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Anthropogenic stressors associated with climate change and shoreline development are increasingly impacting important habitats. But when multiple stressors act simultaneously, the results are often difficult to predict. Effects of multiple anthropogenic stressors and their interactions with community processes are of particular concern in estuarine habitats because of their vulnerability to anthropogenic impacts and their provision of valuable ecosystem services such as fisheries production and sediment stabilization. Two stressors, warming temperatures and reduced light availability due to suspended materials, are expected to simultaneously affect important habitats such as seagrass systems, but the impacts of these stressors may be influenced by community processes such as immigration. Immigration of propagules can alter diversity and relative species abundances, rescue populations from extinction, and modify the capacity of communities to respond to environmental conditions. Using an experimental eelgrass ( Zostera marina ) system, we examined how warming, reduced light availability (shading), and periodic immigration of crustacean grazers influenced diversity, biomass, and structure of this community. Shading generally had large effects, and reduced biomass of all primary producers but did not affect grazer biomass. Warming reduced epiphytic algal biomass but only in the absence of grazers, while it increased sessile invertebrate biomass overall. Immigration of crustacean grazers did not interact significantly with stressors, and had little effect overall. Immigration did not influence diversity of grazers, relative abundance of grazer species, or biomass of primary producers and sessile invertebrates, however, it did increase biomass of grazers. Overall, reduced light availability by shading had strong effects on primary producers, whereas warming generally had weaker effects that differed with grazer presence. Thus, our results show that the impacts of reduced light availability and warming were independent and context specific, while crustacean grazer immigration effects were minimal in this model eelgrass system. [ABSTRACT FROM AUTHOR]

Coastal ecosystems are essential for absorbing and bouncing back from the impacts of climate change, yet accelerating climate change is causing anthropogenically-derived stressors in these ecosystems to grow. The effects of stressors are more difficult to foresee when they act simultaneously, however, predicting these effects is critical for understanding ecological change. Spartina alterniflora (Spartina) , a foundational saltmarsh plant key to coastal resilience, is subject to biological stress such as herbivory, as well as anthropogenic stress such as chemical pollution. Using saltmarsh mesocosms as a model system in a fully factorial experiment, we tested whether the effects of herbivory and two chemicals (oil and dispersant) were mediated or magnified in combination. Spartina responded to stressors asynchronously; ecophysiology responded negatively to oil and herbivores in the first 2–3 weeks of the experiment, whereas biomass responded negatively to oil and herbivores cumulatively throughout the experiment. We generally found mixed multi-stressor effects, with slightly more antagonistic effects compared to either synergistic or additive effects, despite significant reductions in Spartina biomass and growth from both chemical and herbivore treatments. We also observed an indirect positive effect of oil on Spartina , via a direct negative effect on insect herbivores. Our findings suggest that multi-stressor effects in our model system, 1) are mixed but can be antagonistic more often than expected, a finding contrary to previous assumptions of primarily synergistic effects, 2) can vary in duration, 3) can be difficult to discern a priori , and 4) can lead to ecological surprises through indirect effects with implications for coastal resilience. This leads us to conclude that understanding the simultaneous effects of multiple stressors is critical for predicting foundation-species persistence, discerning ecosystem resilience, and managing and mitigating impacts on ecosystem services. • Multi-stressor effects were mixed with marginally more antagonistic than either synergistic or additive effects. • Ecologically surprising indirect and direct effects were observed, in particular indirect positive effect of oil on plants. • Asynchronous ecophysiological and biomass plant responses to anthropogenic stressors. • Unified ecological theory for a predictive understanding of multi-stressor effects remains underdeveloped. [ABSTRACT FROM AUTHOR]