From green to transparent waters : Managing eutrophication and cyanobacterial blooms by geo-engineering

Abstract

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 in the water column, and it should not therefore be used without testing its effects on the cyanobacterial assemblage being targeted to avoid unwanted rapid release of cyanotoxins. In chapter 6, we showed field results from a whole-lake treatment with PAC and Phoslock®. This technique called Flock and Lock aimed to target P from the water column, P-release from the sediment and the ongoing cyanobacterial bloom. The intervention was successful in improving water quality in Lake De Kuil. After two weeks of the treatment, however a surface scum was observed near the shore of the lake, which disappeared spontaneously after two weeks. The lake was open in time for the bathing season without any swimming bans during 2017. Tests to why the scums occurred, and how to avoid their occurrence showed that promising approach to avoid biomass accumulation is to damage the cell first using hydrogen peroxide and later settle them with the Flock and Lock technique. Larger scales tests need still to be performed to shed light on possible limitations of this technique. In chapter 7 I reflected that there is no single magical solution to manage eutrophication and cyanobacterial blooms. Each system is unique and each material/technique (P immobilization, chitosan, Flock and Lock, peroxide) has its limitations. Thus, a broad-scale generalization (copy-paste of methods) will in most cases not lead to a successful restoration. A mitigation plan must always include a proper system analysis and experimental tests under realistic condition on various scales before a field application can be performed.