In a field study we investigated the temporal dynamics of drift-algal assemblages on both small (1 m2) and larger (2500 m2) spatial scales in two shallow (1-3 m) and relatively sheltered locations in Aarhus Bay and Isefjorden, Denmark. Drift-algal cover was estimated every second day in 40 permanent plots (1 m2) randomly positioned in a 2500 m2 study area during three 8-12 days periods of the growth season. Results show that the algal assemblages were highly dynamic on the small spatial scale as cover within individual plots changed regularly between subsequent observations. The change in algal cover was inversely correlated with the cover of eelgrass, Zostera marina, suggesting that algae were retained by the eelgrass leaves. At the larger spatial scale algal cover was less variable and significant changes occurred just a few times during the study periods. Variability was caused either by algal growth, as indicated by a steady increase in cover, or by physical forces moving large aggregations of algae into or out of the study area leading to significant changes in cover within few days. Thus, in shallow coastal ecosystems aggregations of ephemeral macroalgae can be highly dynamic, algae are often moved around within a site and large changes in total site cover can occur within days. This may have implications for the interpretation of experimental set-ups investigating effects of algal mats on seagrasses, where the fixed nature of mats may intensify effects on biogeochemical conditions and make prediction of seagrass loss caused by macroalgal blooms difficult.