• multiple stressors
• local stressors
• global climate change
• disturbance
• coastal systems

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 loading. The results showed that both chronic and pulse nutrient enrichment can compromise the ability of the seagrass to cope with high herbivory, either directly, by altering plant physiology, or, indirectly, by stimulating consumption. High grazing pressure caused a more severe reduction of plant biomass when combined with chronic than pulse nutrient enrichment. These results suggest that taking into account herbivore pressure in necessary to accurately assess the effects of different temporal regimes of nutrient supply on seagrass meadows.
In chapter 4, I used shallow rocky reefs, composed by a mosaic of stands of the canopy-forming macroalga, Cystoseira spp., and barren patches, to experimentally investigate the role of canopy degradation, nutrient enrichment and sea urchin density, in triggering the shift from canopy-dominated to alternative states inside and at the margin of macroalgal forests. High grazing pressure altered the structure of the whole assemblages, possibly leading to the transition from high diverse habitats to low productive barren grounds, in particular at high levels of disturbance (canopy removal). In contrast, under weak control by consumers, moderate or severe events of disturbance on habitat-forming macroalgae did not result in a change of the relative extent of contrasting habitats. Understanding the mechanism that regulate the switches to alternative habitats requires taking into account interactions among species that compose each habitats and the way they are modified by abiotic and biotic stressors.
In chapter 5, I experimentally investigated the cumulative effects of ocean acidification and hypoxia on the organic matter cycling in soft sediments, through a short-term mesocosm experiment. I used isotopically labelled macroalgae as a tracer to assess faunal uptake of organic carbon and carbon incorporation into the sediment under different experimental conditions. The results showed that the effects of elevated [CO2] on C-uptake by fauna varied with different oxygen concentration. Under normoxia, elevated [CO2] significantly enhanced faunal uptake of organic carbon, likely due to the higher energetic cost of living associated to high level of CO2. By contrast, following the hypoxia event, there was limited C-uptake by fauna exposed to high CO2, potentially leading to the metabolic depression of invertebrates. The results suggest that the capacity of invertebrates to maintain vital physiological processes could impair under the combined effects of the two stressors.
This research helps to better understanding the cumulative nature of human impacts on key structural and functional systems and may have important implications for management strategies aimed to sustain the functioning of marine ecosystems in face to multiple anthropogenic stressors.