentrations of nitrogen and phosphorus remain having the potential to stimulate algal blooms. A novel large-scale study was undertaken in a reservoir inlet, adding nutrients at concentrations and ratios relevant to HTRW additions. We tested the effect on phytoplankton, macrophytes and denitrifiers. Phytoplankton cell densities increased by 180 to 280 % over a few days, due to the addition of NO 3 -N and PO 4 -P, but limited PO 4 -P availability appeared to constrain a larger algal response. Our study also showed competition for nutrients between phytoplankton and macrophytes, which also limited the scale of the phytoplankton response. We complemented this field study with calibration and validation of reservoir numerical models at a reservoir and inlet scale, designed to test management scenarios related to HTRW effects on phytoplankton biomass over one-year. Model simulations showed a synergistic effect of HTRW inflow volumes and drawdown in reservoir water levels, which increased nitrogen concentrations and phytoplankton biomass in the inlet. Based on our field and modelling studies, we identified key management actions, particularly regulation of water levels to maintain macrophytes and dilution of the HTRW. Catchment management is also needed to reduce phosphorus loads to the reservoir, and avoid synergies with nitrogen added from the HTRW which would further stimulate algal blooms. This field- and modelling-based study, provides new insights needed to understand to more effectively manage indirect HTRW inputs to reservoirs. • Nutrients in highly treated recycled water stimulated on algal and macrophyte growth in a drinking water reservoir • The scale of the algal response was limited by low phosphate addition, relative to nitrogen, and competition for nutrients by macrophytes • The field results were used to validate a lake model to predict suitable management actions to minimize algal growth