## 4 Conclusions  
In this study, the ERDC CyanoSTUN(tm) vessel was deployed to a cyanoHAB in Massachusetts as part of a field trial to determine whether UV-C could effectively suppress/stun cellular growth, degrade associated cyanotoxins, and inhibit harmful phytoplankton species more readily than beneficial species without the addition of chemicals. The results from this first field trial with the CyanoSTUN(tm) suggest that this treatment method may offer water managers confronted with a CyanoHAB the ability to rapidly and safely pause a bloom for multiple days and reduce the risks posed by its associated cyanotoxins without adding chemicals. The CyanoSTUN UV-C treatment effectively suppressed the growth of the cyanobacteria community for approximately two days at the three tested UV-C doses. This effect did not show a relationship with UV dose; thus, a dose-response relationship between UV-C dose and duration of cyanoHAB suppression could not be established. However, it is unclear whether the cyanobacteria population exposed to the highest treatment dose may have remained suppressed for longer than two days, due to the fact that a key sample was compromised during transport to UF/IFAS. The CyanoSTUN UV-C treatment also demonstrated a sustained, dose-dependent effect on microcystin concentration, possibly as a result of direct photolysis by UV\({}_{254}\) or indirect destruction by UV\({}_{185}\)-induced reactive oxygen species. Additionally, the CyanoSTUN UV-C treatment reduced chlorophyll \(a\) concentration at a statistically significant level for some of the regrowth period, but had no significant effect on phyococyanin, optical density at 680 nm, Ft 450 (in vivo chlorophyll _a_), Ft 620 (in vivo phyococyanin), or quantum yield. Analysis of cell density and community composition revealed that non-cyanobacteria were very scarce in this CyanoHAB (only 0.26% of the community, on average); at such low cell densities, we are unable to conclude whether the CyanoSTUN UV-C inhibits harmful phytoplankton species more readily than beneficial species without the addition of chemicals. As the cell density and microcystin concentration within this particular cyanoHAB were comparatively low, additional field studies with the CyanoSTUN(tm) should be conducted in surface waters experiencing more severe cyanoHABs to validate performance under more challenging conditions. Because the CyanoSTUN treatment does not impact water quality parameters or environmental conditions that are supportive of HABs, it cannot permanently eliminate blooms from a water body. However, the results of this study suggest that the CyanoSTUN may be a useful tool for either mitigating an ongoing bloom (reactive treatment) or preventing a future bloom (proactive treatment) through the short-term suppression of cyanobacterial growth. By targeting the suppression of cyanobacterial cell growth, the CyanoSTUN appears capable of pausing a bloom so it cannot worsen. Extending these findings to the case of proactive treatment, the CyanoSTUN may be able to suppress cyanobacterial growth in a target area that is not experiencing a bloom but is suspected to experience one imminently, such that the suspected bloom's development is delayed or does not occur at all. The results of this study indicate that the \"pause\" period will last for approximately two days before the cells within the bloom recover. During this two-day period, a water manager may take several courses of action including but not limited to using the CyanoSTUN again to extend the \"pause\" period, using a different treatment method to remove or kill the cells while they are stunned, or letting a natural event (e.g., rain event, cold front, etc.) take its course and disrupt the bloom naturally while the cells cannot self-regulate.