Hypolimnetic nitrate treatment to reduce internal phosphorus loading in a stratified lake

Abstract

Nitrate [Ca(NO3)2] was added to the hypolimnion of eutrophic Lake Lyng (10 ha, maximum depth 7.6 m), Denmark, over a 2-year period to study the impact on sediment phosphorus release and to evaluate the potential of hypolimnetic nitrate dosing as a lake restoration method. Dissolved (1995) or granulated (1996) nitrate was added 10 to 11 times at a depth of 5 m during stratification. The total dose was 8 to 10 g N m−2 yr−1. Hypolimnetic nitrate concentration was <0.01 mg N L−1 in years without dosing (1994 and 1997), and reached 1.2 and 2.2 mg N•L−1 in treatment years, respectively. Maximum concentrations of dissolved inorganic phosphorus (DIP) in the hypolimnion were 1.6 to 2.9 mg P•L−1 in untreated years, but were reduced to 0.8 to 1.2 mg P•L−1 in treatment years. Correspondingly, total phosphorus (TP) accumulation in treatment years was 48–77% that of non-treatment years. Dissolved nitrate seemed to reduce TP concentrations more efficiently than granulated nitrate, which sank partly into the loose sediment, and led to lower nitrate as well as lower iron concentrations in the hypolimnion. Lower iron release and/or lower nitrate concentrations resulted from the use of dissolved nitrate, which seemed to reduce precipitation of phosphate with iron and left a larger proportion of particulate phosphorus in the hypolimnion. Ammonia concentrations were higher with nitrate addition, probably due to enhanced mineralization of organic sediment and nitrate ammonification. The results from Lake Lyng suggest that the potential of using nitrate as a lake restoration method in deep lakes suffering from internal loading needs further investigation.