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Blooms and scums of cyanobacteria led to prolonged swimming bans in Lake Rauwbraken (The Netherlands). We mitigated this nuisance by imposing phosphorus limitation on the lake. Our lake system analysis revealed that, over 40 years history, small dispersed sources built up a legacy phosphorus pool in the sediment and that the internal sediment release of phosphorus was the main phosphorus-source to the cyanobacteria (Chapter 2). The practically and economically most feasible approach was to reduce the internal phosphorus load by applying a solid phase phosphorus sorbent (SPS). We combined the flocculent poly aluminium chloride (PAC; Floc; Chapter 3) with the SPS lanthanum modified bentonite (Lock; Phoslock®; Chapter 3) as sinking weight to sink a present bloom, additional lanthanum modified bentonite was applied to the reduce internal phosphorus load. The treatment first stripped the water column of phosphorus using a flocculant (Floc) (Chapters 3, 4), shifting the lake from a (hyper) eutrophic state to a mesotrophic state. It also provided an instantaneous cure to the nuisance and lifted the imposed swimming ban (Chapter 4). Longer term, water quality remained good for more than a decade, but due to ongoing and increased external phosphorus loads, the lake is returning to a eutrophic state (Chapter 5). Directly after its application, the treatment caused a temporarily decrease in the Daphnia population in the lake (Chapter 4). Four years after the treatment Microcystis cells emerged from the lake sediment and produced a minor low-toxic scum. The event did not lead to a proliferation into a nuisance bloom (Chapter 5). Other than this mystery scum, the lake remained free of cyanobacterial nuisances. Directly after the application the Dutch maximum permissible filterable lanthanum concentration was exceeded for after 75 days (Chapter 6). Three years after the application, part of the LMB is relocated to the deeper part in the lake (Chapter 6). The lanthanum from the LM

Eutrophication and cyanobacterial blooms are increasing worldwide. Despite being studied for almost a century, mitigating eutrophication remains a challenge. Motivated by this challenge, we studied potential geo-engineering materials and in-site techniques to manage the eutrophication and cyanobacterial blooms in controlled experiments and a whole-ecosystem intervention. As phosphorus (P) control is essential to manage eutrophication, this thesis started evaluating natural and modified clays and soils for their capacity to adsorb P (chapter 2). We showed that four out of ten materials were able to adsorb P, and that P adsorption differed under varying abiotic conditions. The modified materials (lanthanum (La) modified bentonite, commercially called Phoslock® and Aluminium modified zeolite, commercially called Aqual-P®) were able to adsorb more P than the naturals ones such as Fe-rich soils The need to mitigate eutrophication in coastal areas prompted us to evaluate Phoslock® efficiency and behaviour in saline waters in chapter 3. Phoslock® was able to adsorb P in all salinities tested from brackish to seawater, whilst filterable La concentrations remained very low. We concluded that the use of Phoslock® on saline waters should be considered, yet, ecotoxicological studies must be performed before field applications in saline environments. Beside solid-phase P sorbents, flocculants have also been used in lake restoration. In this context, chitosan has been proposed as an “eco-friendly” flocculant as an alternative to metal based flocculant, such as polyaluminium chloride (PAC). In chapter 4, we tested the effect of chitosan on several cyanobacterial species and showed that chitosan may cause rapid cell lysis. In chapter 5, we looked closer into strain variation whilst also measuring cyanotoxin release. We showed that chitosan was able to cause cyanotoxins release. These effects were, however, strain dependent. Chitosan application might therefore cause toxin release i

Small lakes and ponds are common features in urban areas and they contribute to the quality of citizens’ life. A poor water quality, however, can easily give rise to nuisance. A major cause for a poor water quality is a high concentration of plant-nourishing nutrients, eutrophication. In (semi-)standing waters, eutrophication often results in a high biomass of blue-green algae (cyanobacteria), turbid water and the disappearance of submerged aquatic plants. The cyanobacterial blooms can be accompanied with fish kills due to anoxia, the development of unpleasant surface scums and malodors. As cyanobacteria can produce potent toxins, they impose a serious risk for citizens’ health, pets and wildlife. The cyanobacterial blooms hamper the anthropogenic use of the water and can have negative economic impacts. Water managers experience that the reduction of cyanobacterial nuisance is arduous. As long-term positive effects of management interventions are not often achieved, there is need for effective approaches. The objective of this study was to investigate the efficacy of promising methods to reduce cyanobacterial nuisance in city waters, targeting the clear water state and promoting the growth of aquatic plants. Various methods were tested, in the laboratory, in small and large compartments and were eventually applied in whole ponds and lakes. It is widely accepted that the reduction of nutrient inputs is essential for long-term positive effects. This study focused on the reduction of the input and the availability of the key-nutrient phosphorus. It was shown that cyanobacterial nuisance was wide spread in urban ponds and lakes in the Dutch province of North Brabant. The phosphorus inputs of four urban lakes in this province were addressed. The study lakes differed greatly in the phosphorus sources and loads, depending on site-specific characteristics. These differences affected the selection of measures. It was shown that in-lake measures were effective in realizing th, Deze studie had als doel te onderzoeken welke kansrijke aanpak effectief blauwalgenoverlast bestrijdt en de waterkwaliteit langdurig verbetert, met helder water en goede groeimogelijkheden voor waterplanten.

The Dutch company WiseUse International BV has proposed using a so-called bio stimulator “PoCo” (Pollution Control) in ecological restoration of Lake Ypacarai (Paraguay). In this study results of laboratory assays are reported testing the performance of PoCo regarding the alleged enhanced mineralization of organic matter in the water phase and sediment, the inhibition of cyanobacterial growth (as a clear measure counter acting the symptoms of eutrophication) and effects on survival of the waterflea Daphnia.