Comparative phenology of Sargassum muticum and Halidrys siliquosa (Phaeophyceae : Fucales) in Limfjorden, Denmark

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