Despite the abundance of literature on organismal responses to multiple environmental stressors, most studies have not matched the timing of experimental manipulations with the temporal pattern of stressors in nature. We test the interactive effects of diel-cycling hypoxia with both warming and decreased salinities using ecologically realistic exposures. Surprisingly, we found no evidence of negative synergistic effects on Olympia oyster growth; rather, we found only additive and opposing effects of hypoxia (detrimental) and warming (beneficial). We suspect that dielcycling provided a temporal refuge that allowed physiological compensation. We also tested for latent effects of warming and hypoxia to low-salinity tolerance using a seasonal delay between stressor events. However, we did not find a latent effect, rather a threshold survival response to low salinity that was independent of early life-history exposure to warming or hypoxia. The absence of synergism is likely the result of stressor treatments that mirror the natural timing of environmental stressors. We provide environmental context for laboratory experimental data by examining field time series environmental data from four North American west coast estuaries and find heterogeneous environmental signals that characterize each estuary, suggesting that the potential stressor exposure to oysters will drastically differ over moderate spatial scales. This heterogeneity implies that efforts to conserve and restore oysters will require an adaptive approach that incorporates knowledge of local conditions. We conclude that studies of multiple environmental stressors can be greatly improved by integrating ecologically realistic exposure and timing of stressors found in nature with organismal life-history traits.

Understanding the extent of local adaptation is critical for conservation and restoration planning in an era of environmental change. Adaptive differentiation among populations may mediate a species' response to environmental stress, yet these evolutionary processes are seldom studied in marine systems. We used native Olympia oysters (Ostrea lurida) as a model organism for studying local adaptation with applications to marine restoration. We tested whether populations of oysters in California estuaries are locally adapted to their home sites and to stressful low salinity events, which are predicted to increase in frequency with climate change. We spawned oysters from three sites in San Francisco Bay and raised their offspring under common laboratory conditions. These oysters were then reciprocally transplanted among the three field sites. At two of the three sites, oysters of local origin tended to survive better than those from other populations, suggesting that local adaptation may occur even within a single estuary. In a subsequent experiment, we raised oysters from two sites in San Francisco Bay and one site in Tomales Bay through two generations under common conditions and then subjected these oysters to a low salinity challenge in the laboratory. Oysters with the highest survival originated from the site with the lowest field salinity, suggesting that natural selection may favor stress tolerant phenotypes in certain regions. As interest grows in restoring heavily impacted native species, our results indicate that considering local adaptation may be essential to deciding how and where to conserve and restore species faced with changing conditions.

The Olympia oyster (Ostrea lurida) is a foundation species inhabiting estuaries along the North American west coast. In California estuaries, O. lurida is adapted to local salinity regimes and populations differ in low salinity tolerance. In this study, oysters from three California populations were reared for two generations in a laboratory common garden and subsequently exposed to low salinity seawater. Comparative transcriptomics was then used to understand species-level responses to hyposmotic stress and population-level mechanisms underlying divergent salinity tolerances. Gene expression patterns indicate Olympia oysters are sensitive to hyposmotic stress: All populations respond to low salinity by up-regulating transcripts indicative of protein unfolding, DNA damage and cell cycle arrest after sub-lethal exposure. Among O. lurida populations, transcriptomic profiles differed constitutively and in response to low salinity. Despite two generations in common-garden conditions, transcripts encoding apoptosis modulators were constitutively expressed at significantly different levels in the most tolerant population. Expression of cell death regulators may facilitate cell fate decisions when salinity declines. Following low salinity exposure, oysters from the more tolerant population expressed a small number of mRNAs at significantly higher levels than less tolerant populations. Proteins encoded by these transcripts regulate ciliary activity within the mantle cavity and may function to prolong valve closure and reduce mortality in low salinity seawater. Collectively, gene expression patterns suggest sub-lethal impacts of hyposmotic stress in Olympia oysters are considerable and that even oysters with greater low salinity tolerance may be vulnerable to future freshwater flooding events.