## 1 Introduction
Following a similar mechanism of action that affects waterborne pathogens and SAV, UV-C damages the genetic material within cyanobacterial cells (e.g., through the formation of pyrimidine dimers), resulting in either sublethal (e.g., growth suppression) or lethal effects. [PERSON] et al. (2010) demonstrated in laboratory studies that UV-C at a wavelength of 254 nm (\"UV\({}_{254}\)\") can disrupt intracellular processes within _Microystis aeruginosa_, leading to both reversible and irreversible damage depending on the intensity of the radiation andthe amount of time the cells are exposed. The UV-C dose can be tuned to a low/medium range (10-50 mJ/cm\({}^{2}\)) to either stun _M. aeruginosa_ cells for a period of time without killing the cells or impacting their structural integrity--an important consideration if the cells contain potent cyanotoxins--or a high range (>50 mJ/cm\({}^{2}\)) to destroy the cells and prevent possible regrowth. The same study also revealed that cyanobacteria are more susceptible to damage from UV\({}_{254}\) than other phytoplankton (e.g., green microalgae), suggesting that UV-C may be employed to suppress harmful cyanobacteria without significantly impacting microalgae that are beneficial to the ecosystem [13]. [PERSON] et al. (2015) also support this finding, reporting that the freshwater microalga _Pseudokirchneriella subcapitata_ and the marine microalga _Tetraselmis suecia_ both had a high tolerance to UV\({}_{254}\) (LC\({}_{50}\) = 236 mJ/cm\({}^{2}\) and 353 mJ/cm\({}^{2}\), respectively [14]. Related, bench-scale studies showed that UV\({}_{254}\) photodegrades the cyanotoxin microccystn-LR (MC-LR), transforming it into non-toxic photolytic products [15]. Outside of the laboratory, the authors are only aware of two field-scale studies that attempted to suppress phytoplankton growth using UV-C, both of which were conducted in Japan. In Iseri and Sasaki (1994), a boat equipped with UV lamps was developed to suppress freshwater red tide of the dinoflagellate _Peridinium bipes_. The researchers showed 90-100% lethality of _P. bipes_ depending on in situ exposure time (20 or 50 s) and cell concentration (<5.0 \(\times\) 10\({}^{3}\) or >1.0 \(\times\) 10\({}^{4}\) cells/mL) [16]. In Iseri and Maeda (2004), researchers developed a plankton control ship equipped with a UV irradiation system to examine the relationship between UV\({}_{254}\) dose and inhibition of harmful red tide algae growth in the Ariake Sea, Kyushu, Japan. The study reported that a 40 s UV treatment with the experimental equipment immediately destroyed the cells or stopped activity in about 80% of the _Chattonella_ spp. that dominated the bloom (neither UV intensity nor dose was reported [17]. Given the safety and efficacy that UV-C has proven for water and wastewater disinfection, as well as the promise that it has shown as an herbicide for suppressing invasive SAV, phytoplankton, and cyanobacteria, the goal of this study was to conduct a field trial to determine whether UV-C could be deployed to U.S. surface waters affected by cyanobacteria to sustainably and effectively suppress/stun cellular growth, degrade associated cyanotoxins, and inhibit harmful phytoplankton species more readily than beneficial species without the addition of chemicals. To achieve these ends, the US Army Engineer Research and Development Center (ERDC) engineered the CyanoSTUN\({}^{\text{TM}}\) (Cyanobacterial Suppression Through Ultraviolet-Light-C Neutralization) vessel, which irradiated cyanoHAB-contaminated surface waters with UV-C delivered by a wastewater-grade UV reactor mounted to the underside of a pontoon boat (Figure 1; [18; 19]). The UV-C was therefore contained within the UV reactor (rather than applied to a wider area like many HAB remediation chemicals), so organisms that did not pass through the reactor control volume were unaffected.