## 2 Methods  
### Field Collection  
The CyanoSTUN vessel was deployed to Lake Cochituate (Natick, MA, USA) on 12 September 2023 (Figure 1A-C). At the time of the field trial, all ponds associated with Lake Cochituate were under a Massachusetts Department of Public Health (DPH) \"Harmful Cyanobacterial Bloom Advisory\" and were closed to swimming and wading. The Massachusetts DPH recommends issuing a public health advisory for HABs at recreational freshwater locations when at least one of the following criteria is met: (1) a visible cyanobacteria scum or mat is evident, (2) total cell count of cyanobacteria exceeds 7.00 \(\times\) 10\({}^{4}\) cells/mL, (3) concentration of the toxin microcystin exceeds 8 ug/L, or 4) concentration of the toxin cylindrospermopsin exceeds 15 ug/L [20]. Three treatment areas and a control area were examined for depth and submerged hazards and defined in Pegan Cove near the US Army Natick Soldier Systems Center (approximate coordinates 42\({}^{\circ}\)17\({}^{\prime}\)10.3\({}^{\prime\prime}\) N 71\({}^{\circ}\)21\({}^{\prime}\)35.7\({}^{\prime\prime}\) W, Figure 1D). The depth of the treatment areas ranged from 5 to 10 feet, and the pond bottom in these areas consisted primarily of mud. The three treatment areas corresponded to three UV-C experimental doses: low (15 m)/cm\({}^{2}\)), medium (30 mJ/cm\({}^{2}\)), and high (45 mJ/cm\({}^{2}\)); the control area corresponded to no UV-C irradiation (0 mJ/cm\({}^{2}\)). The CyanoSTUN UV-C reactor lamps (Trojan UV3000 PTP, TrojanUV, London, ON, Canada) were turned on and given a 10 min warm-up period. Prior to field testing, the reactor lamps had been burned-in (i.e., run continuously) for 100 h to burn off impurities and stabilize lamp output, per EPA recommendations for low-pressure mercury vapor lamp operation [21]. Two composite sampler intake strainers (Global Water WS755,  
Figure 1: **(A)** The ERDC CyanoSTUNTM vessel during operation. **(B)** A screen protects the UV reactor from debris, and a bow-mounted trolling motor maximizes mixing within the UV-C reaction zone. **(C)** The UV-C reactor beneath the CyanoSTUN pontoon platform (stern side depicted with weir and effluent sampling device)—water flows through the reactor as the boat moves forward. **(D)** Treatment areas as linear routes (image not to scale). Treatment Area A—low dose = 15 mJ/cm\({}^{2}\). Treatment Area B—medium dose = 30 mJ/cm\({}^{2}\). Treatment Area C—high dose = 45 mJ/cm\({}^{2}\). Control Area D—no UV-C irradiation = 0 mJ/cm\({}^{2}\). Global Water Resources, Phoenix, AZ, USA; size 20 mesh (841 microns)) were fastened at 10.16 cm (4 in.) water depth, one at the influent (herein referred to as front) end and one at the effluent (herein referred to as rear) end of the reactor. Total composite sample collection was set to 2.5 L with a sampling interval of 5 min and a sampling volume of 500 mL. A flow meter (FloWatch FW450, JDC Electronic SA, Yverdon-les-Bains, Switzerland) was fastened to the bow of the boat with the impeller set to 20.32 cm (8 in.) depth to monitor CyanoSTUN velocity. The UV transmittance (UVT) of the water was measured using a Real UV254 P200 (Real Tech Inc., Whitby, ON, Canada). After the 10 min warm-up period, the UV intensity within the reactor was observed on the Trojan UV3000 PTP System Monitor (TrojanUV, London, ON, Canada). Because the observed UV intensity represented a single point value at the exact location of the UV sensor within the CyanoSTUN reactor rather than the desired average UV intensity within the reactor, a conversion factor was applied to derive the average UV intensity from the point value measurement using Equation (1). The conversion factor (f) was determined by modeling the CyanoSTUN reactor design in UV Calc Premium (Bolton Photosciences Inc., Edmonton, AB, Canada) and identifying the relationship between the sensor intensity reading and average reactor intensity at various UV transmittances.