Based on previous studies reporting that biofilms from PPF-1 exposed to Mg 2+ in a shaken culture resulted in an increase in adhered biofilms, we chose to study the impact of Mg 2+ ions on biofilms under dynamic flow conditions. The experiment was repeated four times with PPF-1 exposed to Mg 2+ and under control conditions in the reference channel (no Mg 2+ addition). Due to the differences in the OD versus time profiles, including differential effects from the lag phase, growth rate and timing of biofilm sloughing events, the results from four individual experiments for each condition are plotted in Figures 4a rather than their mean results. Despite the variability in these results, certain observations are possible. First, in the presence of Mg 2+ ions, biofilms accumulated into more optically dense and thicker layers while also demonstrating greater resistance to shear stress and detachment compared to those under control conditions. This is shown in Electronic Supplementary Information, Section S3 for each individual experiment via hb and the cumulative release profiles versus time. Second, preceding the detachment, displacement events occurred that resulted in corrugated topographical structures, as observed previously under control conditions. Such displacements preceded sloughing, but continued nearly continuously between sloughing events for low-ionic-strength nutrient solutions, compared to those with added Mg 2+ , but never showed signs of catastrophic by sloughing ripping (Figure ). Based on the increases to the PPF-1 biofilm surface attachment, stiffness, and accumulated material before reaching the critical volume strain in the presence of Mg 2+ ions, we conclude that the biofilm more closely resembled PAO1 than PPF-1 under the control conditions. Third, after reaching a critical volume (and corresponding τc), the Mg 2+ -exposed biofilms sloughed off abruptly and massively via a tearing process (Figure ). The clean edges at the fracture boundary indicate that the biofilm had become brittle. This evidence points to a viscoplastic deformation behaviour (e.g., as described by Binham and Herschel-Buckley models). Generally, biofilms are treated as viscoelastic materials, which can respond to both long-term shear stresses by viscous flow and to short-term perturbations by a reversible elastic response. However, the concept of biofilm plasticity is a new idea, one by which large internal stresses are irreversibly relieved. In this case, biofilm plasticity can be viewed as the irreversible deformation that occurs by ripping at the stress fracture boundaries as a response to large shear stresses applied to a highly viscous state obtained by the mature biofilm when it is exposed to a high-ionicstrength nutrient. Therefore, this observation may help to accelerate further inquiry, including the creation of predictive models for critical shear stress based on viscoelastic parameters. This type of deformation, which we call catastrophic sloughing, was observed in 3 out of 4 experiments using added MgCl2. In the one experiment in which the sloughing was not observed at all, the biofilm height was only 32.2 µm by the end of the experiment, whereas in the other 3 experiments, the average biofilm height was significantly higher (hb = 39.1 ± 3.0 µm). Therefore, it seems likely that a critical biofilm volume had not been reached before the end of the 68 hours in the experiment in which sloughing was not observed.