Tackling cyanobacterial blooms : Evaluating compounds and techniques for effective eutrophication management across different scales

Abstract

The purpose of this thesis was to investigate various potential compounds and techniques to manage eutrophication and mitigate cyanobacteria blooms. This research was conducted on different scales, beginning with laboratory experiments (Chapters 2, 3 and 4), then processing to in situ enclosure study (Chapter 5), and ultimately culminating in a large-scale intervention in a 114 ha shallow lake (Chapter 6).In-lake measures are gaining attention as complementary measures to speed up recovery in lakes and ponds suffering from nuisance eutrophication issues. In-lake measures must be efficient, easy to apply, and relatively cheap and safe, which means minimizing unwanted consequences, such as cyanotoxin releases. This thesis starts by evaluating the impact of nine commercially available products on a common bloom-forming cyanobacterium (Microcystis aeruginosa) and cyanotoxin (microcystins) releases (Chapter 2).According to Chapter 2, we learned that copper-based algaecides have excellent properties for killing M. aeruginosa, yet they may also be toxic to non-target zooplankton grazers, such as Daphnia magna, which play an important role in the water ecosystem. Thus, in Chapter 3, we conducted experiments to assess the impact of four copper-based algaecides on Daphnia magna. P binders (also called Phosphorus sorbents (PS)) as in-lake measures that immobilize the internal P pool are essential to manage eutrophication in the case. While P sorbents induced a milder response on M. aeruginosa, they were efficient in reducing phosphate concentration (Chapter 2). As abiotic factors might affect their efficiency, PS was tested under a realistic pH and temperature range (Chapter 4).Chapter 5 involved an enclosure study in a pond to investigate dredging, lanthanum-modified bentonite clay (LMB), aluminium-modified zeolite (AMZ) and iron chloride (FeCl2), which was partly studied in Chapters 2 and 4, in mitigating nutrient release from sediment.Building on the findings of Chapters 2, 4, and 5, a whole lake treatment using LMB was performed in Chapter 6. We collected sediment cores from the large-scale lake at pre-, post-3 months, and post-15 months LMB application and tested the sediment nutrients released under different pH.Lastly, Chapter 7 presents the key findings and provides a discussion on water restoration. Compounds to mitigate cyanobacterial blooms affect growth and toxicity of M i c r oc y s t i s ae r ugi nos a Li Kang, M aí r a M ucci , M i quel Lür l i ng