Do macroalgal mats limit microphytobenthos on mudflats?

Background/Question/Methods: Macroalgal biomass is increasing in many coastal ecosystems due to eutrophication and species introductions. Macroalgal mats may compete with microphytobenthos (MPB) for light and nutrients and due to their position in the canopy, have a negative impact on MPB biomass. We tested this effect by conducting a meta-analysis of prior experiments as well a comparative survey and a macroalgal-removal manipulation in the coastal lagoons of the Virginia Coastal Reserve (VCR) on the eastern shore of Virginia (U.S.A.).

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.