Results/Conclusions: While prior individual studies documented impacts of macroalgae, when effect sizes were averaged across studies, there was no significant effect of macroalgal biomass on MPB. In the VCR a non-native red macroalga, Gracilaria vermiculophylla, dominates intertidal mats and attains high biomasses at some sites. Nevertheless, macroalgal biomass was unrelated to MPB based on a survey of mudflats. Further, when macroalgae were removed from a mudflat using a before and after impact design, there was no change in MPB relative to a reference. Based on the meta-analysis, survey, and manipulation we conducted, macroalgal mats do not generally have a negative effect on MPB. This finding is important given the significance of MPB in supporting food webs and other estuarine ecosystem functions, and the increasing frequency and intensity of macroalgal blooms.