Cyanobacterial blooms pose a risk to wild and domestic animals as well as humans due to the toxins they may produce. Humans may be subjected to cyanobacterial toxins through many routes, e.g., by consuming contaminated drinking water, fish, and crop plants or through recreational activities. In earlier studies, cyanobacterial cells have been shown to accumulate on leafy plants after spray irrigation with cyanobacteria-containing water, and microcystin (MC) has been detected in the plant root system after irrigation with MC-containing water. This paper reports a series of experiments where lysis of cyanobacteria in abstracted lake water was induced by the use of hydrogen peroxide and the fate of released MCs was followed. The hydrogen peroxide–treated water was then used for spray irrigation of cultivated spinach and possible toxin accumulation in the plants was monitored. The water abstracted from Lake Köyliönjärvi, SW Finland, contained fairly low concentrations of intracellular MC prior to the hydrogen peroxide treatment (0.04 μg L−1 in July to 2.4 μg L−1 in September 2014). Hydrogen peroxide at sufficient doses was able to lyse cyanobacteria efficiently but released MCs were still present even after the application of the highest hydrogen peroxide dose of 20 mg L−1. No traces of MC were detected in the spinach leaves. The viability of moving phytoplankton and zooplankton was also monitored after the application of hydrogen peroxide. Hydrogen peroxide at 10 mg L−1 or higher had a detrimental effect on the moving phytoplankton and zooplankton.