Seaweeds represent one of the largest groups of marine aliens in Europe and constitute a large percentage of all introduced marine species. In Brittany, the red macroalga Gracilaria vermiculophylla has invaded the bare areas of brackish waters in saltmarshes. In the Bay of Brest, the alga forms dense monospecific mats on the mud surface and occupies an empty ecological niche, in association with the invasive halophyte, Spartina alterniflora. The phenology of G. vermiculophylla was studied through seasonal monitoring of biomass, density and size of fragments, complemented by metabolomic monitoring using 1H HR-MAS NMR chemical footprinting analyses. Moreover, lipids and pigments were quantified, using high-performance thin layer chromatography for the former and high-performance liquid chromatography and spectrophotometry for the latter. This rhodophyte is present throughout the year, never fixed to a substrate on the mud, with a maximum biomass in the summertime. Phenological observations on algal populations demonstrated a high capacity for fragmentation, with a majority of fragments shorter than 3 cm. Metabolomic analyses highlighted a temporal variability of lipids, pigments and osmolytes between seasons. These results, combined with ecological data, improve our understanding of the acclimation of G. vermiculophylla in Brittany, where it is mainly present in a vegetative state throughout the year. Our study represents an important contribution to understanding the ecological strategies used by this invasive seaweed to colonize and persist in the Bay of Brest.