Combined Impacts

Temperature already provides limits to the survival of organisms; it alters many physiological processes by acting on the rates at which these processes occur (e.g. speeding up metabolism, enzyme activity, etc.). However, organisms are acclimatised to a certain temperature range. Acidification may act to narrow these ranges [9]. Increasing temperature will also drive many species polewards, either as a result of biogeographic range expansion (by temperate and tropical species) or as a result of contraction (by boreal and polar species). However, ocean acidification may act in the opposite direction, as the polar waters will be most affected by increasing CO2 [6]. This could lead to a complete disappearance of boreal and polar species and may restrict the ability of temperate and tropical species to migrate.

Available oxygen is also a significant factor in controlling the distribution of organisms in marine environments. Eutrophication events and warming of waters decreases the oxygen content causing hypoxia. As mentioned previously, hypoxia is nearly always accompanied by an elevation of CO2 (and thus a decrease in pH) and will compound the impacts [62].

Corals are again a good example of the effects of multiple stresses. They are affected by both ocean acidification and by warming of ocean surface waters leading to declining calcification and increase in bleaching [7,63]. Other climate change factors (sea-level rise, storm impact, aerosols, ultra-violet irradiation) and non-climate factors (over-fishing, invasion of non-native species, pollution, disease, nutrient and sediment load) add multiple impacts on coral reefs, increasing their vulnerability and reducing their resilience [7,32,63 65]. A recent report shows that about half of the coral reef ecosystem resources within the United States and Pacific Freely Associated States jurisdiction are considered by scientists to be in 'poor' or 'fair' condition and have declined over time due to several natural and anthropogenic threats [66]. Another consensus of opinion is that one-third of reef-building corals face elevated risk of extinction from climate change and local impacts and that the loss of reef ecosystems would lead to large-scale loss of global biodiversity [67].

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