Impact of climate change and ENSO on marine aerobic habitat in the South East Pacific

Overview & Synopsis

On-going climate change (CC) has emerged as an additional anthropogenic threat (on top of pollution and over-fishing pressures) to marine ecosystems across the world's ocean. The oceans are warming at unprecedented rates and a global deoxygenation is materializing. The consequences on marine ecosystems are already observed, with rapid poleward shifts of pelagic organisms and profound transformations of ecosystems, attributed to ocean warming. If a primary concern should naturally be to limit our imprint on natural systems, marine ecosystems support the livelihood of a number of societies and provide precious services, such as food, carbon sequestration and oxygen production. Because inflecting the rise of greenhouse gases emissions remains presently an unresolved issue, it is primordial to (attempt to) anticipate the effect of climate change, with the ambition to mitigate its impacts. The South East Pacific (SEP) hosts the Humboldt Current System (HCS), the most productive system in the world. It is associated with an Eastern Boundary Upwelling System (EBUS), an Oxygen Minimum Zone (OMZ), and is crossed by several seamount systems, hosting unique ecosystems composed of endemic species. Whereas the impact of climate change in the SEP physical processes has been well documented, the impact on marine organisms has been studied only weakly. Climate change impacts marine organisms in a number of ways. The response of organisms is usually attributed to species thermal tolerance levels, which would be reached or exceeded with rising temperature. Temperature is a controlling driver of species physiology and metabolism as a number of functions scale with temperature and/or are limited at some critical temperatures. Rising temperature and changes in oxygen content (or rather oxygen partial pressure) modulate the oxygen supply and demand of aerobic organisms. Species capacity to perform aerobically is possibly correlated to their geographical distribution. Hence, a number of range shifts has been attributed to temperature and oxygen changes. If climate change impacts the SEP, it is also subjected to the largest climatic mode, the El Niño Southern Oscillation (ENSO), for which the effect on the SEP ecosystems are consequential but not well explained. Therefore, the purpose of this study is to assess the pressure of CC and ENSO on marine organisms in the future climate. We use metabolic constraints to infer the impact on organisms' physiology. We hypothesise that changing oxygen and temperature due to CC or ENSO may cause mismatch between oxygen supply and demand, hence modulating aerobic scope and habitat suitability. We show that there are strong regional patterns of changes in habitat suitability. We find that whereas hypoxia tolerant species evolving in the vicinity of the OMZ may find improving conditions, most of the SEP organisms will face pressure from deoxygenation trends and warming, either due to ENSO or CC. Eastern Pacific El Niño events marginally compensate for the deoxygenation trends in remote areas of the seamounts. These changes in habitat suitability and in particular the differences between the epipelagic and mesopelagic realms, could have profound implications at the ecosystem level, triggering a recomposition and reorganisation of fish populations. This poses additional challenges for the design of Marine Protected Areas such as considering whole trophic chains and 3D habitat of target species.