Tackling Cyanobacterial Blooms: Evaluating compounds and techniques for effective eutrophication management across different scales

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. The 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 algicides 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 access the impact of four copper-based algaecides on Daphnia magna. Phosphorus sorbents (PS) as in-lake measures that immobilize the internal P pool are essential to manage eutrophication. 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), the aluminum-modified zeolite (AMZ) and 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 large-scale lake treatment was performed using LMB in Chapter 6. We collected sediment cores from the large lake at pre-, post-3 months, post-15 months LMB application and tested the sediment nutrients released under different pH. Lastly, Chapter 7 presents the key findings of this thesis and provides a discussion on water restoration.