Excessive nutrient loading, primarily linked to anthropogenic stress, is the main driver of lake eutrophication. Eutrophic lakes undergo gradual degradation characterized by poor water quality, decrease biodiversity and loss of area, harming wildlife, reducing ecological services value provided by the lake and posing risks to public health. Among anthropogenic stressors, agriculture and urbanization are particularly significant.
Lake Velnezers (Riga, Latvia, North Eastern Europe) has experienced substantial landscape transformation over recent decades, shifting from predominantly forested to agricultural and subsequently urbanized surroundings. These land-use changes have significantly altered the lake’s nutrient dynamics, leading to persistent eutrophication. While waste water management has substantially reduced external phosphorus input, the vast amount of phosphorus deposited in the sediment has prevented the recovery of the lake. In an effort to mitigate excessive internal phosphorus loading, Lake Velnezers underwent a chemical treatment involving the addition of a polyaluminum chloride (PAC) solution into the sediment, marking the first application of this method in the Baltic States.
Pre-treatment monitoring, conducted monthly for over a year, recorded total phosphorus (TP) concentrations up to 0.11 μg/L in surface waters and 1.15 μg/L in the hypolimnion, contributing to algal blooms and critically low bottom oxygen levels (< 0.5 mg/L) during thermal stratification. Ecological assessments of phytoplankton, zooplankton, macrozoobenthos, macrophytes, and fish confirmed a poor ecological status.
Following watershed analysis and dose modelling, in October 2024 the lake’s sediment was injected with a dosage of PAC equivalent to 56 g Al/m2, leading to a rapid improvement of water quality parameters. Post-treatment monitoring conducted in autumn and winter revealed an approximately 50% reduction in TP concentrations in the water column compared to the same period in the previous year. A substantial part of this decline can be attributed to the reduction in orthophosphate-bound phosphorous, with concentrations decreased by 84% in the epilimnion and by 99% in the meta- and hypolimnion. These changes were accompanied by reductions in chlorophyll-a and dissolved organic carbon (DOC), as well as a marked increase in water transparency from 0.7 m to 2.6 m within one month.
Continued monitoring will assess whether the improvements in water chemistry are reflected in enhanced biological quality elements and a shift in the lake’s trophic status. Additionally, upcoming studies will investigate the potential influence of PAC treatment on microplastic pollution, as Lake Velnezers has been a long-term site for microplastic research.This study is part of the Interreg Baltic Sea Region project TRUST ALUM.