The vertical zonation and temporal dynamics of the marine hydroid Dynamena pumila were assessed across a wave-exposure gradient on five rocky shores in the Bay of Fundy, Atlantic Canada. Hydroid abundance and occupancy (i.e., percentage occurrence in a quadrat) were measured in 0.25-m2 quadrats at eight vertical elevations for each site over four consecutive seasons. Hydroid abundance and occupancy were highly correlated (R2 = 0.877). Estimates of hydroid fertility (percentage of sexually reproductive colonies) and size (stem height and number of branches) were obtained in five quadrats at every elevation over the four seasons. Abundance peaked at about 37.5–62.5% of the mean tidal range, with maximum abundance at the site with moderate wave exposure. Abundance peaked during the summer and dropped dramatically over the winter, particularly at the more wave-exposed sites. Winter ice scour and unfavourable environmental conditions reduced hydroid abundance, fertility, and size during the winter. This study demonstrates dramatic shifts in the distribution, fertility, and size of a marine hydroid. The importance of wave action, ice scour, and seasonal changes in environmental conditions is highlighted to emphasize their roles in regulating intertidal hydroid communities on boreal rocky shores.