Wildfire ash leachates can alter the community composition and abundance of coastal prokaryote plankton and the magnitude of the changes can be proportional to the amount of ash added to seawater. To understand the extent to which community composition and abundance of coastal prokaryotes are affected by ash, two ash leachate addition experiments were performed at the Ria de Vigo in summer and winter. The experiments involved the addition of ash leachates from a wildfire which took place in the watershed of the Ria de Vigo in October 2017, into surface water samples collected in the middle sector of the ria and incubated for 72 h, under natural in situ water temperature and irradiance. Prokaryote DNA was extracted and analyzed using Illumina amplicon sequencing of the 16S rRNA gene at 48 h, while bacterial abundance and chlorophyll a were sampled up to a 72-h incubation. In summer, when initial nutrient concentrations were low, bacterial abundance increased with the addition of ash leachate, along with the proportions of Alteromonadales, Flavobacteriales, Saprospiraceae, Verrucomicrobia, and Vibrio splendidus (which can be pathogenic to bivalves). In winter, when post-fire runoff usually takes place in the Ria de Vigo, and initial nutrient concentrations are high, biomass changes were not detected and only subtle taxonomic alterations were observed, including changes in the proportions of the NS3a clade, Winogradskyella, Psychrobium, Pseudohongiella, and taxa which have been described as hydrocarbon degraders, such as Amphritea, Marinobacterium, and other members of the family Nitrincolaceae. Our findings suggest that the nutrients contained in wildfire ash can alter coastal prokaryote communities.