Effects of Asynchronous Stressors on the Eastern Oyster (Crassostrea virginica)

Population responses to environmental perturbations can be influenced by stressor timing or asynchrony. Since physiological responses often depend on prior conditioning, responses to extreme events may depend on prior exposure to environmental conditions, which could enhance or erode resilience. Yet there are relatively few studies investigating population responses to extreme events in the context of asynchronous stresses. Most multi-stressor experiments impose them simultaneously, which may sometimes poorly reflect seasonal dynamics in nature. For example, hurricanes typically occur over a broad season and may impact populations pre-conditioned with either mild early-season or elevated late-season water temperatures. In the Atlantic, hurricane season begins in June, following mild spring water temperatures, but continues through summer and autumn months, when water temperatures in some estuaries can reach 32 °C. We used the ecologically and commercially important Eastern oyster (Crassostrea virginica) as a model system, investigating how newly settled oysters responded to hurricane-level low-salinity stress (1 ppt for 10 days) after a month of elevated late summer temperatures (32 °C) versus milder early-season conditions (24 °C) typical of Galveston Bay, TX, USA. Oyster mortality under synchronous thermal and salinity stressors was higher than under asynchronous stressor treatments (46% versus 4%). Additionally, oysters without a prior thermal stress had higher mortality under salinity stress than those with prior elevated temperatures (16% versus 4%). Unlike mortality, growth and metabolism were dependent on individual stressors rather than their timing. Results indicate that incorporating temporal dynamics, rather than simply crossing multiple stressors simultaneously, can have important consequences for our understanding of how disturbances influence populations.