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A mechanistic understanding of the factors that influence establishment and secondary spread of introduced species is critical for predicting the spatial extent and magnitude of negative impacts of species invasions. In the northwest Atlantic, an ecologically significant invasive bryozoan (Membranipora membranacea) has expanded its range northwards over the last 30 years. Warm ocean temperature has been linked to population outbreaks of M. membranacea within its established invasive range in southwestern Nova Scotia; however, rates of spread and the physical and biological factors affecting establishment of founding populations have not been explicitly quantified. Here, we use unique baseline data on presence/absence and abundance of this bryozoan near its current northern range limit to quantify rates of spread and identify factors influencing its establishment in three regions in the Gulf of St. Lawrence. Except for one potential incident of human-facilitated long-distance dispersal, the rate of spread of M. membranacea in the northwest Atlantic was comparable to that of other invasive marine invertebrates. Contrary to our predictions based on long-established populations in Nova Scotia, the abundance of M. membranacea was not strongly related to regional differences in ocean temperature at its northern range limit. Our results suggest that colder temperatures are not limiting the northward spread of this bryozoan in the northwest Atlantic. Instead, relationships between temperature and abundance of M. membranacea in the Gulf of St. Lawrence may be moderated by a lack of available kelp substrate. Our findings, therefore, have important implications regarding the potential for M. membranacea to impact vulnerable Arctic ecosystems where laminarian kelps are widespread. [ABSTRACT FROM AUTHOR]