## 4 Discussion
Therefore, in line with previous research, our results show an indication that marine biofilms, even after 14 days of grazing, may be able to develop beneficial relationships with grazers and demonstrate high tolerance for grazing disturbances. Although biofilms can slightly resist chemical exposure through their EPS matrix ([PERSON] et al. 2020), rhamnids are known for their ability to break down biofilms [51], and therefore, we did not expect growth and development of biofilm material to occur on the exposed substrate at a comparable level to the control. Although not significant, loss of C within our biofilms in the rhamnid-treated systems led to a decreased C:N ratio. Rhamolipids can encourage EPS material disintegration [52]. It is likely that, as the EPS matrix of a biofilm is heavily made up of lipids, polysaccharides, proteins, and extracellular DNA [17], and therefore heavy in C content, rhamnids affected the EPS matrix and decreased overall C within the biofilm. Due to the loss of C and altered biofilm stoichiometry within rhamnid-treated mesocosoms, biofilms were not wholly tolerant of the chemical but demonstrated strong resistance, as productivity did not change. There is a chance that resistance developed, as biofilm exposure to rhamnid was diluted with the constant input of fresh seawater; however, a constant exposure study would have to be performed to confirm this possibility. Carrying out additional measurements of macromolecular compositions of the biofilm would be a useful undertaking in similar future studies. The marine environment contains a combination of stressors and disturbances for biofilms and other biotas. As many of the stressors, such as chemical exposure, are fairly new, we hypothesized that our biofilms would not be tolerant to a combination of grazing and chemical exposure. Although productivity did not change within exposed biofilms, stoichiometry did, with a significant decrease in C:N in the presence of rhamnid, indicating resistance but not full tolerance. Our results support the idea that organisms will have different, and potentially negative, reactions to new combinations of stressors and disturbances within the environment. Climate change, leading to increased temperature stress, led to decreases in grazing overall from _L. litutorea_ after a two-week period [53]. Grazing habits are also based on food availability, which can be impacted by stressors, such as pollution and flow alteration [54]. As N increased in mesocosoms exposed to grazing alone due to excretion, likely of ammonium by _L. litutorea_, the lack of differences in N after dual exposure may demonstrate a lack of grazing occurring, thereby indicating damage to a previously beneficial relationship. Prior research has also shown that when the stressor is a chemical, such as atrazine, the combination of grazing and chemical pressure decreased carbon uptake and biofilm density overall, even though when examining the stressor alone, no differences were identified. Grazing pressure reduced the chances of biofilms developing new materials that are resistant to atrazine, thereby preventing recovery [55]. This is very problematic, as biofilms can develop chemical resistance overtime in areas with high chemical stress [56], but this cannot happen if the grazing of new, resistant material occurs. Although we did not perform an analysis to identify changes in chemical resistance, the significant reduction of C and C:N indicate stoichiometric, and likely structural, shifts after dual exposure. Comparing these results to those identified within our disturbance only mesocosoms provides strong evidence to suggest that dual exposure has detrimental effects on biofilms that may otherwise be tolerant to individual disturbances or resistant to stressors. Our experiment supports the idea that biofilms can develop holobiotic relationships with grazers, and that they may be sensitive to combined stress and disturbance events.