| Summary: | Coastal ecosystems are simultaneously exposed to a variety of stressors that operate at different spatial and temporal scales. The effects of global climate changes are likely to be dependent on local settings, yet combined effects of global and regional stressors are poorly understood. Predicting the outcome of multiple stressors is further complicated by variations in their temporal regimes. My thesis aims to investigate the cumulative effects of multiple stressors on coastal benthic systems. I focused on three different benthic habitats: the seagrass, Posidonia oceanica, assemblages dominated by the canopy forming algae, Cystoseira spp., and permeable soft sediment hosting a diverse infaunal invertebrate community. In Chapter 2, I used shallow water CO2 vents to assess how the effects of ocean acidification on the seagrass, P. oceanica, and the associated epiphytic community can be modified by enhanced nutrient loading. The compounded effects of global and local stressors were evaluated across different organization levels, from genes to the whole community. The results showed that nutrient enrichment compensated the negative effects of ocean acidification on the seagrass leaf production. The antagonistic interaction of these stressors was likely the result of direct effects on the physiology of the plant and indirect effects due to changes in species interactions (plant-epiphytes). These results show that the effects of global stressors are likely to be context-dependent and may have important implication for management strategies aimed to sustain the functioning of marine ecosystems in face of climate change. In Chapter 3, I investigated the compounded effects of nutrient supply and simulated herbivory on a shallow P. oceanica bed. To assess the temporal variability of disturbances, chronic and pulse nutrient loading were combined with simulated herbivory, treated as a pulse stress. I evaluated traits underpinning tolerance and resistance to herbivory in P. oceanica under different regimes of nutrients ...
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