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Controlling lake eutrophication is a challenge. A case-specific diagnostics driven approach is recommended that will guide to a suite of measures most promising in restoration of eutrophic lakes as exemplified by the case of the shallow lake Groote Melanen, The Netherlands. A lake system analysis identified external and internal nutrient load as main reasons for poor water quality and reoccurring cyanobacterial blooms in the lake. Based on this analysis, a package of restoration measures was implemented between January 2015 and May 2016. These measures included fish removal, dredging, capping of peat rich sediment with sand and an active barrier (lanthanum-modified bentonite), diversion of two inlet streams, reconstruction of banks, and planting macrophytes. Dredging and sand capping caused temporarily elevated turbidity and suspended solids concentrations, while addition of the lanthanum-modified clay caused a temporary exceedance of the Dutch La standard for freshwaters. Diversion of inflow streams caused 35 % less water inflow and larger water level fluctuations, but the lake remained water transporting with strongly improved water quality as was revealed by comparing five years pre-intervention water quality data with five years' post-intervention data. Total phosphorus concentration in the water column was reduced by 93 % from 0.47 mg P l -1 before the intervention to 0.03 mg P l -1 after the intervention, total nitrogen by 66 % from 1.27 to 0.21 mg N l -1 , total chlorophyll-a by 75 % from 68 to 16 µg l -1 , cyanobacteria chlorophyll-a by 88 % from 32 to 4 µg l -1 . Turbidity had declined by 58 % from 23.5 FTU to on average 9.9 FTU. No cyanobacteria blooms were recorded over the entire post-intervention monitoring period (2016-2021). Submerged macrophytes increased from complete absence before intervention to around 10 %-15 % coverage after intervention. Repeated fish removal lowered the fish stock to below 100 kg ha -1 with 12 % of bream and carp remaining. Hence, the package of cohesive measures that was based on a thorough diagnosis resulted in rapidly, strongly and enduringly improved water quality. This case provides evidence for the power of combining measures in restoring eutrophic lakes., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024. Published by Elsevier Ltd.)

Lanthanum Modified Bentonite (LMB; Phoslock®) is used to mitigate eutrophication by binding phosphate released from sediments. This study investigated the fate of lanthanum (La) from LMB in water, sediment, macrophytes, and chironomid larvae in Lake Rauwbraken (The Netherlands). Before the LMB application, water column filterable La (FLa) was 0.02 µg L -1 , total La (TLa) was 0.22 µg L -1 . In sediment the total La ranged 0.03-1.86 g m -2 . The day after the application the maximum FLa concentration in the water column was 44 µg L -1 , TLa was 528 µg L -1 , exceeding the Dutch Maximum Permissible Concentrations (MPC) of 10.1 µg L -1 by three to fourfold. TLa declined below the MPC after 15 days, FLa after 75 days. After ten years, FLa was 0.4 µg L -1 and TLa was 0.7 µg L -1 . Over the post-application years, FLa and TLa showed statistically significant downward trends. While the LMB settled homogeneously on sediment, after 3 years it redistributed to 0.2-5.4 g La m -2 within shallow zones, and 30.7 g m -2 to 40.0 g La m -2 in deeper zones. In the upper 20 cm of sediment, La concentrations were 7-6702 mg kg -1 dry weight (DW) compared to 0.5-7.0 mg kg -1 before application. Pre-application anaerobic sediment release of FLa was 0.006 mg m -2 day -1 . Three months after the application it was 1.02 mg m -2 day -1 . Three years later it was 0.063 mg m -2 day -1 . Before application La in plants was 0.8-5.1 mg La kg -1 DW, post-application values were up to 2925 mg La kg -1 DW. In chironomid larvae, La increased from 1.7 µg g -1 DW before application to 1421 µg g -1 DW after one month, 3 years later it was 277 µg g -1 DW. Filtration experiments indicate FLa is not truly dissolved free La 3+ cations., (Copyright © 2019 The Author(s). Published by Elsevier B.V. All rights reserved.)