Background/Question/Methods With intensifying global change in both terrestrial and aquatic systems alike, ecologists work to understand the effects of anthropogenic and environmental stressors on organisms. Importantly, prior stress can influence population response to later stress. This is often seen in population response to acute stress during extreme events, which may occur after exposure to other chronic environmental stressors. However, experiments on the effects of asynchrony in which stresses are imposed at different times on organisms are rare. For example, while there are studies on the effects of multiple stressors on Eastern oysters (Crassostrea virginica) in the Gulf of Mexico, few have focused on temporal dynamics of stressors. In Galveston Bay, Texas, the hurricane season begins in May, following mild spring water temperatures, but continues through late summer months, when water temperatures can reach 32°C. Here, we asked if oysters respond differently to hurricane-level low-salinity stress after periods of elevated late summer temperatures relative to more standard, early-season conditions. We exposed newly settled oyster spat to early-season (24°C) or late-season high temperatures (32°C) for one month followed by an acute low salinity (1 ppt) disturbance for 10 days (versus control). We also exposed a subset of oysters to lower temperatures during the low salinity phase to account for temperature drops associated with heavy rainfall events and compared all of these asynchronous-stress treatments to a more standard, 10-day simultaneous heat and low-salinity treatment. Results/Conclusions Oysters experienced the highest mortality (46%) when high-temperature and low salinity stresses were imposed simultaneously with no prior conditioning. This was significantly different from oysters under asynchronous stressor treatments, which saw only 4% mortality. Additionally, prior thermal stress mitigated some of the negative effects of subsequent osmotic stress leading to significantly lower mortalities (4%) compared to treatments under acute salinity stress without prior heat exposure (16%). While any beneficial effects of lower temperatures imposed during acute salinity stress were not immediately apparent, we saw significant differences in mortality between these treatments after one week of recovery at ambient conditions. Oysters exposed to lower temperatures during low-salinity conditions had significantly lower mortality (8%) than those in high temperature treatments (20%). Results suggest that incorporating temporal dynamics, rather than simply crossing multiple stressors simultaneously, can have important consequences for our understanding of likely population impacts from extreme events.
