Effects of Asynchronous Stressors on the Eastern Oyster (Crassostrea virginica)
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With intensifying global change 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. I reviewed this small but growing body of literature and show how results in studies incorporating asynchrony often differ from those that examine the effects of multiple stressors imposed on organisms at the same time. In particular, I found that asynchrony rarely leads to additive effects, but rather elicited responses of cross-tolerance (mitigation of latter stress) or cross-susceptibility (exacerbation of the latter stress). Additionally, the majority of asynchronous stressor studies were conducted in North America and Europe and were rare in the rest of the world. I also conducted an experiment on Eastern oysters (Crassostrea virginica), for which there are few asynchronous stressor studies. 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 32C. I 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. I exposed newly settled oyster spat to early-season (24C) or late-season high temperatures (32C) for one month followed by an acute low salinity (1 ppt) disturbance for 10 days (versus control). 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%). Results suggest that incorporating temporal dynamics, rather than simply crossing multiple stressors simultaneously, can have important consequences for our understanding of impacts from extreme events on early, important life stages of organisms.
