Summary
It is unclear how host‐associated microbial communities will be affected by future environmental change. Characterizing how microbiota differ across sites with varying environmental conditions and assessing the stability of the microbiota in response to abiotic variation are critical steps towards predicting outcomes of environmental change. Intertidal organisms are valuable study systems because they experience extreme variation in environmental conditions on tractable timescales such as tide cycles and across small spatial gradients in the intertidal zone. Here we show a widespread intertidal macroalgae, Fucus distichus, hosts site‐specific microbiota over small (meters to kilometers) spatial scales. We demonstrate stability of site‐specific microbial associations by manipulating the host environment and microbial species pool with common garden and reciprocal transplant experiments. We hypothesized that F. distichus microbiota would readily shift to reflect the contemporary environment due to selective filtering by abiotic conditions and/or colonization by microbes from the new environment or nearby hosts. Instead, F. distichus microbiota were stable for days after transplantation in both the laboratory and field. Our findings expand current understanding of microbiota dynamics on an intertidal foundation species. These results may also point to adaptations in hosts and/or microbes for withstanding short‐term environmental variation as facilitating stable host‐microbial associations.