Abstract: Coastal estuaries are affected by an increasing number of human interventions, including the introduction of invasive species. Baccharis halimifolia is endemic to the east coast of North America and in recent decades has colonised a large number of estuaries in northern Spain. Although hydrodynamic conditions play an important role in the creation of niches for plant species in salt marshes, their interaction with the spatial distribution of B. halimifolia remains poorly studied. This study identified the main hydrodynamic variables controlling the distribution of this species in Oyambre estuary (Cantabria, Spain) and used these variables to model the habitat of the plant. B. halimifolia develops in areas that are inundated < 26 %of the year, with salinity < 25 g/L, and water speed and water flow < 0.1 m/s and < 0.85 m3/s, respectively. Habitat suitability modelling showed that this plant is not equally represented throughout the estuary; preferring areas where hydrodynamic values are lower than average. The plant also showed medium tolerance to the present hydrodynamic conditions (tolerance á 1), preferring areas that are inundated 0 to 21 % of the year, with salinity between 0 and 26 g/L. Changes in hydromorphologic conditions may have a predictable impact on the type and distribution of vegetation in salt marshes. B. halimifolia will remain in areas where tidal influence is reduced, competing with native vegetation.

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