Oyster mortality and human food poisoning events are linked to pathogens from the genus. However, the link between these events, planktonic bacterial dynamics and environmental variables has not yet been resolved. In Port Stephens, Australia, we characterized the microbial community and quantified the abundance of total , and in a (i) 27-month seawater planktonic microbial time-series; (ii) samples of Pacific oysters () during a mortality event and (iii) seawater samples following the mortality event. and exhibited seasonal abundance, peaking during the summer months. Total and in seawater were significantly greater at sites with high levels of oyster mortality and decreased 5-fold in the weeks following oyster mortality. Our findings provide evidence for the role of in oyster mortality events and indicate that ocean warming and elevated phytoplankton may stimulate putative pathogens in the genus.