Sargassum muticum (Fucales: Sargassaceae) has been present in Scandinavia since 1984. In Limfjorden, a shallow Danish sound, it has invaded the habitat of the indigenous Halidrys siliquosa (Fucales: Cystoseiraceae). Studies have suggested a displacement of indigenous relatives by Sargassum muticum and that this could be due to differences in life history strategies. Our study is the first to document the phenology of S. muticum at the northern part of its European distribution range (Scandinavia) and the first to document the phenology of Halidrys siliquosa. Thallus height and weight, species cover and tissue carbon, nitrogen and phosphorus were measured. For Sargassum muticum a strong seasonality with midsummer maxima, similar to previously published results from other European sites, was found. Halidrys siliquosa only exhibited moderate seasonality, if any. Hence, the two species were found to be functionally different and have different life histories. Our results support the idea that the competitive success of Sargassum muticum is caused by its pseudo-perennial life history compared to the truly perennial life histories of many indigenous relatives. Conversion factors between commonly used biometric measures were derived from the phenological data.