Marine macroalgae (i.e. seaweeds) are coastal foundation species, colonized by diverse and variable microbial communities. Microbes influence macroalgal health and ecosystem functioning but the factors that drive microbiome variation across a host population are unknown. Understanding the tempo and drivers of microbiome turnover on coastal foundation species is important for identifying and predicting how host-microbe interactions will be impacted by changing ocean conditions. Time series studies offer unique ecological insights into community stability and responses to change in the host or abiotic conditions that cannot be achieved otherwise. Here we sampled the bacterial and eukaryotic taxa on focal individuals of a widespread, intertidal macroalga, Fucus distichus, over time to characterize microbiome turnover within a single host individual and to compare individual temporal dynamics to population-wide patterns. We show that seasonal turnover is the dominant factor shaping the F. distichus microbiome, paralleling turnover of microbial communities in the surrounding environment. With shotgun metagenomics, we show that the functional gene potential of the microbiome changes seasonally but is much more stable over time and across the population than the taxonomic composition. This suggests hosts are colonized by seasonally-adapted microbes that have functional redundancies. Within the context of seasonal turnover, we find that inter-individual variation in the F. distichus microbiome is shaped by the local environment, host individual, and host developmental stage. Predictable temporal turnover across the host population, despite microbiome variation between individuals, suggests priority effects contribute to inter-individual differences. This work improves our understanding of multi-trophic microbial succession and turnover on an abundant temperate macroalga. It further demonstrates seasonal and predictable microbiome responses to environmental and host physiological cues, which may be key in maintaining ecologically important host-microbial interactions in changing coastal ecosystems.