Effects of co-varying diel-cycling hypoxia and pH on disease susceptibility in the eastern oyster Crassostrea virginica

Diel-cycling hypoxia co-occurs with diel-cycling pH in shallow waters that are typically considered as refuge from deep-water hypoxia and are, therefore, targeted for restoration. These areas also tend to be heavily impaired by eutrophication from nutrient over-enrichment which increases the occurrence and severity of hypoxia and pH cycles. We used laboratory experiments to investigate the effects of diel-cycling dissolved oxygen (DO) and co-varying diel-cycling pH on infections of Perkinsus spp. and hemocyte activity in the eastern oyster Crassostrea virginica. Perkinsus marinus is the protistan parasite that causes Dermo disease in oysters. Severe diel-cycling DO increased the acquisition and progression of Perkinsus infections during exposure, and had a legacy effect the next year. Diel-cycling pH did not significantly affect infection dynamics either on its own or in combination with diel-cycling DO. Diel-cycling DO and pH both individually and in conjunction stimulated hemocyte activity, although this stimulated activity may not be effective at preventing Perkinsus infection. The magnitude of cycling conditions is an important consideration when choosing restoration sites, as severe cycling may hinder the re-establishment of oysters by creating areas that serve as reservoirs for parasites that can infect nearby populations.